The Scrotum and Testes, and the Ducts of the Testes.—The scrotum contains the two testes, one in each of its compartments. Each testis lies in a diverticulum of the abdominal cavity, which is lined by an extension of the peritoneum. The testis has the same relation to this peritoneal diverticulum that the intestine has to the abdominal cavity; i.e., it does not lie within the cavity of the diverticulum, but is suspended apparently within it by means of a fold of its wall which acts as a mesentery. The peritoneal diverticulum is called the tunica vaginalis propria and consists thus of a parietal layer and a visceral layer.
The tunica vaginalis propria consists of a slender proximal part through which the blood-vessels pass to the testis and the vas deferens from it, and of an expanded distal part in which lies the testis. Only the distal part lies within the scrotum. The blood-vessels and vas deferens are suspended in the narrow part of the tunica vaginalis propria by means of a mesenterial fold similar to that which suspends the testis, and continuous with it. This fold and the blood-vessels and vas deferens contained within it form the spermatic cord ([Fig. 111], d) which passes from the abdomen to the scrotal sac in the narrow part of the tunica vaginalis propria. In the formation of the human tunica vaginalis the various layers of the body wall are carried out by it and form the tunics, or coats of the testis. The one of these coats next the tunica vaginalis propria (which is reckoned as one of the coats) is the tunica vaginalis communis (or fascia propria) and is formed by the transversalis fascia. Outside of the tunica vaginalis communis is the cremaster muscle, an incomplete layer formed from the fibres of the internal oblique muscle. Next is the cremasteric (or intercolumnar) fascia from the aponeurosis of the external oblique muscle, and outside of this is the integument forming the scrotum. In this integument there is a layer of smooth muscle which is sometimes described as the tunica dartos. In the cat the tunica dartos and the cremaster muscle are wanting. The cremaster is replaced by the elevator scroti muscle. The coats of the testis are thus five, as follows:
1. The scrotum (the integument).
2. Cremasteric fascia (subcutaneous fascia).
3. The levator scroti muscle (subcutaneous muscle-layer) ([Fig. 113], j).
4. Tunica vaginalis communis (transversalis fascia).
5. Tunica vaginalis propria (peritoneum).
The tunica vaginalis communis is inseparably united with the parietal layer of the tunica vaginalis propria. Where the spermatic cord passes from the abdominal wall to the scrotum it is covered by integument and cremasteric fascia on its ventral surface only, but is entirely surrounded by the tunica vaginalis propria and tunica vaginalis communis. The canal by which the spermatic cord passes through the body wall is known as the inguinal canal. The end by which it opens into the abdominal cavity is the internal inguinal ring, and the opposite end is called the external inguinal ring.
The internal inguinal ring is merely the point of connection between the proximal tubular portion of the tunica vaginalis propria and the abdominal cavity. It is circular, and is situated close against the lateral side of the lateral ligament of the bladder at its middle.
The external inguinal ring is an oval opening in the aponeurosis of insertion of the external oblique muscle. The aponeurosis of this muscle ends caudally in a free border along the cranial edge of the pubis, from the ilium to the pubic tubercle. The external ring is just craniad of the end of this aponeurosis.