_Action._—To raise the tunica vaginalis, and with it the testicle. _Structure._—The muscle arises by a thin aponeurosis which is succeeded by a flat muscular belly about one and a half to two inches (ca. 4 to 5 cm.) in width.[52] It passes down the inguinal canal on the postero-external surface of the tunica vaginalis, to which it is rather loosely attached. On reaching the point where the tunic is reflected on to the tail of the epididymis, the muscle is firmly attached to the outer surface of the tunic by short tendinous fibers. _Relations._—The muscle lies between the peritoneum and the fascia transversalis in front and the iliac fascia and inguinal ligament behind. On reaching the internal ring it descends the inguinal canal on the postero-external surface of the tunica vaginalis communis. _Blood-supply._—External spermatic or cremasteric artery. _Nerve-supply._—External spermatic nerve. =The Inguinal Canal.=—This term (Canalis inguinalis) is applied to an oblique passage through the posterior part of the abdominal wall.[53] It begins at the internal inguinal or abdominal ring, and extends obliquely downward, inward, and somewhat forward, to end at the external inguinal or subcutaneous ring. Its anterior wall is formed by the fleshy posterior part of the internal oblique muscle, and the posterior wall by the strong tendinous inguinal (Poupart’s) ligament. The average length of the canal, measured along the spermatic cord, is about four inches (ca. 10 cm.). The =internal inguinal ring= (Annulus inguinalis abdominalis) is bounded in front by the thin margin of the internal oblique muscle, and behind by the inguinal ligament. It is directed approximately from the edge of the prepubic tendon toward the external angle of the ilium. Its length is about four or five inches (ca. 10 to 12 cm.). The edge of the muscle is attached to the surface of the ligament here by delicate connective tissue, except where structures intervene between the walls of the canal. Consequently the limits of the ring are not very clearly defined. The =external inguinal ring= (Annulus inguinalis subcutaneus) is a well defined slit in the aponeurosis of the external oblique muscle, situated lateral to the prepubic tendon. Its long axis is directed from the edge of the prepubic tendon outward and forward, and its average length is about four inches (ca. 10 cm.). The canal contains in the male the spermatic cord, the tunica vaginalis, the external cremaster muscle, the external pudic artery and a small satellite vein, and the inguinal lymph vessels and nerves. In the female it contains the mammary vessels and nerves; in the bitch it also lodges the round ligament of the uterus, inclosed in a tubular process of peritoneum.