Chapter V: Muscles of the Leg and Foot (9)

The term spermatic cord is to a certain extent misleading as applied
to most animals, while in man the structure is distinctly cord-like.
In the horse, when the tunica vaginalis is slit open and the “cord”
stretched out, the latter is seen to have the form of a wide sheet,
the mesorchium, which has a thick, rounded anterior edge, the
so-called “vascular part” of the cord. The posterior edge of the
mesorchium is continuous with the parietal layer of the tunic; its
inner surface presents posteriorly the deferential fold (Plica ductus
deferentis). Between the two layers of the mesorchium are bundles of
unstriped muscle (cremaster internus) and small vessels.

THE TUNICA VAGINALIS

The =tunica vaginalis= is a somewhat pyriform serous sac which extends from the internal inguinal ring through the inguinal canal to the bottom of the scrotum. Like the abdominal peritoneum, of which it is an evagination, it consists of two layers—parietal and visceral. The =parietal layer=, or =tunica vaginalis communis= (s. reflexa), lines the scrotum below; its narrow, tubular part lies in the inguinal canal and is directly continuous with the parietal peritoneum of the abdomen at the internal inguinal ring. The =cavity= of the tunica vaginalis (Cavum vaginale) is a diverticulum of the general peritoneal cavity, with which it communicates through the =vaginal ring= (Annulus vaginalis). It contains normally a small quantity of serous fluid. The parietal layer is reflected from the posterior wall of the inguinal canal around the structures of the cord, forming the =mesorchium=, a fold analogous to the mesentery of the intestine. The =visceral layer=, or =tunica vaginalis propria=, covers the spermatic cord, testicle, and epididymis.

The =external cremaster muscle= (M. cremaster externus) lies on the outer and posterior part of the tunic, to the scrotal part of which it is attached.

Confusion has arisen from the use of the term internal abdominal or
inguinal ring in two senses. The term is used to designate the upper
or abdominal opening of the inguinal canal, but it is also applied to
the upper opening of the cavity of the tunica vaginalis. It should
only be applied to the abdominal opening of the canal, the
=subperitoneal ring=. The =peritoneal ring= at which the cavity of the
tunica vaginalis opens into the general peritoneal sac is
distinguished by the name =vaginal ring=. It is placed about four or
five inches (ca. 10 to 12 cm.) from the linea alba, and about two or
three inches (ca. 6 to 8 cm.) in front of the ilio-pectineal eminence.
In stallions it will usually admit the end of the finger readily, but
it may be abnormally large and allow a loop of bowel to enter the
cavity of the tunica vaginalis. In the gelding it is smaller and
sometimes partially occluded. In man the cavity is almost always
obliterated early, except in its scrotal portion, thus abolishing the
vaginal ring.

FIG. 393.—DIAGRAM OF CROSS-SECTION OF SPERMATIC CORD AND TUNICA
VAGINALIS; LATTER REPRESENTED AS DISTENDED.
]

DESCENT OF THE TESTICLES

During early fœtal life the testicle is situated against the dorsal wall of the abdominal cavity, in contact with the ventral surface of the corresponding kidney. As growth proceeds it gradually migrates from this primitive position, and finally passes down the inguinal canal into the scrotum. Previous to its descent through the abdominal wall the testicle is suspended by a fold of peritoneum, termed the =mesorchium=. This fold contains the vessels and nerves of the testicle in its anterior border. In its posterior edge is the elongated tail of the epididymis, and two cords of fibrous tissue and unstriped muscle. One of these cords is short and connects the tail of the epididymis with the testicle; later it becomes shorter, and is termed the ligament of the epididymis. The other cord, the =gubernaculum testis=, extends from the tail of the epididymis to the subperitoneal tissue in the vicinity of the future vaginal ring. The =deferential fold= (Plica ductus deferentis) is given off from the inner face of the mesorchium, and joins the urogenital fold posteriorly. The body of the epididymis at this time lies in the edge of an oblique fold formed by the outer layer of the mesorchium. After the middle of fœtal life, a pouch or diverticulum of the peritoneum, the =processus vaginalis=, grows downward through the inguinal canal, carrying with it cremaster fibers derived from the internal oblique muscle and a layer from the transversalis fascia. It is accompanied by an inguinal extension of the gubernaculum testis. The latter blends below with the subcutaneous tissue which later becomes the dartos. The tail of the epididymis first enters the processus vaginalis, followed by the testicle with its mesorchium, which descends within this diverticulum of the peritoneum until it reaches the scrotum. The vas deferens and its fold descend synchronously with the epididymis and testicle.

The mechanical factors concerned in the migration of the testicle are
matters on which much uncertainty still exists. That the gubernaculum
exerts sufficient traction to guide the epididymis and testicle to the
inguinal canal seems plausible. The internal inguinal ring may
constitute a locus minoris resistentiæ in the abdominal wall,
especially after the descent of the processus vaginalis. Progressive
shortening of the gubernaculum was formerly considered to be the chief
cause of the descent through the abdominal wall. Increase in the
intra-abdominal pressure is probably an important factor.

In the foal the descent of the testicles is often complete at birth, but it frequently happens that one testicle or both may be retained in the inguinal canal or in the abdomen for some months. In other cases the testicle may return into the canal or abdomen, since in the young foal the vaginal ring is large and the testicle small and soft, and not yet closely anchored by the scrotal ligament. In rare cases the descent may be completed as late as the fourth year (personal observation).

Indefinite retention of one testicle or both in the abdominal cavity
or inguinal canal is not rare in horses; this condition is termed
cryptorchism. Abdominal retention is the more usual form of
cryptorchism in adult horses, inguinal retention being usually
temporary. The retained testicle is usually, but not always, small,
thin, soft, and flabby, and is non-spermiogenic. The processus
vaginalis and the inguinal part of the gubernaculum are usually
present, but may be rudimentary. The ligament of the epididymis and
the corresponding part of the mesorechium are often so long that the
tail of the epididymis may be several inches distant from the
testicle. The abdominal part of the gubernaculum may be 8 to 10 inches
(ca. 20 to 25 cm.) long, and the ligament of the epididymis much
elongated (10 to 15 cm. in length according to Vennerholm); thus the
testicle may have a wide range. The vaginal ring is sometimes closed.

In many mammals the testicles normally remain in the abdominal cavity;
such animals are termed testiconda, and include the elephant, some
insectivora, hyrax, sloths, ant-eaters, armadillos, and cetacea. In
others the testicles descend periodically during the period of œstrum,
and then return into the abdomen, or they may be extruded and
retracted voluntarily; this is true of most rodents, many insectivora
(moles, shrews, hedgehog), and bats.

THE VESICULAE SEMINALES

The =vesiculæ seminales= (Fig. 394) are two elongated and somewhat pyriform sacs, which lie on either side of the posterior part of the dorsal surface of the bladder. They are partly inclosed in the urogenital fold, and are related to the rectum dorsally. Their long axes are parallel with the vas deferentia and converge posteriorly. Each consists of a rounded blind end, the =fundus=, a middle, slightly narrower part, the =body=, and a posterior constricted part, the =neck= or =duct=.

In the stallion they are about six to eight inches (ca. 15 to 20 cm.) long, and their greatest diameter is about two inches (ca. 5 cm.); in the gelding they are usually much smaller.[145]

The vesiculæ are, for the most part, retroperitoneal, but the fundus extends forward into the urogenital fold and hence has a serous covering. The =neck= or =duct= (Ductus excretorius) dips under the prostate, and usually unites with the end of the vas deferens to form the =ejaculatory duct=.

FIG. 394.—INTERNAL GENITAL ORGANS OF STALLION, DORSAL VIEW.

On left side urethral muscle has been removed over bulbo-urethral
gland. Cornua of uterus masculinus are indicated in urogenital fold.
]

=Structure.=—The wall, exclusive of the partial serous coat, consists of a fibrous =adventitia=, a middle =muscular coat=, and a =mucous lining=. The muscular coat is thickest at the fundus, and consists of two planes of longitudinal fibers with a circular layer between them. The mucous membrane is thin, and is arranged in numerous folds which form a network; the spaces so inclosed present the openings of tubulo-alveolar glands. The epithelium is columnar. The =blood-supply= is derived from the =internal pudic artery=.

THE PROSTATE

The =prostate= (Prostata) is a musculo-glandular organ which lies on the neck of the bladder and the beginning of the urethra, beneath the rectum. It consists of two lateral lobes and a connecting isthmus.

The =lateral lobes=, right and left (Lobus dexter et sinister), are somewhat prismatic in form, and are directed forward, outward, and somewhat upward. The antero-internal surface of each lobe is concave and partly embraces the corresponding vesicula seminalis. The dorsal surface is concave and is in relation with the rectum. The ventral surface is convex and lies on the obturator internus muscle and fat. The apex is pointed and lies near the posterior end of the superior ischiatic spine.

The =isthmus= is a thin, transverse band, about four-fifths of an inch (ca. 2 cm.) wide. It lies over the junction of the bladder with the urethra, the uterus masculinus, the terminal parts of the vasa deferentia, and the necks of the vesiculæ seminales. Dorsally it is partly covered by transverse fibers of the urethral (Wilson’s) muscle.

=Structure.=—The prostate is inclosed in a fibro-muscular =capsule=, which sends thick, somewhat radially arranged =trabeculæ= into the gland substance, dividing it into spheroidal or ovoid =lobules=. The lobules contain a central space in which the prostatic secretion (Succus prostaticus) collects. The walls of these spaces are pouched out to form tubular diverticula, and the latter present saccular dilatations. These cavities are all lined by cubical epithelium. There are 15 to 20 =prostatic ducts= (Ductus prostatici) on either side, which perforate the urethra and open lateral to the colliculus seminalis. The =blood-supply= is derived from the =internal pudic artery=.

The surface of the prostate is commonly tuberculate in old subjects,
and amyloid bodies and calcareous concretions may be found in it.

THE UTERUS MASCULINUS

The =uterus masculinus= or =prostatic utricle= is a rudimentary structure of variable size and form, situated centrally on the posterior part of the dorsal surface of the bladder. When well developed, it consists of a median flattened tube, some three or four inches (ca. 7.5 to 10 cm.) long, and about half an inch (ca. 1 to 1.5 cm.) wide, the anterior part of which lies in the urogenital fold and gives off two slender processes or cornua; the latter curve forward and outward in the fold a variable distance, being sometimes traceable as far as the anterior end of the ampulla of the vas deferens. The posterior extremity of the tube passes under the isthmus of the prostate, and opens into the urethra on the summit of the colliculus or joins an ejaculatory duct. It has a muscular coat and a mucous lining. In many cases it consists merely of a very small central tubule with a blind anterior end, or a band, not at all sharply marked off from the adjacent tissue; in other cases it cannot be recognized. It is interesting chiefly as being a remnant of the ducts of Müller and the homologue of the uterus and vagina.

THE BULBO-URETHRAL GLANDS

The =bulbo-urethral= (or Cowper’s) =glands= (Glandulæ bulbourethrales) are two in number, and are situated on either side of the pelvic part of the urethra close to the ischial arch (Fig. 452). They are covered by the urethral (Wilson’s) muscle. They are ovoid in form, somewhat depressed dorso-ventrally, and their long axes are directed obliquely forward and outward. In the stallion they may measure about two inches (ca. 5 cm.) in length, and an inch or more (ca. 3 cm.) in width. In the gelding they are about the size of an average hazel nut.

=Structure.=—They resemble the prostate in general structure, but the interstitial tissue is much less abundant, especially in regard to its muscular elements. In the peripheral part there are striped muscle-fibers. Each gland has six to eight =excretory ducts= (Ductus excretorii) which open into the urethra on a series of small papillæ behind the prostatic ducts and close to the median plane. The =blood-supply= comes from the =internal pudic artery=.

THE PENIS

The =penis=, the male organ of copulation, is composed essentially of erectile tissue, and incloses the extrapelvic part of the urethra. It extends from the ischial arch forward between the thighs on to the umbilical region of the abdominal wall. It is supported by the fascia penis and the skin, and its prescrotal portion is situated in a cutaneous pouch, the prepuce or sheath. It is cylindrical in form, but much compressed laterally in the greater part of its extent.

FIG. 395.—PENIS OF HORSE, LATERAL VIEW.
]

In the quiescent state it is about 20 inches (ca. 50 cm.) long; of
this, about 6 to 8 inches (ca. 15 to 20 cm.) is free in the prepuce.
In erection it increases 50 per cent. or more in length.

It may be divided into a root, a body, and a terminal enlargement, the glans.

The =root= (Radix penis) is attached to the lateral parts of the ischial arch by two =crura=, which converge and unite below the arch (Fig. 452). The urethra passes over the ischial arch between the crura, and curves forward to become incorporated with the penis.

The =body= (Corpus penis) begins at the junction of the crura and constitutes the bulk of the organ. At its origin it is attached to the symphysis ischii by two strong flat bands, the =suspensory ligaments= of the penis (Ligamenta suspensoria penis), which blend with the tendon of origin of the graciles muscles (Figs. 451, 456). This part of the penis is flattened laterally for the most part, but becomes rounded and smaller anteriorly. It presents four surfaces. The =dorsal surface= or =dorsum penis=, is narrow and rounded; on it are the dorsal arteries and nerves of the penis and a rich venous plexus. The =ventral= or =urethral surface= (Facies urethralis) is rounded, and along it runs the urethra, embedded in the deep =urethral groove= of the corpus cavernosum. The =lateral surfaces= are high and flattened, except anteriorly, where they are lower and rounded.

The =glans penis= is the enlarged free end of the organ. It is marked off from the body by a faintly marked =neck= (Collum glandis). In front of this is a prominent circular ridge, the =corona glandis=, which is notched below. The base of the glans is rounded and extends further backward dorsally than ventrally; it is marked in its lower part by a deep depression, the =fossa glandis=, in which the urethra protrudes for about an inch (ca. 2.5 cm.) as a free tube, the =urethral process= (Processus urethræ), covered by a thin integument. The urethra is thus surrounded by a circular fossa, which opens superiorly into the =urethral sinus=, a bilocular diverticulum lined by thin black skin. This diverticulum is filled sometimes with a caseous mass of sebaceous matter and epithelial débris.

=Structure.=—The penis consists essentially of two erectile bodies, the corpus cavernosum penis and the corpus spongiosum.

The =corpus cavernosum penis= forms the greater part of the bulk of the penis except at its free extremity. It arises from each side of the ischial arch by a =crus=, which is embedded in the ischio-cavernosus muscle. Below the ischial arch the crura fuse completely to form the laterally compressed body of the corpus cavernosum; this presents ventrally the =urethral groove= (Sulcus urethralis), which contains the urethra and corpus spongiosum. Anteriorly, the corpus cavernosum divides into three processes, a long central one, which is capped by the glans penis, and two short blunt lateral ones. The corpus cavernosum is inclosed by the =tunica albuginea=, a thick capsule of fibrous tissue which contains many elastic fibers and some unstriped muscle. Externally the fibers are chiefly longitudinal; internally they are mainly circular and are looser in arrangement. Numerous =trabeculæ= pass inward from the tunic and form a sponge-like framework in the interior of the corpus cavernosum, which is thus divided into numerous spaces (Cavernæ). These spaces may be regarded as greatly enlarged capillaries; they contain blood, are lined with flat endothelial cells, and are directly continuous with the veins of the penis. Their walls are composed very largely of unstriped muscle. Erection is produced by distention of these spaces with blood; at other times the spaces are mere slits.

FIG. 396.—CROSS-SECTION OF BODY OF PENIS OF HORSE.
]

In man there are two distinct corpora cavernosa, separated by a
complete septum penis, except in the middle part of the organ, where
the septum is composed of vertical trabeculæ, between which are
slit-like intervals; through the latter the blood-spaces of the two
corpora cavernosa communicate. In the horse no distinct septum exists
except near the root, but in the proximal and distal parts of the
corpus cavernosum there are vertical trabeculæ.

The =corpus spongiosum= or =corpus cavernosum urethræ= forms a tube around the urethra, and is much enlarged at its free end to form the bulk of the glans penis; throughout the remainder of its course it is practically uniform in size, and does not form any very distinct bulb (Bulbus urethræ) at its origin, as in man and many animals. The glans has been described in part; it should be noted that the corpus spongiosum extends backward dorsally a distance of about four inches (ca. 10 cm.) on the middle process of the corpus cavernosum penis, forming the processus dorsalis glandis. The urethral process is covered by a thin layer of corpus spongiosum.

In structure the corpus spongiosum has a general resemblance to the corpus cavernosum, but the trabeculæ are finer and more elastic. In the glans the trabeculæ are highly elastic, and the spaces are large and very extensible; the latter are continuous at the extremity of the processus dorsalis with large veins on the dorsum penis. There is a partial septum glandis. The skin covering the glans is thin, destitute of glands, and richly supplied with nerves and special nerve-endings.

=Vessels and Nerves.=—The penis is supplied with blood by three =arteries=, viz., the =internal pudic=, =obturator=, and =external pudic=. The =veins= form a rich plexus on the dorsum and sides of the penis, which is drained by the =external pudic= and =obturator veins=; from the root the blood is carried by the =internal pudic veins=. The =lymphatics= run with the veins and go to the inguinal and lumbar glands. The =nerves= are derived chiefly from the =pudic nerves= and the =pelvic plexus= of the sympathetic.

MUSCLES OF THE PENIS (Figs. 183, 451, 452, 456)

1. The =ischio-cavernosus= (or erector penis) is a short but strong paired muscle, which arises from the tuber ischii and the adjacent part of the sacro-sciatic ligament, and is inserted on the crus and adjacent part of the body of the penis. It is somewhat fusiform, incloses the crus as in a sheath, and is situated in a deep depression in the semimembranosus muscle. It pulls the penis against the pelvis, and assists in producing and maintaining erection by compressing the dorsal veins of the penis. Its blood-supply is derived from the obturator artery, and the nerve-supply from the pudic nerve.

2. The =retractor penis= is an unstriped muscle which arises on the ventral surface of the first two or three coccygeal vertebræ. It divides into two flat bands, half an inch or more (ca. 1.5 cm.) in width, which pass downward over the sides of the rectum to meet below the anus. Here there is a decussation of fibers, thus forming a sort of suspensory apparatus for the posterior part of the rectum and the anus. From the decussation the muscle passes for a short distance between superficial and deep layers of the bulbo-cavernosus, and then along the ventral surface of the penis, to which it is loosely attached. Near the glans penis it splits up into bundles which pass through the bulbo-cavernosus and are attached to the tunica albuginea. Below the anus the muscle is attached to the sphincter ani externus. On the penis the two muscles are intimately united to each other. Their action is to withdraw the penis into the sheath after erection or protrusion. The proximal part would draw the anus upward and forward and support it during defecation.

THE PREPUCE

The =prepuce= (Præputium), popularly called the “sheath,” is a double invagination of the skin which contains and covers the free or prescrotal portion of the penis when not erect. It consists of two parts, external and internal. The external part or sheath extends from the scrotum forward close to the umbilicus, where the external layer is reflected ventrally and laterally, forming the thick margin of the =preputial orifice= (Ostium præputiale); dorsally it is directly continuous with the integument of the abdominal wall. It is marked by a median =raphé præputii=, a continuation of the scrotal raphé. At the lower margin of the preputial orifice there are often in the stallion two papillæ, which are rudimentary teats or nipples. The internal layer =passes= backward from the preputial orifice a distance of about six to eight inches (ca. 15 to 20 cm.), lining the cavity of the external prepuce, and is then reflected forward until it approaches the orifice, where it is again reflected backward. It thus forms within the cavity of the sheath a secondary tubular invagination, the prepuce proper, in which the anterior part of the penis lies. This tubular cavity is closed behind by the reflection of the internal layer on to the penis to form the penile layer of the prepuce. Its orifice is surrounded by a thick margin, the =preputial ring= (Annulus præputialis), which is connected ventrally with the outer part of the sheath by the =preputial frenum= (Frenulum præputii).

The arrangement differs from that found in man in the fact that the
inner part of the prepuce as described above is equivalent to the
entire human prepuce. This part, the prepuce proper, is well seen on
sagittal sections, and can be demonstrated by pulling the penis,
inclosed in this prepuce, out of the cavity of the sheath; the
arrangement of the free part of the penis and prepuce is then like
that in man. (In paraphimosis the penis is strangulated by the
preputial ring.)

FIG. 397.—SAGITTAL SECTION OF PREPUCE AND PART OF PENIS OF HORSE.

_P_, Internal part of prepuce or prepuce proper; _P′_, external part
of prepuce or sheath; _c.p._, preputial cavity; _F.g._, fossa
glandis; _D_, diverticulum of fossa glandis; _p.u._, processus
urethræ.
]

=Structure.=—The =external skin= of the prepuce resembles that of the scrotum. The =inner layers= of skin are hairless, variable in color, and often irregularly pigmented; they form irregular folds, and are supplied with numerous large sebaceous glands and coil glands, which reach their greatest size at the preputial ring. Beyond this the glands are absent, and the skin resembles a non-glandular mucous membrane. The secretion of the =preputial glands= (Glandulæ præputiales), together with desquamated epithelial cells, forms the fatty =smegma præputii=, which has a strong unpleasant odor, and often accumulates in considerable amount. Beneath the skin is a large amount of loose connective tissue, except over the glans penis, where the skin is closely attached to the tunic of the corpus spongiosum. The external part of the prepuce is strengthened by a layer of elastic tissue, derived from the abdominal tunic, and termed the suspensory ligament.

=Vessels and Nerves.=—The =arteries= are branches of the =external pudic artery=, and the =veins= go chiefly to the =external pudic vein=. The =lymphatics= go to the superficial inguinal and lumbar lymph glands. The =nerves= are derived from the =pudic=, =ilio-hypogastric=, and =ilio-inguinal= nerves.

THE MALE URETHRA

The =urethra= of the male (Urethra masculina) is the long mucous tube which extends from the bladder to the glans penis. It passes backward on the floor of the pelvis, turns around the ischial arch, forming an acute angle, and passes forward inclosed in the corpus spongiosum. It may, therefore, be divided into two segments, pelvic and extrapelvic.

The =pelvic part= (Pars pelvina) is four or five inches (ca. 10 to 12 cm.) long. At its origin it is not distinguishable from the neck of the bladder in size or structure; in fact no line of demarcation exists between the two. Behind the prostate the tube dilates to a potential width of two inches or more (ca. 5 to 6 cm.). Near the ischial arch, between the bulbo-urethral (Cowper’s) glands, it contracts again, forming the =isthmus urethræ=. It is related dorsally to the rectum and the prostate, ventrally to the internal obturator muscles, and laterally to the bulbo-urethral glands. It is inclosed, except at its origin, by the urethral (Wilson’s) muscle. Beneath this is a rich venous plexus, forming a sort of erectile tissue.[146]

FIG. 398.—PELVIC URETHRA AND POSTERIOR PART OF BLADDER OF HORSE SLIT
VENTRALLY AND LAID OPEN.

_e.d._, Ejaculatory ducts.
]

The =extrapelvic part= (Pars cavernosa) passes between the two crura of the penis and runs along the groove on the ventral surface of the corpus cavernosum penis, inclosed by the corpus spongiosum and the bulbo-cavernosus muscle. It passes through the glans penis and projects forward about an inch in the fossa glandis as a free tube, the =processus urethræ=; this part is covered by a delicate integument, under which there is a thin layer of erectile tissue.

The lumen of the tube is largely obliterated in the inactive condition
of the parts. When moderately distended its dimensions in a horse of
medium size are as follows: At its origin the diameter is about half
an inch (1 to 1.5 cm.). The pelvic dilatation at its widest part
measures one and a half to two inches (ca. 3.5 to 5 cm.) transversely,
and about an inch (2 to 3 cm.) vertically; it is elliptical in
cross-section when fully distended. The isthmus at the ischial arch is
a little smaller than the initial part. Beyond this the lumen is about
one-half to three-fourths of an inch (ca. 1.5 cm.) in diameter, and is
fairly uniform to the glans penis. Here there is a slight fusiform
dilatation (Fossa navicularis), beyond which the tube contracts.

The opening from the bladder into the urethra is termed the =internal urethral orifice= (Orificium urethræ internum); it is closed except during urination. The terminal opening is the =external urethral orifice= (Orificium urethræ externum) or meatus urinarius. The two orifices of the =ejaculatory ducts= are situated dorsally on either side of the colliculus seminalis, about two inches (ca. 5 cm.) behind the internal urethral orifice; they are slit-like and close together. The small orifice of the uterus masculinus is placed centrally on the colliculus; it is inconstant. The orifices of the =prostatic ducts= are on two groups of small papillæ, placed lateral to the openings of the ejaculatory ducts. The =ducts= of the =bulbo-urethral= (Cowper’s) =glands= open on two lateral series of small papillæ, about an inch (ca. 2.5 cm.) further back and close to the median line. The small orifices of the =urethral glands= are situated laterally in the wide pelvic portion.

=Structure.=—The =mucous membrane= contains an unusually large amount of fine elastic fibers, and in its pelvic part there are alveolar =glands= (Glandulæ urethrales). The epithelium is at first like that of the bladder, then becomes cylindrical, and in the terminal part is stratified squamous. In the dorsal wall the membrane forms a median ridge, the =urethral crest= (Crista urethralis); this terminates about two inches from the internal urethral orifice in a rounded elevation, the =colliculus seminalis=, on the sides of which the ejaculatory ducts open.

At the origin of the urethra there is a layer of circular =unstriped muscle-fibers= outside of the mucous coat. Beyond this the latter is inclosed by a layer of =erectile tissue=, composed of plexuses of veins supported by trabeculæ of elastic and smooth muscular tissue. This is continued by the corpus spongiosum. Outside of the erectile tissue there is a continuation of the intrinsic unstriped muscular coat, consisting of external and internal longitudinal strata, with a layer of circular fibers between them.

Except at its origin and termination the urethra is provided with a continuous layer of striped muscle, placed outside of the erectile tissue. This is described as consisting of two parts or muscles.

(1) The =urethral= (or Wilson’s) =muscle= (M. urethralis) consists of dorsal and ventral layers of transverse fibers on the wide pelvic part of the urethra, and covers the bulbo-urethral (Cowper’s) glands. It is a compressor of the pelvic part of the urethra and the bulbo-urethral glands. By its forcible contraction it plays an important rôle in the ejaculation of the seminal fluid, and also in evacuating the last of the urine in micturition.[147]

(2) The =bulbo-cavernosus muscle= (or accelerator urinæ) is the continuation of the urethral muscle on the extrapelvic part of the urethra; it extends from the ischial arch to the glans penis. At the root of the penis it is the thickest, and forms a complete layer of circular fibers which inclose the corpus spongiosum and the urethra. Beyond this it diminishes very gradually in thickness, and consists of fibers which arise on a median ventral raphé and curve around the corpus spongiosum to end on the tunica albuginea of the corpus cavernosum. Its action is to empty the extrapelvic part of the urethra.

The =ischio-urethral muscles= (Fig. 452) are small bands which arise
on the ischial arch and crura of the penis and pass forward to become
lost on the ventral layer of the urethral muscle. They may assist in
erection of the penis by exerting pressure on the dorsal veins.

MALE GENITAL ORGANS OF THE OX

The =scrotum= is situated somewhat further forward than in the horse. It is long, pendulous, and has a well-marked neck. The skin here is usually flesh-colored, but in some breeds is more or less pigmented; it is sparsely covered with short hairs. Just in front of it are four (sometimes only two) rudimentary teats.

The =testicles= are relatively larger than those of the horse, and have an elongated, oval outline. The long axis is vertical. The inner surface is somewhat flattened.

A testicle of an adult bull measures on the average about four or five
inches (10 to 12 cm.) in length, exclusive of the epididymis; with the
latter the length is about six inches (15 cm.). The width is about two
and a half to three inches (ca. 6 to 8 cm.), and the antero-posterior
diameter about the same. The weight is about nine or ten ounces (ca.
250 to 300 grams).

FIG. 399.—GENERAL VIEW OF GENITAL ORGANS OF BULL.
]

The tunica albuginea is thin. The parenchyma is yellowish in color. The mediastinum testis descends from the anterior part of the upper extremity deeply into the gland as an axial strand about 5 mm. in thickness. The vasa efferentia leave at the upper end of the mediastinum.

The =epididymis= is very closely attached to the testicle. The head is long; it curves over the upper extremity and nearly half-way down the anterior border of the testicle. The body is very narrow, and lies along the outer part of the posterior border of the testicle, to which it is attached by a narrow peritoneal fold. The tail is large and is closely attached to the lower extremity of the testicle.

The =spermatic cord= and the =tunica vaginalis= are much longer than in the horse; the extra-inguinal part of the cord is about eight to ten inches (20 to 25 cm.) in length. The =cremaster externus= is well developed, and almost completely incloses the tunic to the nook of the scrotum; it does not extend so far down as in the horse, but is inserted about at the level of the upper pole of the testicle. The =vaginal ring= is relatively very small, and is about four inches (ca. 10 cm.) from the linea alba. The cremaster internus is feeble.

FIG. 400.—INTERNAL GENITAL ORGANS OF BULL; DORSAL VIEW.

The right vesicula is sectioned frontally. The dotted line indicates
the backward extension of the peritoneum.
]

The spermatic cords emerge through the outer angles of the external
inguinal rings, and curve downward and inward to the neck of the
scrotum. The external rings are about four inches (ca. 10 cm.) long,
and are two and a half to three inches (6 to 8 cm.) apart. In animals
in good condition there is a large mass of fat above the neck of the
scrotum, between the spermatic cords. The mesorchium extends to the
bottom of the tunica vaginalis forming a narrow fold (ca. 1 cm. wide)
which attaches the epididymis to the posterior part of the tunic.

The inguinal canal presents several special features in the bull. The
internal ring is very long. Its anterior border, formed by the edge of
the internal oblique, is decidedly concave and is tendinous in its
inner part. The long axis of the external ring is directed outward,
forward, and downward. The spermatic cord lies in its outer part, the
external pudic vessels in its middle. A muscular band about an inch
wide, detached from the internal oblique muscle, crosses the outer
side of the vaginal ring.

The =vas deferens= is small in caliber. The urogenital fold is narrow, so that the vasa are closer together in it than in the horse. On reaching the posterior part of the bladder they are in apposition for a distance of about four inches (10 cm.), flanked and overlapped by the vesiculæ seminales. They form ampullæ about four or five inches (10 to 12 cm.) long and half an inch (1.2 to 1.5 cm.) wide, the mucous lining of which is plicated. They then pass under the body of the prostate and unite with the ducts of the vesiculæ seminales to form the ejaculatory ducts. The latter have slit-like openings on either side of the colliculus seminalis.

The =vesiculæ seminales= are not bladder-like sacs, as in the horse, but are compact glandular organs with a lobulated surface. In the adult they measure about four or five inches (ca. 10 to 12 cm.) in length, two inches (5 cm.) in width in their largest part, and an inch or more (ca. 3 cm.) in thickness. The dorsal surface of each faces upward and inward and is partially covered with peritoneum. The ventral surface faces in the opposite direction and is non-peritoneal. Each may be regarded as consisting of a very thick-walled, sacculated tube, bent on itself in a tortuous manner. This tube, if straightened out, would be about ten inches (25 cm.) in length. They are commonly unsymmetrical in size and shape. Short branches are often given off from the chief tube. The excretory duct or neck joins the outer side of the vas deferens at an acute angle to form the ejaculatory duct.

FIG. 401.—ANTERIOR EXTREMITY OF PENIS OF BULL, LEFT VIEW. (After
Böhm.)
]

FIG. 402.—CROSS-SECTION OF PELVIC URETHRA OF BULL.

_1_, Prostate gland (pars disseminata); _2_, urethra; _3_, urethral
muscle; _4_, aponeurosis. The lumen of the urethra is black.
]

The structure of the vesicula is masked by a thick capsule of fibrous
tissue and unstriped muscle, which maintains it in its bent condition
and also sends trabeculæ between the alveoli. There is a central canal
into which the secretion formed in the alveoli passes. The cavities
(central canal and alveoli) are lined with columnar epithelium. On
account of this structure the term glandulæ vesiculares is preferred
by some anatomists.

The =prostate= is pale yellow in color, and consists of two parts. The =body= (Corpus prostatæ) is a mass which stretches across the dorsal surface of the neck of the bladder and the origin of the urethra. It measures about an inch and a half (ca. 3.5 to 4 cm.) transversely, and about half an inch (ca. 1 to 1.5 cm.) in width and thickness. The =pars disseminata= surrounds the pelvic part of the urethra; dorsally it forms a layer about half an inch (ca. 10 to 12 mm.) thick, but ventrally it is quite thin. It is concealed by the urethral (Wilson’s) muscle; hence it often escapes notice, but is very evident on cross-section (fig. 402). The =prostatic ducts= open into the urethra in rows between two folds of the mucous membrane which proceed backward from the colliculus seminalis.

The =uterus masculinus= appears to be absent frequently. Ellenberger states that it opens between the ejaculatory ducts, while Martin says that it usually has two orifices at the colliculus, but that it often unites with the vasa deferentia.

The =bulbo-urethral= (Cowper’s) =glands= are somewhat smaller than in the stallion. They are liable to escape notice, since they are covered by a thick layer of dense fibrous tissue and also partially by the bulbo-cavernosus muscle. Each has a single duct which opens into the urethra under cover of a fold of the mucous membrane.

The =penis= is cylindrical, and is longer and of very much smaller diameter than in the horse. Just behind the scrotum it forms an =S=-shaped curve, the =sigmoid flexure=; thus about one foot is folded up when the penis is fully retracted. The flexure is effaced during erection. The =glans penis= is about three inches (ca. 8 cm.) in length. It is flattened dorso-ventrally, and its extremity is pointed and twisted. The =external urethral orifice= is situated at the end of a groove formed by this twist; it is only large enough to admit a probe of medium size. Even in the non-erect state the penis is remarkably dense and firm. The =tunica albuginea= is very thick, and incloses the urethra. In the first part, as far as the first curve, there is a thick median =septum penis=. Beyond this there is a central axial rod of dense fibrous tissue from which numerous strong trabeculæ radiate. The erectile tissue is small in amount, so that the organ undergoes very little enlargement in erection.

The length of the penis in the adult is about three feet (ca. 90 cm.).
The crura are flattened laterally. The suspensory ligaments are
attached to the ventral ridge on the symphysis. The body is somewhat
flattened dorso-ventrally beyond the first curve. The extremity of the
glans is unsymmetrical, the urethral orifice being situated
ventro-laterally. The glans contains little erectile tissue, and hence
undergoes little enlargement in erection.

The =bulbo-cavernosus muscle= presents several remarkable features. It is for the greater part an inch or more (ca. 3 cm.) in thickness, but its length is only six to eight inches (15 to 20 cm.). It is covered by a strong aponeurosis, and is divided by a median raphé into two lateral halves except at its origin. It diminishes in size from behind forward, and its anterior extremity is pointed.

The =ischio-cavernosus muscle= resembles in general that of the horse, but is flattened laterally. It is covered by a close aponeurosis.

The =retractor penis= muscle resembles that of the horse in origin. Its two parts are about an inch (ca. 2 to 5 cm.) apart on the root of the penis, where they lie in a groove on either side of the bulbo-cavernosus. They then come close together and pass on either side of the ventral curve of the sigmoid flexure. Further forward they lie along the ventral aspect of the penis, and end about five or six inches (ca. 12 to 15 cm.) behind the glans.

The =prepuce= is very long and narrow. Its orifice is about two inches (ca. 5 cm.) behind the umbilicus; it is only large enough to admit a finger readily, and is surrounded by long hairs. The cavity of the sheath is about fifteen inches (35 to 40 cm.) long, and a little over an inch (ca. 3 cm.) in diameter. The lining membrane forms longitudinal folds; it is covered with squamous stratified epithelium, and has coiled tubular glands. The penile layer is glandless, and is reddish in color. It presents lymph nodules in its posterior part.

There are two pairs of =preputial muscles= which are derivatives of the panniculus carnosus. The anterior pair, the =protractors= of the prepuce (Mm. præputiales craniales), are two flat bands, two inches or more (5 to 6 cm.) in width, which arise close together in the xiphoid region, about eight inches (20 cm.) in front of the preputial orifice. Traced backward they diverge, leaving the umbilicus and an area about one and a half inches wide free, and then unite behind the preputial orifice. They draw the prepuce forward. The posterior pair, the =retractors= of the prepuce (Mm. præputiales caudales), arise in the inguinal region and converge on the anterior part of the prepuce. They draw the prepuce backward.

These muscles are subject to a good deal of variation. The retractor
may be absent. Many fibers come in from the panniculus on either side,
dip under the protractor, and are inserted into the skin just behind
the preputial opening. The homologue of the protractor is present in
the cow.

The =urethra= in its pelvic part is about five inches (12 cm.) long, and is of relatively small and uniform caliber. The =urethral= (Wilson’s) =muscle= incloses the tube ventrally and laterally; it is very thick, crescentic in cross-section, and is covered by an aponeurosis. Dorsally, the aponeurosis is thick and the muscle absent. Inside of these the pars disseminata of the prostate surrounds the urethra as far back as the bulbo-urethral glands. The =colliculus seminalis= is short, but prominent; from it the urethral crest extends forward to the trigonum vesicæ. The extrapelvic part has an enlargement, the =bulb=, at its origin; it then gradually diminishes in diameter, and is relatively very small at its termination. Other features have been mentioned in preceding paragraphs.

FIG. 403.—ANTERIOR PART OF PENIS OF RAM.

_A_, Right side; _B_, left side. (After Böhm.)
]

In the ram the urethra lies in a groove on the ventral surface of the corpus cavernosum. Its terminal part projects commonly 3 to 4 cm. beyond the glans penis, forming a twisted processus urethræ.

MALE GENITAL ORGANS OF THE PIG

The =scrotum= is situated a short distance from the anus, and is not so sharply defined from the surrounding parts as in the other animals.

The =testicles= are very large and are soft in texture. Their long axis is almost vertical, the free border being posterior and the tail of the epididymis dorsal. The mediastinum testis is well developed. The epididymis is closely attached to the testicle and the tunica vaginalis; the tail is specially large.

The =spermatic cord= is necessarily very long (20 to 25 cm. in a boar of medium size). The =vas deferens= in its testicular part is flexuous, and is closely attached to the tunica vaginalis; it forms no ampullæ. The =cremaster externus= is well developed, and extends to about the middle of the scrotal part of the tunic.

The =vesiculæ seminales= are exceedingly large, and extend into the abdominal cavity. They are three-sided pyramidal masses, are in apposition with each other medially, and cover the posterior part of the bladder and the ureters, the vasa deferentia, the body of the prostate, and the anterior part of the urethra. They are pale pink in color, distinctly lobulated and glandular in structure, and contain large alveoli, but no central cavity. There are two excretory ducts which open into the urethra close to the vasa deferentia; the two ducts frequently unite.

In the adult boar the vesiculæ are about five to six inches (ca. 12 to
15 cm.) long, two to three inches (ca. 5 to 8 cm.) wide, and one and a
half to two inches (ca. 4 to 5 cm.) thick; they weigh about 6 to 8
ounces each.

The =prostate= consists of two parts, as in the ox. The =body= is about an inch (2.5 cm.) wide, and embraces the neck of the bladder and the urethra at their junction dorsally and laterally. It is concealed by the vesiculæ seminales. The =pars disseminata= forms a layer which surrounds the pelvic part of the urethra, and is covered by the urethral muscle.

The uterus masculinus is small.

The =bulbo-urethral= (Cowper’s) =glands= are very large and dense. They are somewhat cylindrical, and lie on either side of and upon the posterior two-thirds of the pelvic urethra. In a large boar they are about five inches (ca. 12 cm.) in length and an inch or more (ca. 2.5 to 3 cm.) in diameter. They are partially covered with a layer of striated muscle (M. ischio-glandularis), and have a lobulated surface. Each gland has a large excretory duct which leaves at the posterior extremity, perforates the dorsal wall of the urethra at the ischial arch, and opens in a cul-de-sac covered by a fold of mucous membrane.

FIG. 404.—GENITAL ORGANS OF BOAR.

_a_, Testicle; _b_, epididymis; _c_, vas deferens; _d_, spermatic
artery; _e_, vesicula seminalis reflected to show the body of the
prostate, _f_; _e′_, excretory ducts of vesiculæ; _g_,
bulbo-urethral (Cowper’s) gland; _h_, urinary bladder; _i_, urethral
muscle; _k_, bulbo-cavernosus muscle; _l_, penis; _l′_, sigmoid
flexure of penis; _l″_, spiral anterior part of penis, exposed by
slitting open prepuce; _m_, orifice of preputial pouch; _n_,
retractor penis muscle. (Ellenberger-Baum, Anat. d. Haustiere.)
]

It is to be noted that these accessory glands are very small in
animals which have been castrated early.

The =penis= resembles in general that of the ox. The sigmoid flexure is, however, prescrotal. The anterior portion has no glans, and is spirally twisted, especially in erection. The =urethral orifice= is slit-like and is situated ventro-laterally, close to the pointed extremity. The penis in the adult boar measures about 18 to 20 inches (ca. 45 to 50 cm.) in length. Its muscles resemble those of the bull; the bulbo-cavernosus is very strong but short.

The =prepuce= has a narrow orifice, around which there are a number of stiff hairs. The cavity is partially divided by a circular fold into a posterior narrow portion and a much wider anterior portion. The lining membrane of the posterior part is papillated, and is in close contact with the penis. In the dorsal wall of the wide part there is a circular opening which leads into a cul-de-sac, the =preputial diverticulum=. This pouch is ovoid in form (when distended), and varies greatly in size in different subjects. It extends for the most part backward over the narrow part of the prepuce. Its cavity is partially divided by a narrow median septum. It contains usually decomposing urine and macerated epithelium, which have a characteristic and very unpleasant odor. Concrements have been found in it.

Oehmke found that a cast of the pouch in a Yorkshire boar weighing
about 500 pounds measured 9 cm. in length, 12½ cm. in breadth, and 6
cm. in height. The opening into the prepuce will admit two fingers in
the adult, but is ordinarily closed by folds of the lining membrane.
The sac is much smaller in animals which were castrated young, and the
opening is vertical and further back; in them it is often empty or
contains only a little clear urine. The pouch is covered by a layer of
striped muscle which is mainly derived from the homologue of the
protractor of the prepuce of ruminants.

The =urethra= has a very long pelvic portion (ca. 15 to 18 cm. long in the adult); it is inclosed (with the pars disseminata of the prostate) by a thick =urethral muscle=. There is a distinct =bulb= at the root of the penis. The penile part is of small caliber, and is surrounded by erectile tissue which, however, does not extend to the extremity of the penis.

MALE GENITAL ORGANS OF THE DOG

The =scrotum= is situated about half-way between the inguinal region and the anus. The skin covering it is pigmented and is covered sparsely with fine hairs. The raphé is not very distinct.

The =testicles= are relatively small, and have a round-oval form. The long axis is oblique, and is directed upward and backward. The mediastinum testis is central and is well developed. The epididymis is large, and is closely attached along the outer part of the dorsal border of the testicle.

The =spermatic cord= and the =tunica vaginalis= are long; they cross the side of the penis very obliquely. The upper end of the tunic is sometimes closed, so that there is then no vaginal ring. The vasa deferentia have very narrow ampullæ.

The =vesiculæ seminales= are absent; hence there are no ejaculatory ducts.

The =prostate= is relatively large; it is yellowish in color, dense in structure, and lies at the anterior border of the pubis. It is globular, and surrounds the neck of the bladder and the urethra at their junction. A faint median furrow indicates a division into two lateral lobes. The capsule and stroma contain a large amount of unstriped muscle. The ducts are numerous. Lobules of prostatic tissue (pars disseminata) are also found on the urethra further back. The gland is subject to much variation in size, and is often enlarged, especially in old subjects.

The =uterus masculinus= is a very small saccule (Utriculus prostaticus), often difficult to find.

The =bulbo-urethral= (Cowper’s) =glands= are absent.

The =penis= presents several special features. In its posterior part there are two distinct corpora cavernosa, separated by a median septum penis. In its anterior part there is a bone, the =os penis=, which in large dogs reaches a length of four inches (ca. 10 cm.) or more. It is regarded as a continuation forward of the corpus cavernosum. Ventrally it is grooved for the urethra; dorsally it is convex, and anteriorly it becomes smaller and has a curved fibrous prolongation. The =glans penis= is very long, extending over the entire length of the os penis; posteriorly it forms a rounded enlargement, the =bulbus glandis=; anteriorly it is cylindrical, smaller, and pointed at the extremity. Just in front of the bulbus glandis there is a thick circular layer of glands, forming the =annulus glandularis= (Schmaltz). The two dorsal veins arise from the bulbus glandis, pass backward on the dorsum penis, and unite at the ischial arch. A small muscle (M. compressor venæ dorsalis penis) arises from the tuber ischii on either side; the two converge on the dorsum penis near the bulbus glandis. They compress the dorsal veins, and may also tend to elevate the penis and thus assist in copulation. The other muscles offer no features worthy of special description.

The =prepuce= forms a complete sheath around the anterior part of the penis. The outer layer is ordinary integument. The inner layer has the character of a glandular mucous membrane; it is thin and reddish in color. The penile layer is glandless. In the fundus of the præputial cavity there are numerous lymph follicles. The protractor muscles arise in the xiphoid region and decussate posteriorly around the extremity of the penis.

FIG. 405.—EXTERNAL GENITAL ORGANS OF MALE DOG, VENTRO-LATERAL VIEW OF
PREPARATION.

_1_, Scrotum; _1′_, tunica vaginalis; _2_, epididymis; _3_, vascular
part of spermatic cord; _4_, vas deferens; _5_, prepuce; _6_, glans
penis; _6′_, corpus cavernosum penis; _7_, bulbus glandis; _7′_,
contour of bulbus in erection; _8_, ischio-cavernosus muscle; _a_,
acetabulum; _b_, obturator foramen. (After Ellenberger, in
Leisering’s Atlas.)
]

FIG. 406.—INTERNAL GENITAL ORGANS OF MALE DOG.

_1_, Urinary bladder; _2_, ureter; _3_, vas deferens; _4_, prostate;
_5_, urethral muscle; _6_, bulb of urethra; _a_, abdominal wall;
_b_, ilium; _c_, ischium. (After Ellenberger, in Leisering’s Atlas.)
]

The pelvic part of the =urethra= is relatively long. Its first part is inclosed in the prostate.[148] At the ischial arch the urethra has a well-developed bilobed =bulb=, formed by an enlargement here of the corpus spongiosum. It is covered by the strong but short bulbo-cavernosus muscle. The other erectile bodies have been described.

The =urethral= (Wilson’s) =muscle= is very strong; it encircles the urethra and has a median raphé dorsally.

THE FEMALE GENITAL ORGANS

The =female genital organs= (Organa genitalia feminina) are: (1) The two =ovaries=, the essential reproductive glands, in which the ova are produced; (2) the =uterine= or =Fallopian tubes=, which convey the ova to the uterus; (3) the =uterus=, in which the ovum develops; (4) the =vagina=, a dilatable passage through which the fœtus is expelled from the uterus; (5) the =vulva=, the terminal segment of the genital tract, which serves also for the expulsion of the urine; (6) the =mammary glands=, which are glands of the skin, but are so closely associated functionally with the generative organs proper as to be described with them.

GENITAL ORGANS OF THE MARE

THE OVARIES

FIG. 407.—LATERAL VIEW OF GENITAL ORGANS AND ADJACENT STRUCTURES OF
MARE.