The Mammæ-Mammary Gland; Breasts

The mammæ secrete the milk, and are accessory glands of the generative system. They exist in the male as well as in the female.

In the female they are two large hemispherical eminences lying within the superficial fascia and situated on the front and sides of the chest; each extends from the second rib above to the sixth rib below, and from the side of the sternum to near the midaxillary line. Their weight and dimensions differ at different periods of life, and in different individuals. Before puberty they are of small size, but enlarge as the generative organs become more completely developed. They increase during pregnancy and especially after delivery, and become atrophied in old age. The left mamma is generally a little larger than the right. The deep surface of each is nearly circular, flattened, or slightly concave, and has its long diameter directed upward and lateralward toward the axilla; it is separated from the fascia covering the Pectoralis major, Serratus anterior, and Obliquus externus abdominis by loose connective tissue. The subcutaneous surface of the mamma is convex, and presents, just below the center, a small conical prominence, the papilla.

The Mammary Papilla or Nipple (papilla mammæ) is a cylindrical or conical eminence situated about the level of the fourth intercostal space. It is capable of undergoing a sort of erection from mechanical excitement, a change mainly due to the contraction of its muscular fibers. It is of a pink or brownish hue, its surface wrinkled and provided with secondary papillæ; and it is perforated by from fifteen to twenty orifices, the apertures of the lactiferous ducts. The base of the mammary papilla is surrounded by an areola. In the virgin the areola is of a delicate rosy hue; about the second month after impregnation it enlarges and acquires a darker tinge, and as pregnancy advances it may assume a dark brown or even black color. This color diminishes as soon as lactation is over, but is never entirely lost throughout life. These changes in the color of the areola are of importance in forming a conclusion in a case of suspected first pregnancy. Near the base of the papilla, and upon the surface of the areola, are numerous large sebaceous glands, the areolar glands, which become much enlarged during lactation, and present the appearance of small tubercles beneath the skin. These glands secrete a peculiar fatty substance, which serves as a protection to the integument of the papilla during the act of sucking. The mammary papilla consists of numerous vessels, intermixed with plain muscular fibers, which are principally arranged in a circular manner around the base: some few fibers radiating from base to apex.

Structure—The mamma consists of gland tissue; of fibrous tissue, connecting its lobes; and of fatty tissue in the intervals between the lobes. The gland tissue, when freed from fibrous tissue and fat, is of a pale reddish color, firm in texture, flattened from before backward and thicker in the center than at the circumference. The subcutaneous surface of the mamma presents numerous irregular processes which project toward the skin and are joined to it by bands of connective tissue. It consists of numerous lobes, and these are composed of lobules, connected together by areolar tissue, bloodvessels, and ducts. The smallest lobules consist of a cluster of rounded alveoli, which open into the smallest branches of the lactiferous ducts; these ducts unite to form larger ducts, and these end in a single canal, corresponding with one of the chief subdivisions of the gland. The number of excretory ducts varies from fifteen to twenty; they are termed the tubuli lactiferi. They converge toward the areola, beneath which they form dilatations or ampullæ, which serve as reservoirs for the milk, and, at the base of the papillæ, become contracted, and pursue a straight course to its summit, perforating it by separate orifices considerably narrower than the ducts themselves. The ducts are composed of areolar tissue containing longitudinal and transverse elastic fibers; muscular fibers are entirely absent; they are lined by columnar epithelium resting on a basement membrane. The epithelium of the mamma differs according to the state of activity of the organ. In the gland of a woman who is not pregnant or suckling, the alveoli are very small and solid, being filled with a mass of granular polyhedral cells. During pregnancy the alveoli enlarge, and the cells undergo rapid multiplication. At the commencement of lactation, the cells in the center of the alveolus undergo fatty degeneration, and are eliminated in the first milk, as colostrum corpuscles. The peripheral cells of the alveolus remain, and form a single layer of granular, short columnar cells, with spherical nuclei, lining the basement membrane. The cells, during the state of activity of the gland, are capable of forming, in their interior, oil globules, which are then ejected into the lumen of the alveolus, and constitute the milk globules. When the acini are distended by the accumulation of the secretion the lining epithelium becomes flattened.

Development.—The mamma is developed partly from mesoderm and partly from ectoderm—its bloodvessels and connective tissue being derived from the former, its cellular elements from the latter. Its first rudiment is seen about the third month, in the form of a number of small inward projections of the ectoderm, which invade the mesoderm; from these, secondary tracts of cellular elements radiate and subsequently give rise to the epithelium of the glandular follicles and ducts. The development of the follicles, however, remains imperfect, except in the parous female

The Female Genital Organs

The female genital organs consist of an internal and an external group. The internal organs are situated within the pelvis, and consist of the ovaries, the uterine tubes, the uterus, and the vagina. The external organs are placed below the urogenital diaphragm and below and in front of the pubic arch. They comprise the mons pubis, the labia majora et minora pudendi, the clitoris, the bulbus vestibuli, and the greater vestibular glands.

The ovaries are homologous with the testes in the male. They are two nodular bodies, situated one on either side of the uterus in relation to the lateral wall of the pelvis, and attached to the back of the broad ligament of the uterus, behind and below the uterine tubes .The ovaries are of a grayish-pink color, and present either a smooth or a puckered uneven surface. They are each about 4 cm. in length, 2 cm. in width, and about 8 mm. in thickness, and weigh from 2 to 3.5 gm. Each ovary presents a lateral and a medial surface, an upper or tubal and a lower or uterine extremity, and an anterior or mesovarion and a posterior free border. It lies in a shallow depression, named the ovarian fossa, on the lateral wall of the pelvis; this fossa is bounded above by the external iliac vessels, in front by the obliterated umbilical artery, and behind by the ureter. The exact position of the ovary has been the subject of considerable difference of opinion, and the description here given applies to the ovary of the nulliparous woman. The ovary becomes displaced during the first pregnancy, and probably never again returns to its original position. In the erect posture the long axis of the ovary is vertical. The tubal extremity is near the external iliac vein; to it are attached the ovarian fimbria of the uterine tube and a fold of peritoneum, the suspensory ligament of the ovary, which is directed upward over the iliac vessels and contains the ovarian vessels. The uterine end is directed downward toward the pelvic floor, it is usually narrower than the tubal, and is attached to the lateral angle of the uterus, immediately behind the uterine tube, by a rounded cord termed the ligament of the ovary, which lies within the broad ligament and contains some non-striped, muscular fibers. The lateral surface is in contact with the parietal peritoneum, which lines the ovarian fossa; the medial surface is to a large extent covered by the fimbriated extremity of the uterine tube. The mesovarian border is straight and is directed toward the obliterated umbilical artery, and is attached to the back of the broad ligament by a short fold named the mesovarium. Between the two layers of this fold the bloodvessels and nerves pass to reach the hilum of the ovary. The free border is convex, and is directed toward the ureter. The uterine tube arches over the ovary, running upward in relation to its mesovarian border, then curving over its tubal pole, and finally passing downward on its free border and medial surface.

Structure—The surface of the ovary is covered by a layer of columnar cells which constitutes the germinal epithelium of Waldeyer. This epithelium gives to the ovary a dull gray color as compared with the shining smoothness of the peritoneum; and the transition between the squamous epithelium of the peritoneum and the columnar cells which cover the ovary is usually marked by a line around the anterior border of the ovary. The ovary consists of a number of vesicular ovarian follicles imbedded in the meshes of a stroma or frame-work.
The stroma is a peculiar soft tissue, abundantly supplied with bloodvessels, consisting for the most part of spindle-shaped cells with a small amount of ordinary connective tissue. These cells have been regarded by some anatomists as unstriped muscle cells, which, indeed, they most resemble; by others as connective-tissue cells. On the surface of the organ this tissue is much condensed, and forms a layer (tunica albuginea) composed of short connective-tissue fibers, with fusiform cells between them. The stroma of the ovary may contain interstitial cells resembling those of the testis.
Vesicular Ovarian Follicles (Graafian follicles).—Upon making a section of an ovary, numerous round transparent vesicles of various sizes are to be seen; they are the follicles, or ovisacs containing the ova. Immediately beneath the superficial covering is a layer of stroma, in which are a large number of minute vesicles, of uniform size, about 0.25 mm. in diameter. These are the follicles in their earliest condition, and the layer where they are found has been termed the cortical layer. They are especially numerous in the ovary of the young child. After puberty, and during the whole of the child-bearing period, large and mature, or almost mature follicles are also found in the cortical layer in small numbers, and also “corpora lutea,” the remains of follicles which have burst and are undergoing atrophy and absorption. Beneath this superficial stratum, other large and more or less mature follicles are found imbedded in the ovarian stroma. These increase in size as they recede from the surface toward a highly vascular stroma in the center of the organ, termed the medullary substance (zona vasculosa of Waldeyer). This stroma forms the tissue of the hilum by which the ovary is attached, and through which the bloodvessels enter: it does not contain any follicles.

Discharge of the Ovum.—The follicles, after attaining a certain stage of development, gradually approach the surface of the ovary and burst; the ovum and fluid contents of the follicle are liberated on the exterior of the ovary, and carried into the uterine tube by currents set up by the movements of the cilia covering the mucous membrane of the fimbriæ.

Corpus Luteum.—After the discharge of the ovum the lining of the follicle is thrown into folds, and vascular processes grow inward from the surrounding tissue. In this way the space is filled up and the corpus luteum formed. It consists at first of a radial arrangement of yellow cells with bloodvessels and lymphatic spaces, and later it merges with the surrounding stroma.

The uterine tubes convey the ova from the ovaries to the cavity of the uterus. They are two in number, one on either side, situated in the upper margin of the broad ligament, and extending from the superior angle of the uterus to the side of the pelvis. Each tube is about 10 cm. long, and is described as consisting of three portions: (1) the isthmus, or medial constricted third; (2) the ampulla, or intermediate dilated portion, which curves over the ovary; and (3) the infundibulum with its abdominal ostium, surrounded by fimbriæ, one of which, the ovarian fimbria is attached to the ovary. The uterine tube is directed lateralward as far as the uterine pole of the ovary, and then ascends along the mesovarian border of the ovary to the tubal pole, over which it arches; finally it turns downward and ends in relation to the free border and medial surface of the ovary. The uterine opening is minute, and will only admit a fine bristle; the abdominal opening is somewhat larger. In connection with the fimbriæ of the uterine tube, or with the broad ligament close to them, there are frequently one or more small pedunculated vesicles. These are termed the appendices vesiculosæ.

The uterus is a hollow, thick-walled, muscular organ situated deeply in the pelvic cavity between the bladder and rectum.

Cervix (cervix uteri; neck).—The cervix is the lower constricted segment of the uterus. It is somewhat conical in shape, with its truncated apex directed downward and backward, but is slightly wider in the middle than either above or below.

The vagina extends from the vestibule to the uterus, and is situated behind the bladder and in front of the rectum.Its is a female sexual organ.

The external genital organs of the female are: the mons pubis, the labia majora et minora pudendi, the clitoris, the vestibule of the vagina, the bulb of the vestibule, and the greater vestibular glands.

The Mons Pubis (commissura labiorum anterior; mons Veneris), the rounded eminence in front of the pubic symphysis, is formed by a collection of fatty tissue beneath the integument. It becomes covered with hair at the time of puberty.
The Labia Majora (labia majora pudendi) are two prominent longitudinal cutaneous folds which extend downward and backward from the mons pubis and form the lateral boundaries of a fissure or cleft, the pudendal cleft or rima, into which the vagina and urethra open. The Labia Minora (labia minora pudendi; nymphæ) are two small cutaneous folds, situated between the labia majora, and extending from the clitoris obliquely downward,lateralward, and backward for about 4 cm. on either side of the orifice of the vagina, between which and the labia majora they end.

The Clitoris is an erectile structure, homologous with the penis. It is situated beneath the anterior labial commissure, partially hidden between the anterior ends of the labia minora. It consists of two corpora cavernosa, composed of erectile tissue enclosed in a dense layer of fibrous membrane, united together along their medial surfaces by an incomplete fibrous pectiniform septum; each corpus is connected to the rami of the pubis and ischium by a crus; the free extremity (glans clitoridis) is a small rounded tubercle, consisting of spongy erectile tissue, and highly sensitive. The clitoris is provided like the penis, with a suspensory ligament, and with two small muscles, the Ischiocavernosi, which are inserted into the crura of the clitoris.

The hymen is a thin fold of mucous membrane situated at the orifice of the vagina; the inner edges of the fold are normally in contact with each other, and the vaginal orifice appears as a cleft between them. The hymen varies much in shape

Male Genitals Organ

The male genitals include the testes, the ductus deferentes, the vesiculæ seminales, the ejaculatory ducts, and the penis, together with the following accessory structures, viz., the prostate and the bulbourethral glands.
1. The Testes and their Coverings—The testes are two glandular organs, which secrete the semen; they are suspended in the scrotum by the spermatic cords. At an early period of fetal life the testes are contained in the abdominal cavity, behind the peritoneum. Before birth they descend to the inguinal canal, along which they pass with the spermatic cord, and, emerging at the subcutaneous inguinal ring, they descend into the scrotum, becoming invested in their course by coverings derived from the serous, muscular, and fibrous layers of the abdominal parietes, as well as by the scrotum.
The coverings of the testes are, the
Skin
Scrotum.
Cremaster.
Dartos tunic
Infundibuliform fascia.
Intercrural fascia.
Tunica vaginalis.

The Scrotum is a cutaneous pouch which contains the testes and parts of the spermatic cords. It is divided on its surface into two lateral portions by a ridge or raphé, which is continued forward to the under surface of the penis, and backward, along the middle line of the perineum to the anus.

The Integument is very thin, of a brownish color, and generally thrown into folds or rugæ. It is provided with sebaceous follicles, the secretion of which has a peculiar odor, and is beset with thinly scattered, crisp hairs, the roots of which are seen through the skin.

The epididymis consists of a central portion or body; an upper enlarged extremity, the head (globus major); and a lower pointed extremity, the tail (globus minor), which is continuous with the ductus deferens, the duct of the testis.

Appendages of the Testis and Epididymis.—On the upper extremity of the testis, just beneath the head of the epididymis, is a minute oval, sessile body, the appendix of the testis (hydatid of Morgagni); it is the remnant of the upper end of the Müllerian duct. On the head of the epididymis is a second small stalked appendage (sometimes duplicated); it is named the appendix of the epididymis (pedunculated hydatid), and is usually regarded as a detached efferent duct.
The testis is invested by three tunics: the tunica vaginalis, tunica albuginea, and tunica vasculosa.

The Tunica Vaginalis (tunica vaginalis propria testis) is the serous covering of the testis. It is a pouch of serous membrane, derived from the saccus vaginalis of the peritoneum, which in the fetus preceded the descent of the testis from the abdomen into the scrotum. After its descent, that portion of the pouch which extends from the abdominal inguinal ring to near the upper part of the gland becomes obliterated; the lower portion remains as a shut sac, which invests the surface of the testis, and is reflected on to the internal surface of the scrotum; hence it may be described as consisting of a visceral and a parietal lamina.
The visceral lamina (lamina visceralis) covers the greater part of the testis and epididymis, connecting the latter to the testis by means of a distinct fold. From the posterior border of the gland it is reflected on to the internal surface of the scrotum.
The parietal lamina (lamina parietalis) is far more extensive than the visceral, extending upward for some distance in front and on the medial side of the cord, and reaching below the testis. The inner surface of the tunica vaginalis is smooth, and covered by a layer of endothelial cells. The interval between the visceral and parietal laminæ constitutes the cavity of the tunica vaginalis.

The Tunica Albuginea is the fibrous covering of the testis. It is a dense membrane, of a bluish-white color, composed of bundles of white fibrous tissue which interlace in every direction. It is covered by the tunica vaginalis, except at the points of attachment of the epididymis to the testis

The ductus deferens, the excretory duct of the testis, is the continuation of the canal of the epididymis

vesiculæ seminales -

The vesiculæ seminales are two lobulated membranous pouches, placed between the fundus of the bladder and the rectum, serving as reservoirs for the semen, and secreting a fluid to be added to the secretion of the testes. Each sac is somewhat pyramidal in form, the broad end being directed backward, upward and lateralward.

The Ejaculatory Ducts-

The ejaculatory ducts are two in number, one on either side of the middle line. Each is formed by the union of the duct from the vesicula seminalis with the ductus deferens, and is about 2 cm. long. They commence at the base of the prostate, and run forward and downward between its middle and lateral lobes, and along the sides of the prostatic utricle, to end by separate slit-like orifices close to or just within the margins of the utricle. The ducts diminish in size, and also converge, toward their terminations.

The Penis-

The penis is a pendulous organ suspended from the front and sides of the pubic arch and containing the greater part of the urethra. In the flaccid condition it is cylindrical in shape, but

when erect assumes the form of a triangular prism with rounded angles, one side of the prism forming the dorsum. It is composed of three cylindrical masses of cavernous tissue bound together by fibrous tissue and covered with skin. Two of the masses are lateral, and are known as the corpora cavernosa penis; the third is median, and is termed the corpus cavernosum urethræ.

The corpora cavernosa penis are surrounded by a strong fibrous envelope consisting of superficial and deep fibers.The Corpus Cavernosum Urethræ (corpus spongiosum) contains the urethra. Behind, it is expanded to form the urethral bulb, and lies in apposition with the inferior fascia of the urogenital diaphragm, from which it receives a fibrous investment.

The prostate-- is a firm, partly glandular and partly muscular body, which is placed immediately below the internal urethral orifice and around the commencement of the urethra. It is situated in the pelvic cavity, below the lower part of the symphysis pubis, above the superior fascia of the urogenital diaphragm, and in front of the rectum, through which it may be distinctly felt, especially when enlarged. It is about the size of a chestnut and somewhat conical in shape, and presents for examination a base, an apex, an anterior, a posterior and two lateral surfaces.

The bulbourethral glands--are two small, rounded, and somewhat lobulated bodies, of a yellow color, about the size of peas, placed behind and lateral to the membranous portion of the urethra, between the two layers of the fascia of the urogenital diaphragm. They lie close above the bulb, and are enclosed by the transverse fibers of the Sphincter urethræ membranaceæ. Their existence is said to be constant: they gradually diminish in size as age advances.The excretory duct of each gland

Microscopic Struture of Nephron

Minute Anatomy.—The renal tubules , of which the kidney is for the most part made up, commence in the cortical substance, and after pursuing a very circuitous course through the cortical and medullary substances, finally end at the apices of the renal pyramids by open mouths, so that the fluid which they contain is emptied, through the calyces, into the pelvis of the kidney. If the surface of one of the papillæ be examined with a lens, it will be seen to be studded over with minute openings, the orifices of the renal tubules, from sixteen to twenty in number, and if pressure be made on a fresh kidney, urine will be seen to exude from these orifices. The tubules commence in the convoluted part and renal columns as the renal corpuscles, which are small rounded masses of a deep red color, varying in size, but of an average of about 0.2 mm. in diameter. Each of these little bodies is composed of two parts: a central glomerulus of vessels, and a membranous envelope, the glomerular capsule (capsule of Bowman------------------------->)

which is the small pouch-like commencement of a renal tubule.
The glomerulus is a lobulated net-work of convoluted capillary bloodvessels, held together by scanty connective tissue. This capillary net-work is derived from a small arterial twig, the afferent vessel, which enters the capsule, generally at a point opposite to that at which the latter is connected with the tubule; and the resulting vein, the efferent vessel, emerges from the capsule at the same point. The afferent vessel is usually the larger.The glomerular or Bowman’s capsule, which surrounds the glomerulus, consists of a basement membrane, lined on its inner surface by a layer of flattened epithelial cells, which are reflected from the lining membrane on to the glomerulus, at the point of entrance or exit of the afferent and efferent vessels. The whole surface of the glomerulus is covered with a continuous layer of the same cells, on a delicate supporting membrane. Thus between the glomerulus and the capsule a space is left, forming a cavity lined by a continuous layer of squamous cells; this cavity varies in size according to the state of secretion and the amount of fluid present in it. In the fetus and young subject the lining epithelial cells are polyhedral or even columnar.

The renal tubules, commencing in the renal corpuscles, present, during their course, many changes in shape and direction, and are contained partly in the medullary and partly in the cortical substance. At their junction with the glomerular capsule they exhibit a somewhat constricted portion, which is termed the neck. Beyond this the tubule becomes convoluted, and pursues a considerable course in the cortical substance constituting the proximal convoluted tube. After a time the convolutions disappear, and the tube approaches the medullary substance in a more or less spiral manner; this section of the tubule has been called the spiral tube. Throughout this portion of their course the renal tubules are contained entirely in the cortical substance, and present a fairly uniform caliber. They now enter the medullary substance, suddenly become much smaller, quite straight in direction, and dip down for a variable depth into the pyramids, constituting the descending limb of Henle’s loop. Bending on themselves, they form what is termed the loop of Henle, and reascending, they become suddenly enlarged, forming the ascending limb of Henle’s loop, and reënter the cortical substance. This portion of the tubule ascends for a short distance, when it again becomes dilated, irregular, and angular. This section is termed the zigzag tubule; it ends in a convoluted tube, which resembles the proximal convoluted tubule, and is called the distal convoluted tubule. This again terminates in a narrow junctional tube, which enters the straight or collecting tube.
The straight or collecting tubes commence in the radiate part of the cortex, where they receive the curved ends of the distal convoluted tubules. They unite at short intervals with one another, the resulting tubes presenting a considerable increase in caliber, so that a series of comparatively large tubes passes from the bases of the rays into the renal pyramids. In the medulla the tubes of each pyramid converge to join a central tube (duct of Bellini) which finally opens on the summit of one of the papillæ; the contents of the tube are therefore discharged into one of the calyces.
Structure of the Renal Tubules.—The renal tubules consist of a basement membrane lined with epithelium. The epithelium varies considerably in different sections of the tubule. In the neck the epithelium is continuous with that lining the glomerular capsule, and like it consists of flattened cells each containing an oval nucleus.The two convoluted tubules, the spiral and zigzag tubules and the ascending limb of Henle’s loop, are lined by a type of epithelium which is histologically the same in all. The cells are somewhat columnar in shape and dovetail into one another of their lateral aspect. Each has a striated border next the lumen of the tube, its inner part is granular and its outer portion vertically striated. The nucleus is spherical and situated about the center of the cell. In the descending limb of Henle’s loop the epithelium resembles that found in the glomerular capsule and the commencement of the tube, consisting of flat, clear epithelial plates, each with an oval nucleus.The nuclei alternate on opposite surfaces of the tubule so that the lumen remains fairly constant.

Internal Structure of the Kidney

Internal Structure of the Kidney.—The kidney is invested by a fibrous tunic, which forms a firm, smooth covering to the organ. The tunic can be easily stripped off, but in doing so numerous fine processes of connective tissue and small bloodvessels are torn through. Beneath this coat a thin, wide-meshed net-work of unstriped muscular fiber forms an incomplete covering to the organ. When the capsule is stripped off, the surface of the kidney is found to be smooth and even and of a deep red color. In infants fissures extending for some depth may be seen on the surface of the organ, a remnant of the lobular construction of the gland. The kidney is dense in texture, but is easily lacerable by mechanical force. If a vertical section of the kidney be made from its convex to its concave border, it will be seen that the hilum expands into a central cavity, the renal sinus, this contains the upper part of the renal pelvis and the calyces, surrounded by some fat in which are imbedded the branches of the renal vessels and nerves. The renal sinus is lined by a prolongation of the fibrous tunic, which is continued around the lips of the hilum. The renal calyces, from seven to thirteen in number, are cup-shaped tubes, each of which embraces one or more of the renal papillæ; they unite to form two or three short tubes, and these in turn join to form a funnel-shaped sac, the renal pelvis. The renal pelvis, wide above and narrow below where it joins the ureter, is partly outside the renal sinus. The renal calyces and pelvis form the upper expanded end of the excretory duct of the kidney.
The kidney is composed of an internal medullary and an external cortical substance.
The medullary substance (substantia medullaris) consists of a series of red-colored striated conical masses, termed the renal pyramids, the bases of which are directed toward the circumference of the kidney, while their apices converge toward the renal sinus, where they form prominent papillæ projecting into the interior of the calyces.
The cortical substance (substantia corticalis) is reddish brown in color and soft and granular in consistence. It lies immediately beneath the fibrous tunic, arches over the bases of the pyramids, and dips in between adjacent pyramids toward the renal sinus. The parts dipping in between the pyramids are named the renal columns (Bertini), while the portions which connect the renal columns to each other and intervene between the bases of the pyramids and the fibrous tunic are called the cortical arches (indicated between A and A’ in Fig. 1127). If the cortex be examined with a lens, it will be seen to consist of a series of lighter-colored, conical areas, termed the radiate part, and a darker-colored intervening substance, which from the complexity of its structure is named the convoluted part. The rays gradually taper toward the circumference of the kidney, and consist of a series of outward prolongations from the base of each renal pyramid.

Kidney

The urinary organs comprise the kidneys, which secrete the urine, the ureters, or ducts, which convey urine to the urinary bladder, where it is for a time retained; and the urethra, through which it is discharged from the body
1. The Kidneys (Renes)—The kidneys are situated in the posterior part of the abdomen, one on either side of the vertebral column, behind the peritoneum, and surrounded by a mass of fat and loose areolar tissue. Their upper extremities are on a level with the upper border of the twelfth thoracic vertebra, their lower extremities on a level with the third lumbar. The right kidney is usually slightly lower than the left, probably on account of the vicinity of the liver. The long axis of each kidney is directed downward and lateralward; the transverse axis backward and lateralward.
Each kidney is about 11.25 cm. in length, 5 to 7.5 cm. in breadth, and rather more than 2.5 cm. in thickness. The left is somewhat longer, and narrower, than the right. The weight of the kidney in the adult male varies from 125 to 170 gm., in the adult female from 115 to 155 gm. The combined weight of the two kidneys in proportion to that of the body is about 1 to 240.
The kidney has a characteristic form, and presents for examination two surfaces, two borders, and an upper and lower extremity.

Organs Of Digestion

The apparatus for the digestion of the food consists of the digestive tube and of certain accessory organs.
The Digestive Tube (alimentary canal) is a musculomembranous tube, about 9 metres long, extending from the mouth to the anus, and lined throughout its entire extent by mucous membrane. It has received different names in the various parts of its course: at its commencement is the mouth, where provision is made for the mechanical division of the food (mastication), and for its admixture with a fluid secreted by the salivary glands (insalivation); beyond this are the organs of deglutition, the pharynx and the esophagus, which convey the food into the stomach, in which it is stored for a time and in which also the first stages of the digestive process take place; the stomach is followed by the small intestine, which is divided for purposes of description into three parts, the duodenum, the jejunum, and ileum. In the small intestine the process of digestion is completed and the resulting products are absorbed into the blood and lacteal vessels. Finally the small intestine ends in the large intestine, which is made up of cecum, colon, rectum, and anal canal, the last terminating on the surface of the body at the anus.
The accessory organs are the teeth, for purposes of mastication; the three pairs of salivary glands—the parotid, submaxillary, and sublingual—the secretion from which mixes with the food in the mouth and converts it into a bolus and acts chemically on one of its constituents; the liver and pancreas, two large glands in the abdomen, the secretions of which, in addition to that of numerous minute glands in the walls of the alimentary canal, assist in the process of digestion.

Sense of Hear

The external ear consists of the expanded portion named the auricula or pinna, and the external acoustic meatus.

The Auricula or Pinna is of an ovoid form, with its larger end directed upward. Its lateral surface is irregularly concave, directed slightly forward, and presents numerous eminences and depressions to which names have been assigned. The prominent rim of the auricula is called the helix; where the helix turns downward behind, a small tubercle, the auricular tubercle

The lobule, composed of tough areolar and adipose tissues, and wanting the firmness and elasticity of the rest of the auricula

The middle ear or tympanic cavity is an irregular, laterally compressed space within the temporal bone. It is filled with air, which is conveyed to it from the nasal part of the pharynx through the auditory tube. It contains a chain of movable bones, which connect its lateral to its medial wall, and serve to convey the vibrations communicated to the tympanic membrane across the cavity to the internal ear.

The internal ear is the essential part of the organ of hearing, receiving the ultimate distribution of the auditory nerve. It is called the labyrinth, from the complexity of its shape, and consists of two parts: the osseous labyrinth, a series of cavities within the petrous part of the temporal bone, and the membranous labyrinth, a series of communicating membranous sacs and ducts, contained within the bony cavities.

(Right osseous labyrinth. Lateral view)

(Interior of right osseous labyrinth)

The Osseous Labyrinth (labyrinthus osseus)-The osseous labyrinth consists of three parts: the vestibule, semicircular canals, and cochlea. These are cavities hollowed out of the substance of the bone, and lined by periosteum; they contain a clear fluid, the perilymph, in which the membranous labyrinth is situated.

The Vestibule (vestibulum).—The vestibule is the central part of the osseous labyrinth, and is situated medial to the tympanic cavity, behind the cochlea, and in front of the semicircular canals. It is somewhat ovoid in shape, but flattened transversely

Organ of Vision

From without inward the three tunics are: (1) A fibrous tunic, in the shown figure consisting of the sclera behind and the cornea in front; (2) a vascular pigmented tunic, comprising, from behind forward, the choroid, ciliary body, and iris; and (3) a nervous tunic, the retina.

The Fibrous Tunic (tunica fibrosa oculi).—The sclera and cornea form the fibrous tunic of the bulb of the eye; the sclera is opaque, and constitutes the posterior five-sixths of the tunic; the cornea is transparent, and forms the anterior sixth.

The Sclera.—The sclera has received its name from its extreme density and hardness; it is a firm, unyielding membrane, serving to maintain the form of the bulb.

The Cornea.—The cornea is the projecting transparent part of the external tunic, and forms the anterior sixth of the surface of the bulb. It is almost circular in outline, occasionally a little broader in the transverse than in the vertical direction. It is convex anteriorly and projects like a dome in front of the sclera. Its degree of curvature varies in different individuals, and in the same individual at different periods of life, being more pronounced in youth than in advanced life. The cornea is dense and of uniform thickness throughout; its posterior surface is perfectly circular in outline, and exceeds the anterior surface slightly in diameter. Immediately in front of the sclero-corneal junction the cornea bulges inward as a thickened rim, and behind this there is a distinct furrow between the attachment of the iris and the sclero-corneal junction.

The Vascular Tunic (tunica vasculosa oculi) —The vascular tunic of the eye is formed from behind forward by the choroid, the ciliary body, and the iris.
The choroid invests the posterior five-sixths of the bulb, and extends as far forward as the ora serrata of the retina. The ciliary body connects the choroid to the circumference of the iris. The iris is a circular diaphragm behind the cornea, and presents near its center a rounded aperture, the pupil.

The Choroid (chorioidea).—The choroid is a thin, highly vascular membrane, of a dark brown or chocolate color, investing the posterior five-sixths of the globe; it is pierced behind by the optic nerve, and in this situation is firmly adherent to the sclera. It is thicker behind than in front. Its outer surface is loosely connected by the lamina suprachorioidea with the sclera; its inner surface is attached to the pigmented layer of the retina.

The Ciliary Body (corpus ciliare).—The ciliary body comprises the orbiculus ciliaris, the ciliary processes, and the Ciliaris muscle.

The Iris.—The iris has received its name from its various colors in different individuals. It is a thin, circular, contractile disk, suspended in the aqueous humor between the cornea and lens, and perforated a little to the nasal side of its center by a circular aperture, the pupil.
The orbiculus ciliaris is a zone of about 4 mm. in width, directly continuous with the anterior part of the choroid; it presents numerous ridges arranged in a radial manner in above figure.The ciliary processes (processus ciliares) are formed by the inward folding of the various layers of the choroid, i.e., the choroid proper and the lamina basalis, and are received between corresponding foldings of the suspensory ligament of the lens. They are arranged in a circle, and form a sort of frill behind the iris, around the margin of the lens

Organ of Smell

The peripheral olfactory organ or organ of smell consists of two parts: an outer, the external nose, which projects from the center of the face; and an internal, the nasal cavity, which is divided by a septum into right and left nasal chambers.

The External Nose (Nasus Externus; Outer Nose)—The external nose is pyramidal in form, and its upper angle or root is connected directly with the forehead; its free angle is termed the apex. Its base is perforated by two elliptical orifices, the nares, separated from each other by an antero-posterior septum, the columna.

The Nasal Cavity (Cavum Nasi; Nasal Fossa)—The nasal chambers are situated one on either side of the median plane. They open in front through the nares, and communicate behind through the choanæ with the nasal part of the pharynx. The nares are somewhat pear-shaped apertures, each measuring about 2.5 cm. antero-posteriorly and 1.25 cm. transversely at its widest part. The choanæ are two oval openings each measuring 2.5 cm. in the vertical, and 1.25 cm. in the transverse direction in a well-developed adult skull.
For the description of the bony boundaries of the nasal cavities, see pages 194 and 195.Inside the aperture of the nostril is a slight dilatation, the vestibule, bounded laterally by the ala and lateral crus of the greater alar cartilage, and medially by the medial crus of the same cartilage. It is lined by skin containing hairs and sebaceous glands, and extends as a small recess toward the apex of the nose. Each nasal cavity, above and behind the vestibule, is divided into two parts: an olfactory region, consisting of the superior nasal concha and the opposed part of the septum, and a respiratory region, which comprises the rest of the cavity.

The Mucous Membrane (membrana mucosa nasi).—The nasal mucous membrane lines the nasal cavities, and is intimately adherent to the periosteum or perichondrium. It is continuous with the skin through the nares, and with the mucous membrane of the nasal part of the pharynx through the choanæ. From the nasal cavity its continuity with the conjunctiva may be traced, through the nasolacrimal and lacrimal ducts; and with the frontal, ethmoidal, sphenoidal, and maxillary sinuses, through the several openings in the meatuses. The mucous membrane is thickest, and most vascular, over the nasal conchæ. It is also thick over the septum; but it is very thin in the meatuses on the floor of the nasal cavities, and in the various sinuses.

The Maxillary Sinus (sinus maxillaris; antrum of Highmore), the largest of the accessory sinuses of the nose, is a pyramidal cavity in the body of the maxilla. Its base is formed by the lateral wall of the nasal cavity, and its apex extends into the zygomatic process.

The Organs of Taste

The peripheral gustatory or taste organs consist of certain modified epithelial cells arranged in flask-shaped groups termed gustatory calyculi (taste-buds), which are found on the tongue and adjacent parts. They occupy nests in the stratified epithelium, and are present in large numbers on the sides of the papillæ vallatæ and to a less extent on their opposed walls. They are also found on the fungiform papillæ over the back part and sides of the tongue, and in the general epithelial covering of the same areas. They are very plentiful over the fimbriæ linguæ, and are also present on the under surface of the soft palate, and on the posterior surface of the epiglottis.

FIG– Vertical section of papilla foliata of the rabbit, crossing the folia--------------------------------->

Structure.—

Each taste bud is flask-like in shape

its broad base resting on the corium, and its neck opening by an orifice, the gustatory pore, between the cells of the epithelium. The bud is formed by two kinds of cells: supporting cells and gustatory cells. The supporting cells are mostly arranged like the staves of a cask, and form an outer envelope for the bud. Some, however, are found in the interior of the bud between the gustatory cells. The gustatory cells occupy the central portion of the bud; they are spindle-shaped, and each possesses a large spherical nucleus near the middle of the cell. The peripheral end of the cell terminates at the gustatory pore in a fine hair-like filament, the gustatory hair. The central process passes toward the deep extremity of the bud, and there ends in single or bifurcated varicosities. The nerve fibrils after losing their medullary sheaths enter the taste bud, and end in fine extremities between the gustatory cells; other nerve fibrils ramify between the supporting cells and terminate in fine extremities; these, however, are believed to be nerves of ordinary sensation and not gustatory.
3

FIG. 851– Taste-bud------>

Nerves of Taste.—The chorda tympani nerve, derived from the sensory root of the facial, is the nerve of taste for the anterior two-thirds of the tongue; the nerve for the posterior third is the glossopharyngeal.

The sympathetic nervous system innervates all the smooth muscles and the various glands of the body, and the striated muscle of the heart. The efferent sympathetic fibers which leave the central nervous system in connection with certain of the cranial and spinal nerves all end in sympathetic ganglia and are known as preganglionic fibers. From these ganglia postganglionic fibers arise and conduct impulses to the different organs. In addition, afferent or sensory fibers connect many of these structures with the central nervous system.
The peripheral portion of the sympathetic nervous system is characterized by the presence of numerous ganglia and complicated plexuses. These ganglia are connected with the central nervous system by three groups of sympathetic efferent or preganglionic fibers, i. e., the cranial, the thoracolumbar, and the sacral. These outflows of sympathetic fibers are separated by intervals where no connections exist. The cranial and sacral sympathetics are often grouped together owing to the resemblance between the reactions produced by stimulating them and by the effects of certain drugs. Acetyl-choline, for example, when injected intravenously in very small doses, produces the same effect as the stimulation of the cranial or sacral sympathetics, while the introduction of adrenalin produces the same effect as the stimulation of the thoracolumbar sympathetics. Much of our present knowledge of the sympathetic nervous system has been acquired through the application of various drugs, especially nicotine which paralyzes the connections or synapses between the preganglionic and postganglionic fibers of the sympathetic nerves. When it is injected into the general circulation all such synapses are paralyzed; when it is applied locally on a ganglion only the synapses occurring in that particular ganglion are paralyzed.
The Sympathetic Efferent Fibers of the Facial Nerve are supposed to arise from the small cells of the facial nucleus. According to some authors the fibers to the salivary glands arise from a special nucleus, the superior salivatory nucleus, consisting of cells scattered in the reticular formation, dorso-medial to the facial nucleus. These preganglionic fibers are distributed partly through the chorda tympani and lingual nerves to the submaxillary ganglion where they terminate about the cell bodies of neurons whose axons as postganglionic fibers conduct secretory and vasodilotar impulses to the submaxillary and sublingual glands. Other preganglionic fibers of the facial nerve pass via the great superficial petrosal nerve to the sphenopalatine ganglion where they form synapses with neurons whose postganglionic fibers are distributed with the superior maxillary nerve as vasodilator and secretory fibers to the mucous membrane of the nose, soft palate, tonsils, uvula, roof of the mouth, upper lips and gums, parotid and orbital glands

The Spinal Nerves

Distribution of cutaneous nerves:

(Ventral aspect) (Dorsal aspect)

The spinal nerves spring from the medulla spinalis, and are transmitted through the intervertebral foramina. They number thirty-one pairs, which are grouped as follows: Cervical, 8; Thoracic, 12; Lumbar, 5; Sacral, 5; Coccygeal, 1.
The first cervical nerve emerges from the vertebral canal between the occipital bone and the atlas, and is therefore called the suboccipital nerve; the eighth issues between the seventh cervical and first thoracic vertebræ.
Nerve Roots.—Each nerve is attached to the medulla spinalis by two roots, an anterior or ventral, and a posterior or dorsal, the latter being characterized by the presence of a ganglion, the spinal ganglion.
The Anterior Root (radix anterior; ventral root) emerges from the anterior surface of the medulla spinalis as a number of rootlets or filaments (fila radicularia), which coalesce to form two bundles near the intervertebral foramen.
The Posterior Root (radix posterior; dorsal root) is larger than the anterior owing to the greater size and number of its rootlets; these are attached along the posterolateral furrow of the medulla spinalis and unite to form two bundles which join the spinal ganglion. The posterior root of the first cervical nerve is exceptional in that it is smaller than the anterior; it is occasionally wanting.
The Spinal Ganglia (ganglion spinale) are collections of nerve cells on the posterior roots of the spinal nerves. Each ganglion is oval in shape, reddish in color, and its size bears a proportion to that of the nerve root on which it is situated; it is bifid medially where it is joined by the two bundles of the posterior nerve root. The ganglia are usually placed in the intervertebral foramina, immediately outside the points where the nerve roots perforate the dura mater, but there are exceptions to this rule; thus the ganglia of the first and second cervical nerves lie on the vertebral arches of the atlas and axis respectively, those of the sacral nerves are inside the vertebral canal, while that on the posterior root of the coccygeal nerve is placed within the sheath of dura mater

Structure—The ganglia consist chiefly of unipolar nerve cells, and from these the fibers of the posterior root take origin—the single process of each cell dividing after a short course into a central fiber which enters the medulla spinalis and a peripheral fiber which runs into the spinal nerve. Two other forms of cells are, however, present, viz.: (a) the cells of Dogiel, whose axons ramify close to the cell (type II, of Golgi), and are distributed entirely within the ganglion; and (b) multipolar cells similar to those found in the sympathetic ganglia.
The ganglia of the first cervical nerve may be absent, while small aberrant ganglia consisting of groups of nerve cells are sometimes found on the posterior roots between the spinal ganglia and the medulla spinalis.
Each nerve root receives a covering from the pia mater, and is loosely invested by the arachnoid, the latter being prolonged as far as the points where the roots pierce the dura mater. The two roots pierce the dura mater separately, each receiving a sheath from this membrane; where the roots join to form the spinal nerve this sheath is continuous with the epineurium of the nerve.

The Cranial Nerves

There are twelve pairs of cranial nerves; they are attached to the brain and are transmitted through foramina in the base of the cranium. The different pairs are named from before backward as follows:
1st. Olfactory Type:Sensory Function:Smell
2d. Optic. Type:sensory Function:Vision

3d. Oculomotor. Type:mixed-motor Function:Proprioception

4th.Trochlear. Type:mixed-motor Function:Proprioception

5th. Trigeminal. Type:mixed Function:touch,pain and temperature impulse

6th. Abducent. Type:mixed-motor Function:Proprioception

7th. Facial. Type: Mixed Function:Proprioception and taste
8th. Acoustic. Type:mixed-sensory Function:Conveys Equilibrium impulse
9th. Glossopharyngeal. Type:Mixed Function:Taste and somatic sensation
10th. Vagus. Type:mixed Function:Taste and somatic sensation
11th. Accessory. Type:Mixed -motor Function:Proprioception
12th. Hypoglossal. Type:Mixed-motor Function:Proprioception

The area of attachment of a cranial nerve to the surface of the brain is termed its superficial or apparent origin. The fibers of the nerve can be traced into the substance of the brain to a special nucleus of gray substance. The motor or efferent cranial nerves arise within the brain from groups of nerve cells which constitute their nuclei of origin. The sensory or afferent cranial nerves arise from groups of nerve cells outside the brain; these nerve cells may be grouped to form ganglia on the trunks of the nerves or may be situated in peripheral sensory organs such as the nose and eye. The central processes of these cells run into the brain, and there end by arborizing around nerve cells, which are grouped to form nuclei of termination. The nuclei of origin of the motor nerves and the nuclei of termination of the sensory nerves are brought into relationship with the cerebral cortex, the former through the geniculate fibers of the internal capsule, the latter through the lemniscus. The geniculate fibers arise from the cells of the motor area of the cortex, and, after crossing the middle line, end by arborizing around the cells of the nuclei of origin of the motor cranial nerves. On the other hand, fibers arise from the cells of the nuclei of termination of the sensory nerves, and after crossing to the opposite side, join the lemniscus, and thus connect these nuclei, directly or indirectly, with the cerebral cortex.

The Brain or Encephalon

General Considerations and Divisions.

Fig 1 Fig 2

—The brain, is contained within the cranium, and constitutes the upper, greatly expanded part of the central nervous system. In its early embryonic condition it consists of three hollow vesicles, termed the hind-brain or rhombencephalon, the mid-brain or mesencephalon, and the fore-brain or prosencephalon; and the parts derived from each of these can be recognized in the adult (shown in fig 1 and 2).Thus in the process of development the wall of the hind-brain undergoes modification to form the medulla oblongata, the pons, and cerebellum, while its cavity is expanded to form the fourth ventricle. The mid-brain forms only a small part of the adult brain; its cavity becomes the cerebral aqueduct (aqueduct of Sylvius), which serves as a tubular communication between the third and fourth ventricles; while its walls are thickened to form the corpora quadrigemina and cerebral peduncles. The fore-brain undergoes great modification: its anterior part or telencephalon expands laterally in the form of two hollow vesicles, the cavities of which become the lateral ventricles, while the surrounding walls form the cerebral hemispheres and their commissures; the cavity of the posterior part or diencephalon forms the greater part of the third ventricle, and from its walls are developed most of the structures which bound that cavity.

The Spinal Cord or Medulla Spinalis

The medulla spinalis or spinal cord forms the elongated, nearly cylindrical, part of the central nervous system which occupies the upper two-thirds of the vertebral canal. Its average length in the male is about 45 cm., in the female from 42 to 43 cm., while its weight amounts to about 30 gms. It extends from the level of the upper border of the atlas to that of the lower border of the first, or upper border of the second, lumbar vertebra. Above, it is continuous with the brain; below, it ends in a conical extremity, the conus medullaris, from the apex of which a delicate filament, the filum terminale, descends as far as the first segment of the coccyx
The position of the medulla spinalis varies with the movements of the vertebral column, its lower extremity being drawn slightly upward when the column is flexed. It also varies at different periods of life; up to the third month of fetal life the medulla spinalis is as long as the vertebral canal, but from this stage onward the vertebral column elongates more rapidly than the medulla spinalis, so that by the end of the fifth month the medulla spinalis terminates at the base of the sacrum, and at birth about the third lumbar vertebra.

The medulla spinalis does not fill the part of the vertebral canal in which it lies; it is ensheathed by three protective membranes, separated from each other by two concentric spaces. The three membranes are named from without inward, the dura mater, the arachnoid, and the pia mater. The dura mater is a strong, fibrous membrane which forms a wide, tubular sheath; this sheath extends below the termination of the medulla spinalis and ends in a pointed cul-de-sac at the level of the lower border of the second sacral vertebra. The dura mater is separated from the wall of the vertebral canal by the epidural cavity, which contains a quantity of loose areolar tissue and a plexus of veins; between the dura mater and the subjacent arachnoid is a capillary interval, the subdural cavity, which contains a small quantity of fluid, probably of the nature of lymph. The arachnoid is a thin, transparent sheath, separated from the pia mater by a comparatively wide interval, the subarachnoid cavity, which is filled with cerebrospinal fluid. The pia mater closely invests the medulla spinalis and sends delicate septa into its substance; a narrow band, the ligamentum denticulatum, extends along each of its lateral surfaces and is attached by a series of pointed processes to the inner surface of the dura mater.
Thirty-one pairs of spinal nerves spring from the medulla spinalis, each nerve having an anterior or ventral, and a posterior or dorsal root, the latter being distinguished by the presence of an oval swelling, the spinal ganglion, which contains numerous nerve cells. Each root consists of several bundles of nerve fibers, and at its attachment extends for some distance along the side of the medulla spinalis. The pairs of spinal nerves are grouped as follows: cervical 8, thoracic 12, lumbar 5, sacral 5, coccygeal 1, and, for convenience of description, the medulla spinalis is divided into cervical, thoracic, lumbar and sacral regions, corresponding with the attachments of the different groups of nerves.
The relative inequality in the rates of growth of the medulla spinalis and vertebral column, the nerve roots, which in the early embryo passed transversely outward to reach their respective intervertebral foramina, become more and more oblique in direction from above downward, so that the lumbar and sacral nerves descend almost vertically to reach their points of exit. From the appearance these nerves present at their attachment to the medulla spinalis and from their great length they are collectively termed the cauda equina. Fig shown ------->
The filum terminale is a delicate filament, about 20 cm. in length, prolonged downward from the apex of the conus medullaris. It consists of two parts, an upper and a lower. The upper part, or filum terminale internum, measures about 15 cm. in length and reaches as far as the lower border of the second sacral vertebra. It is contained within the tubular sheath of dura mater, and is surrounded by the nerves forming the cauda equina, from which it can be readily recognized by its bluish-white color. The lower part, or filum terminale externum, is closely invested by, and is adherent to, the dura mater; it extends downward from the apex of the tubular sheath and is attached to the back of the first segment of the coccyx. The filum terminale consists mainly of fibrous tissue, continuous above with that of the pia mater. Adhering to its outer surface, however, are a few strands of nerve fibers which probably represent rudimentary second and third coccygeal nerves; further, the central canal of the medulla spinalis extends downward into it for 5 or 6 cm.

Roots of the Spinal Nerves- Each spinal nerve possesses two roots, an anterior and a posterior, which are attached to the surface of the medulla spinalis opposite the corresponding column of gray

These fibers become medullated about the fifth month of fetal life.

The Anterior Nerve Root (radix anterior) consists of efferent fibers, which are the axons of the nerve cells in the ventral part of the anterior and lateral columns. A short distance from their origins, these axons are invested by medullary sheaths and, passing forward, emerge in two or three irregular rows over an area which measures about 3 mm. in width.
The Posterior Root (radix posterior) comprises some six or eight fasciculi, attached in linear series along the postero-lateral sulcus. It consists of afferent fibers which arise from the nerve cells in a spinal ganglion. Each ganglion cell gives off a single fiber which divides in a T-shaped manner into two processes, medial and lateral. The lateral processes extend to the sensory end-organs of the skin, muscles, tendons, joints, etc. (somatic receptors), and to the sensory end-organs of the viscera (visceral receptors). The medial processes of the ganglion cells grow into the medulla spinalis as the posterior roots of the spinal nerves.
The posterior nerve root enters the medulla spinalis in three chief bundles, medial, intermediate, and lateral. The medial strand passes directly into the fasciculus cuneatus: it consists of coarse fibers, which acquire their medullary sheaths about the fifth month of intrauterine life; the intermediate strand consists of coarse fibers, which enter the gelatinous substance of Rolando; the lateral is composed of fine fibers, which assume a longitudinal direction in the tract of Lissauer, and do not acquire their medullary sheaths until after birth. In addition to these medullated fibers there are great numbers of non-medullated fibers which enter with the lateral bundle. They are more numerous than the myelinated fibers. They arise from the small cells of the spinal ganglia by T-shaped axons similar to the myelinated. They are distributed with the peripheral nerves chiefly to the skin, only a few are found in the nerves to the muscles.
Having entered the medulla spinalis, all the fibers of the posterior nerve roots divide into ascending and descending branches, and these in their turn give off collaterals which enter the gray substance. The descending fibers are short, and soon enter the gray substance. The ascending fibers are grouped into long, short, and intermediate: the long fibers ascend in the fasciculus cuneatus and fasciculus gracilis as far as the medulla oblongata, where they end by arborizing around the cells of the cuneate and gracile nuclei; the short fibers run upward for a distance of only 5 or 6 mm. and enter the gray substance; while the intermediate fibers, after a somewhat longer course, have a similar destination. All fibers entering the gray substance end by arborizing around its nerve cells or the dendrites of cells, those of intermediate length being especially associated with the cells of the dorsal nucleus.

Structure of the Nervous System

The nervous tissues are composed of nerve cells and their various processes, together with a supporting tissue called neuroglia, which, however, is found only in the brain and medulla spinalis. Certain long processes of the nerve cells are of special importance, and it is convenient to consider them apart from the cells; they are known as nerve fibers.

To the naked eye a difference is obvious between certain portions of the brain and medulla spinalis, viz., the gray substance and the white substance. The gray substance is largely composed of nerve cells, while the white substance contains only their long processes, the nerve fibers. It is in the former that nervous impressions are received, stored, and transformed into efferent impulses, and by the latter that they are conducted. Hence the gray substance forms the essential constituent of all the ganglionic centers, both those in the isolated ganglia and those aggregated in the brain and medulla spinalis; while the white substance forms the bulk of the commissural portions of the nerve centers and the peripheral nerves.

Nerve Cells-Nerve cells are largely aggregated in the gray substance of the brain and medulla spinalis, but smaller collections of these cells also form the swellings, called ganglia, seen on many nerves. These latter are found chiefly upon the spinal and cranial nerve roots and in connection with the sympathetic nerves.
The nerve cells vary in shape and size, and have one or more processes. They may be divided for purposes of description into three groups, according to the number of processes which they possess: (1) Unipolar cells, which are found in the spinal ganglia; the single process, after a short course, divides in a T-shaped manner (Fig. E).

(2) Bipolar cells, also found in the spinal ganglia,when the cells are in an embryonic condition. They are best demonstrated in the spinal ganglia of fish. Sometimes the processes come off from opposite poles of the cell, and the cell then assumes a spindle shape; in other cells both processes emerge at the same point. In some cases where two fibers are apparently connected with a cell, one of the fibers is really derived from an adjoining nerve cell and is passing to end in a ramification around the ganglion cell, or, again, it may be coiled spirally around the nerve process which is issuing from the cell.

(3) Multipolar cells-

Fig 1 Fig 2 Fig 3

which are pyramidal or stellate in shape, and characterized by their large size and by the numerous processes which issue from them. The processes are of two kinds: one of them is termed the axis-cylinder process or axon because it becomes the axis-cylinder of a nerve .The others are termed the protoplasmic processes or dendrons; they begin to divide and subdivide soon after they emerge from the cell, and finally end in minute twigs and become lost among the other elements of the nervous tissue.As in Fig 1,2 and 3 above

NERVOUS SYSTEM

THE NERVOUS SYSTEM is the most complicated and highly organized of the various systems which make up the human body. It is the mechanism concerned with the correlation and integration of various bodily processes and the reactions and adjustments of the organism to its environment. In addition the cerebral cortex is concerned with conscious life. It may be divided into two parts, central and peripheral.

The central nervous system consists of the encephalon or brain, contained within the cranium, and the medulla spinalis or spinal cord, lodged in the vertebral canal; the two portions are continuous with one another at the level of the upper border of the atlas vertebra.
The peripheral nervous system consists of a series of nerves by which the central nervous system is connected with the various tissues of the body. For descriptive purposes these nerves may be arranged in two groups, cerebrospinal and sympathetic, the arrangement, however, being an arbitrary one, since the two groups are intimately connected and closely intermingled. Both the cerebrospinal and sympathetic nerves have nuclei of origin (the somatic efferent and sympathetic efferent) as well as nuclei of termination (somatic afferent and sympathetic afferent) in the central nervous system. The cerebrospinal nerves are forty-three in number on either side—twelve cranial, attached to the brain, and thirty-one spinal, to the medulla spinalis. They are associated with the functions of the special and general senses and with the voluntary movements of the body. The sympathetic nerves transmit the impulses which regulate the movements of the viscera, determine the caliber of the bloodvessels, and control the phenomena of secretion. In relation with them are two rows of central ganglia, situated one on either side of the middle line in front of the vertebral column; these ganglia are intimately connected with the medulla spinalis and the spinal nerves, and are also joined to each other by vertical strands of nerve fibers so as to constitute a pair of knotted cords, the sympathetic trunks, which reach from the base of the skull to the coccyx. The sympathetic nerves issuing from the ganglia form three great prevertebral plexuses which supply the thoracic, abdominal, and pelvic viscera; in relation to the walls of these viscera intricate nerve plexuses and numerous peripheral ganglia are found.

Lymphatic System

THE LYMPHATIC SYSTEM consists
(1) of complex capillary networks which collect the lymph in the various organs and tissues;
(2) of an elaborate system of collecting vessels which conduct the lymph from the capillaries to the large veins of the neck at the junction of the internal jugular and subclavian veins, where the lymph is poured into the blood stream; and
(3) lymph glands or nodes which are interspaced in the pathways of the collecting vessels filtering the lymph as it passes through them and contributing lymphocytes to it. The lymphatic capillaries and collecting vessels are lined throughout by a continuous layer of endothelial cells, forming thus a closed system. The lymphatic vessels of the small intestine receive the special designation of lacteals or chyliferous vessels; they differ in no respect from the lymphatic vessels generally excepting that during the process of digestion they contain a milk-white fluid, the chyle.
Lymphatic Capillaries.—The complex capillary plexuses which consist of a single layer of thin flat endothelial cells lie in the connective-tissue spaces in the various regions of the body to which they are distributed and are bathed by the intercellular tissue fluids.

Distribution.—The Skin.—Lymphatic capillaries are abundant in the dermis where they form superficial and deep plexuses, the former sending blind ends into the dermal papillæ. The plexuses are especially rich over the palmar surface of the hands and fingers and over the plantar surface of the feet and toes. The epidermis is without capillaries. The conjunctiva has an especially rich plexus.

1)The subcutaneous tissue is without capillaries.
2)The tendons of striated muscle and muscle sheaths are richly supplied. In muscle, however, their existence is still disputed.
3)The periosteum of bone is richly supplied and they have been described in the Haversian canals. They are absent in cartilage and probably in bone marrow.
4)The joint capsules are richly supplied with lymphatic capillaries, they do not, however, open into the joint cavities.
5)Beneath the mesothelium lining of the pleural, peritoneal and pericardial cavities are rich plexuses; they do not open into these cavities.
6)The alimentary canal is supplied with rich plexuses beneath the epithelium, often as a superficial plexus in the mucosa and a deeper submucosal plexus. Cul-de-sacs extend into the filiform papillæ of the tongue and the villi of the small intestine. Those portions of the alimentary canal covered by peritoneum, have in addition a subserous lymphatic capillary plexus beneath the mesothelium.

Lymphatic Vessels.—The lymphatic vessels are exceedingly delicate, and their coats are so transparent that the fluid they contain is readily seen through them. They are interrupted at intervals by constrictions, which give them a knotted or beaded appearance; these constrictions correspond to the situations of valves in their interior. Lymphatic vessels have been found in nearly every texture and organ of the body which contains bloodvessels. Such non-vascular structures as cartilage, the nails, cuticle, and hair have none, but with these exceptions it is probable that eventually all parts will be found to be permeated by these vessels.

Lymph Glands (lymphoglandulæ).—The lymph glands are small oval or bean-shaped bodies, situated in the course of lymphatic and lacteal vessels so that the lymph and chyle pass through them on their way to the blood. Each generally presents on one side a slight depression—the hilus—through which the bloodvessels enter and leave the interior. The efferent lymphatic vessel also emerges from the gland at this spot, while the afferent vessels enter the organ at different parts of the periphery.A lymph gland displays two different structures: an external, of lighter color—the cortical; and an internal, darker—the medullary. The cortical structure does not form a complete investment, but is deficient at the hilus, where the medullary portion reaches the surface of the gland; so that the efferent vessel is derived directly from the medullary structures, while the afferent vessels empty themselves into the cortical substance

Veins

THE VEINS convey the blood from the capillaries of the different parts of the body to the heart. They consist of two distinct sets of vessels, the pulmonary and systemic.
The Pulmonary Veins, unlike other veins, contain arterial blood, which they return from the lungs to the left atrium of the heart.
The Systemic Veins return the venous blood from the body generally to the right atrium of the heart.
The Portal Vein, an appendage to the systemic venous system, is confined to the abdominal cavity, and returns the venous blood from the spleen and the viscera of digestion to the liver. This vessel ramifies in the substance of the liver and there breaks up into a minute network of capillary-like vessels, from which the blood is conveyed by the hepatic veins to the inferior vena cava.
The veins commence by minute plexuses which receive the blood from the capillaries. The branches arising from these plexuses unite together into trunks, and these, in their passage toward the heart, constantly increase in size as they receive tributaries, or join other veins. The veins are larger and altogether more numerous than the arteries; hence, the entire capacity of the venous system is much greater than that of the arterial; the capacity of the pulmonary veins, however, only slightly exceeds that of the pulmonary arteries. The veins are cylindrical like the arteries; their walls, however, are thin and they collapse when the vessels are empty, and the uniformity of their surfaces is interrupted at intervals by slight constrictions, which indicate the existence of valves in their interior. They communicate very freely with one another, especially in certain regions of the body; and these communications exist between the larger trunks as well as between the smaller branches. Thus, between the venous sinuses of the cranium, and between the veins of the neck, where obstruction would be attended with imminent danger to the cerebral venous system, large and frequent anastomoses are found. The same free communication exists between the veins throughout the whole extent of the vertebral canal, and between the veins composing the various venous plexuses in the abdomen and pelvis, e. g., the spermatic, uterine, vesical, and pudendal.
The systemic venous channels are subdivided into three sets, viz., superficial and deep veins, and venous sinuses.The Superficial Veins (cutaneous veins) are found between the layers of the superficial fascia immediately beneath the skin; they return the blood from these structures, and communicate with the deep veins by perforating the deep fascia.
The Deep Veins accompany the arteries, and are usually enclosed in the same sheaths with those vessels. With the smaller arteries—as the radial, unlar, brachial, tibial, peroneal—they exist generally in pairs, one lying on each side of the vessel, and are called venæ comitantes. The larger arteries—such as the axillary, subclavian, popliteal, and femoral—have usually only one accompanying vein. In certain organs of the body, however, the deep veins do not accompany the arteries; for instance, the veins in the skull and vertebral canal, the hepatic veins in the liver, and the larger veins returning blood from the bones.
Venous Sinuses are found only in the interior of the skull, and consist of canals formed by a separation of the two layers of the dura mater; their outer coat consists of fibrous tissue, their inner of an endothelial layer continuous with the lining membrane of the veins.