Part 156

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.

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.