THE ABDOMINAL MUSCLES 249 



Action. — Similar to that of the ol)hque muscles. 



Structure. — The muscular part is a sheet of parallel l)un(lles of fibers, directed 

 downward and inward. It is thickest over the cartilages of the ribs, and from 

 here it thins out ii;i''':^tl>' toward the aponeurosis and the lumbar region. The 

 fibers of the aponeurosis directly continue those of the fleshy part. Posteriorly 

 it becomes extremely thin and blends with the aponeuroses of the oblique muscles. 

 It covers the deep face of the rectus, so forming the internal rectus sheath. 



Relations. — ^Superficially, the oblique and straight muscles, the retractor 

 costse, the cartilages of the asternal ribs, and the internal intercostal muscles; 

 deeply, the transversalis fascia and the peritoneum. The transversalis fascia is 

 little developed in the horse, and is very thin in emaciated subjects, but in animals 

 in good condition it contains a good deal of fat. It blends with the iliac fascia 

 and descends into the inguinal canal. The asternal artery runs along the inter- 

 val between the origin of the transversus and the costal part of the diaphragm. 

 The intercostal nerves pass down over the external surface of the muscle, to 

 which they give branches. Branches of the first three lumbar nerves are simi- 

 larly disposed further back. 



Blood-supply. — Intercostal, lumbar, and asternal arteries. 



Nerve-supply. — Intercostal and lumbar nerves. 



5. Cremaster extemus. — This small muscle may be regarded as a detached 

 portion of the internal oblique, with which it blends at its origin (Figs. 272, 450). 



Origin. — The iliac fascia, near the origin of the sartorius. 



Insertion. — The tunica vaginalis communis. 



Action. — -To raise the tunica vaginalis, and with it the testicle. 



Structure. — The muscle arises by a thin aponeurosis which is succeeded by 

 a fiat muscular belly about one and a half to two inches (ca. 4 to5 cm.) in width.^ 

 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 firmty attached 

 to the outer surface of the tunic by short tendinous fibers. 



Relations. — -The muscle lies between the peritoneum and the fascia trans- 

 versalis 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.-' 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) liga- 

 ment. 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 



1 As might be expected, tlie cremaster usually undergoes more or less atrophy, and is paler 

 in the castrated subject. In the mare the muscle is small, and ends in the connective tissue in 

 the lower part of the inguinal canal. 



^ The term canal is somewhat misleading; it is rather a slit-like passage or space between 

 the tw^o oblique muscles, since the inguinal ligament is that part of the aponeurosis of the exter- 

 nal oblique muscle which stretches between the external angle of the ilium and the prepubic 

 tendon. 



