Part 154

It has been customary to describe three layers of fascia, in conformity with the accounts given in text-books of human anatomy. These are: (1) the intercolumnar or spermatic fascia, derived from the margin of the external inguinal ring; (2) the cremasteric fascia, derived from the internal oblique muscle; (3) the infundibuliform fascia, derived from the fascia transversalis. The first two cannot be distinguished by dissection and the third is (in the scrotum) fused with the parietal peritoneum of the tunica vaginalis.

(4) =The parietal layer of the tunica vaginalis.=—This is a fibro-serous sac which is continuous with the parietal peritoneum of the abdomen at the internal inguinal ring. It is thin above, but is thick in its scrotal part, where it is strengthened by fibrous tissue (Lamina fibrosa) derived from the transversalis fascia. It will be described further under the caption tunica vaginalis.

THE VAS DEFERENS

This tube (Ductus deferens) extends from the tail of the epididymis to the ejaculatory duct. It passes upward in the inguinal canal, inclosed in a fold detached from the inner surface of the mesorchium, near the posterior (attached) border of the latter. At the vaginal ring it separates from the other constituents of the spermatic cord, and turns backward and inward into the pelvic cavity (Fig. 272). For some distance it lies in the free edge of the urogenital fold, by which it is attached to the lower part of the lateral wall of the pelvis. In its further course (over the dorsal surface of the bladder) it leaves the edge of the fold and inclines inward between its layers, coming in contact with the inner face of the vesicula seminalis. Over the neck of the bladder the two vasa lie very close together, flanked laterally by the necks of the vesiculæ seminales, and having the uterus masculinus between them. They then disappear under the isthmus of the prostate, and are continued through the wall of the urethra by the =ejaculatory ducts=. The latter are formed in the urethral wall by the union of the vas deferens with the duct (or neck) of the corresponding vesicula seminalis; they are thin-walled tubes, about 2 to 3 mm. long and 6 to 7 mm. wide, which open into the urethra on either side of the colliculus seminalis. In about 15 per cent. of subjects the vas deferens and duct of the seminal vesicle do not unite, but open side by side into the urethra. From its origin until it reaches the dorsal surface of the bladder the vas deferens has a uniform diameter of about a quarter of an inch (ca. 6 mm.). It then forms a fusiform dilatation, the =ampulla ductus deferentis= (Fig. 394); this part is about six to eight inches (ca. 15 to 20 cm.) long, and in its largest part nearly an inch (ca. 2 cm.) in diameter in the stallion; in geldings the dilatation is usually not very pronounced. Beyond the ampulla the duct abruptly contracts.

=Structure.=—The wall of the vas deferens is thick and the lumen relatively small, so that the tube has a firm and cord-like character. It is covered with peritoneum, except in the last few inches of its course. The loose =adventitia= contains numerous vessels and nerves. The thick =muscular coat= consists of longitudinal and circular layers. The =mucous membrane= has an epithelium of short columnar cells. In the posterior part of the tube, and especially in the ampulla, there are numerous tubulo-alveolar glands.

=Blood-supply.=—Spermatic and umbilical arteries.

=Nerve-supply.=—Spermatic and pelvic plexuses.

THE SPERMATIC CORD

The =spermatic cord= (Funiculus spermaticus) consists of the structures carried down by the testicle in its migration through the inguinal canal from the abdominal cavity to the scrotum. It begins at the internal inguinal ring, where its constituent parts come together, extends obliquely downward through the inguinal canal, passes over the side of the penis, and ends at the attached border of the testicle. It consists of the following structures:

(1) The =spermatic artery=.

(2) The =spermatic veins=, which form the pampiniform plexus around the artery.

(3) The =lymphatics=, which accompany the veins.

(4) Sympathetic =nerves=, which run with the artery.

(5) The =vas deferens=.

(6) The =internal cremaster muscle=, which consists of bundles of unstriped muscular tissue.

(7) The =visceral layer= of the =tunica vaginalis= or =mesorchium=.

The first four of these constituents are gathered into a rounded mass which forms the anterior part of the cord; they are united by connective tissue, interspersed with which are bundles of the cremaster internus. The vas deferens is situated posteriorly, inclosed in a special fold detached from the inner surface of the mesorchium; hence it is not visible externally.

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.