The scrotum () is a pouch of skin divided by a median septum into two components, each of which is occupied by a testis, an epididymis, and the distal part of the spermatic cord. The scrota! septum (septum scroti) is a median partition that is made up of all the layers of the scrotum except the skin. In the dog, the scrotum is located approximately two thirds of the distance from the preputial opening to the anus. It lies between the thighs and has a spherical shape, indented in an oblique craniocaudal direction by an indistinct raphe scroti. The left testis is usually farther caudad than the right, allowing the surfaces of the testes to glide on each other more easily and with less pressure.

The scrotal integument is pigmented and covered with fine scattered hairs. Sebaceous and tubular (sudoriparous) glands are well developed. Deep to the outer integument of the scrotum is a poorly developed layer of smooth muscle mixed with coUagenous and elastic fibers that is sometimes spoken of as the tunica dartos. Dorsally, the tissue forming the septum blends with the abdominal fascia. Contraction of the dartos causes the integument of the scrotum to retract and draw the testes close to the body.

Extending into each scrotal sac is an evaginated pouch of peritoneum, the vaginal tunic (tunica vaginalii) (), covered by spermatic fascia of the abdominal wall. The vaginal tunic and fascia wrap the descended testis and spermatic cord in such a way as to result in a double-walled extension of abdominal peritoneum. This was a vaginal process before the descent of the testis with its duct system, vessels, and nerves () Zietzsehman (1928). The outer wall, or parietal layer of the vaginal tunic, is separated by a space, the vaginal canal (canalis vaginalii) or vaginal cavity (cavum vaginale), from the visceral layer of the vaginal tunic. The vaginal canal surrounds the spermatic cord and the vaginal cavity surrounds the testis. The vaginal canal is continuous with the peritoneal cavity at the vaginal ring.

The development of the vaginal tunic in the male and the vaginal process in the female is similar. As the evaginating peritoneum passes through the deep inguinal ring, it is invested by the transversalis fascia; as it emerges from the superficial inguinal ring it is joined by the superficial and deep abdominal fascia. The combined fascias form the spermatic fascia, which covers the parietal layer of the vaginal tunic ().

The cremaster muscle () arises from the caudal free border of the internal abdominal oblique (or occasionally from the transversus abdominis) and inserts on the spermatic fascia and parietal layer of the vaginal tunic. The action of the muscle is protective in that it reflexly pulls the testis closer to the body in response to cold.

The scrotum, because of its thin, hairless skin, its lack of subcutaneous fat, and its ability to contract toward the body, functions as a temperature regulator for the tail of the epididymis. Evidence indicates that the epididymis, as the site of sperm storage, is the most heat-sensitive region of the male reproductive tract (). When the question is raised as to why a scrotum exists in some animals and not in others (there are approximately 1500 ascrotal species) we still do not have a satisfactory answer. Freeman (1990) has reviewed the question and came to the conclusion that the scrotum evolved to provide a cool environment for sperm storage, and testicular descent evolved because it improves sperm quality so that fewer are needed. He provides tables that show the proportional size of the testes in many species of animals. There are six mammalian orders that have species with internal testes as well as species with external testes.

Scrotum: Vessels and Nerves

The principal blood vessel to the scrotum is the ventral scrotal branch of the external pudendal artery. The cremasteric artery arises from the deep femoral artery. The scrotal arteries run along the cranioventral surface of the testis, superficial to the parietal layer of the vaginal tunic. The perineal branches of the internal pudendal artery supply dorsal scrotal arteries. The draining veins follow the same course in reverse.

The genital rami, branches of the genitofemoral nerve from the ventral branches of the third and fourth lumbar nerves, innervate the skin of the prepuce. The superficial perineal nerve, a branch of the pudendal from sacral nerves 1, 2, and 3, supplies all of the scrotum, according to Spurgeon and Kitchell (1982). Postganglionic sympathetic axons supplying the tunica dartos enter via the sacral plexus and the pudendal and superficial perineal nerves.