Male Genital Organs of the Horse

The Testicles

The testicles (Testes) are situated in the inguinal region, inclosed in a diverticulum of the abdomen termed the scrotum. Their long axes are nearly longitudinal. They are ovoid in form, but considerably compressed from side to side. Each presents two surfaces, two borders, and two extremities. The internal and external surfaces (Facies medialis, lateralis) are convex and smooth. The free or ventral border (Margo liber) is convex in both directions. The attached or dorsal border (Margo epididymidis) is nearly straight, and is the one by which the gland is suspended in the scrotum by the spermatic cord; the epididymis is attached to this border and overlies it externally. The anterior and posterior extremities (Extremitas capitata, caudalis) are rounded.

At the anterior extremity there is often a sessile or pedunculated sac which contains a clear fluid; this is the appendix testis or hydatid of Morgagni, from which a thread-like process extends backward toward the vas deferens. It is regarded as a remnant of the Müllerian duct of the embryo.

A testicle of average size of an adult stallion is about four or five inches (ca. 10 to 12 cm.) long, two and a half inches (ca. 6 to 7 cm.) high, and one and a half to two inches (ca. 4 to 5 cm.) wide; it weighs about ten and one-half ounces (ca. 300 grams). They vary much in size in different subjects, and are commonly of unequal size, the left one being more often the larger.

The epididymis is adherent to the attached border of the testicle, and overlaps somewhat the external surface. Its anterior enlarged end is termed the head or globus major (Caput epididymidis), and its posterior, slightly enlarged end is the tail or globus minor (Cauda epididymidis); while the intermediate narrow part is the body (Corpus epididymidis). The head is closely connected with the testicle by the efferent ducts of the latter, by connective tissue, and by the serous membrane. The body is less closely attached by the serous covering, which forms externally a pocket beneath the epididymis termed the digital fossa (Sinus epididymidis). The tail is firmly attached to the posterior extremity of the testicle by a short ligament (Lig. epididymidis), formed by a thick fold of the tunica vaginalis, which contains smooth muscle-fibers; it is continued by the vas deferens.

Structure of the Testicle and Epididymis. The greater part of the surface of the testicle is covered by a serous membrane, the tunica vaginalis propria, which is the visceral layer of the serous envelop of the cord and testicle; this is reflected from the attached border of the gland, leaving an uncovered area at which the vessels and nerves in the spermatic cord reach the testicle. Beneath this serous covering is the tunica albuginea, a strong capsule composed of dense white inelastic fibrous tissue and unstriped muscle-fibers. From the deep surface of this tunic delicate membranous septa (Septula testis) pass into the gland, dividing it into pyramidal spaces, the bases of which are peripheral. At the anterior part of the attached border of the testicle connective tissue and smooth muscle-fibers pass into the substance of the gland, forming the mediastinum testis; from this the septa radiate. The mediastinum contains a network of fine canals, the rete testis; it is traversed by blood-vessels, which radiate on the septa to the deep face of the tunica albuginea, forming a delicate network—the so-called tunica vasculosa. The spaces imperfectly marked off by the septa contain the parenchyma testis, which consists of seminiferous tubules (Tubuli seminiferi), supported by loose intertubular connective tissue. The tubules are collected into small masses or lobules (Lobuli testis). They are at first very tortuous (Tubuli contorti); toward the apex of the lobule they unite with other tubules, forming larger straight tubules (Tubuli recti); these pass into the mediastinum testis, where they form the rete testis. From the latter a number (ca. 15 to 20) of larger efferent tubules (Vasa efferentia) pierce the albuginea and enter the head of the epididymis.

The epididymis is covered by the tunica vaginalis propria and a thin albuginea. Its head consists of lobules (Lobuli epididymidis), each of which consists of a coiled tubule or conus vasculosus, which succeeds a vas efferens. By the union of these tubules is formed a single tube, the duct of the epididymis (Ductus epididymidis), which, by its complex coils, forms the body and tail of the epididymis and terminates in the vas deferens. The tubules and the coils of the duct of the epididymis are held together by connective tissue and unstriped muscle-fibers. The tubules and duct are lined with ciliated epithelium, and the duct has a muscular coat which consists of longitudinal and circular fibers.

Vessels and Nerves.—The testicle is richly supplied with blood by the spermatic artery, a branch of the posterior aorta. The artery descends in the anterior part of the spermatic cord, and is very tortuous near the testicle; on reaching the attached border of the gland it passes backward in a flexuous manner, giving branches to the testicle and epididymis, turns around the posterior extremity, and runs forward on the free border to the anterior extremity. It is partially embedded in the tunica albuginea, and detaches lateral branches which ascend in a tortuous fashion on each surface of the testicle; these give off small branches which enter the gland on the septa.

The spermatic veins on leaving the testicle, form the pampiniform plexus around the artery in the spermatic cord. The vein which issues from this plexus usually joins the posterior vena cava on the right side, the left renal vein on the left side.

The nerves, derived from the renal and posterior mesenteric plexuses, form the spermatic plexus around the vessels.