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. The =inguinal canal= between these rings lies along the lateral border of the rectus muscle. It is one to one and a half centimeters long. Its medial wall rests on the rectus muscle; its dorsal wall on the fat contained within the lateral ligament of the bladder. The lateral and ventral walls lie on the transversus muscle proximally, on the internal oblique near the distal end. The wall itself is composed of tunica vaginalis propria and communis. As it passes within the caudal border of the internal oblique muscle it receives some aponeurotic fibres from it. A thin aponeurosis is also continued from the border of the external ring onto the tunica vaginalis propria. [Illustration: FIG. 110.--TESTIS. _a_, testis; _b_, caput epididymis; _c_, epididymis; _d_, cauda epididymis; _e_, vas deferens; _f_, spermatic cord.] The =testes= (Fig. 110) are the organs which produce the spermatozoa. They lie one in each compartment of the scrotal sac, enveloped in the membranes or tunics described when treating of the scrotum. Each is attached to the dorsal wall of its peritoneal pouch by a mesenteric fold. The testis is surrounded by the visceral layer of the tunica vaginalis propria, and within this, by a dense fibrous covering, =the tunica albuginea=, which sends septa into its interior. Within the fibrous covering it is made up of numerous coils of seminiferous tubules which are readily seen by the naked eye.