DISSECTION OF THE DOG 13 



medial branches ; the former piercing the two oblique muscles to pass ventral- 

 wards on the surface of the external oblique about midway between the last 

 rib and the ilium. The medial branch is disposed between the internal oblique 

 and the transverse muscles in the same manner as are the thoracic nerves. 



N. ilio-inguinalis. — The ilio-inguinal nerve has a distribution very similar 

 to that of the ilio-hypogastric. 



M. tkansvebstts abdominis. — As the name suggests, the fibres of the 

 transverse muscle of the abdomen run, for the most part, in a direction at right 

 angles to the long axis of the body. Towards the inguinal region, however, the 

 fibres assume a caudo- ventral obliquity. The margin of the fleshy part of the 

 muscle is mainly continued towards the middle line dorsal to the rectus muscle, 

 but about midway between the umbilicus and the pubis the margin of the 

 transverse overlaps the ventral surface of the straight muscle. 



The fibres of the transverse muscle arise from the extremities of the 

 transverse processes of the lumbar vertebrae, and from the medial surface of the 

 last four or five ribs and their cartilages. 



The aponeurotic tendon which succeeds the fleshy part of the muscle, and by 

 which it finds attachment to the linea alba, is blended with the deep layer of 

 the internal oblique aponeurosis cranial to the umbilicus. Thus it shares in the 

 formation of the deep sheath of the rectus muscle. Caudal to the umbilicus 

 the aponeurosis becomes thin, and, about midway between the umbilicus and 

 the pubis, suddenly changes from the dorsal to the ventral side of the rectus. 



Vagina m. recti abdominis. — The constitution of the sheath of the 

 straight muscle can now be fully determined. The superficial or ventral 

 layer of the sheath is mainly formed by the aponeuroses of the external and 

 internal oblique muscles ; but, in the inguinal region, to these is added the 

 aponeurosis of the transverse muscle. The superficial layer, therefore, is 

 complete and of considerable stoutness. The deep or dorsal layer of the sheath, 

 on the contrary, is incomplete. Where the straight muscle lies over the costal 

 cartilages no sheath interposes. From the region of the xiphoid process to a 

 variable distance from the umbilicus the sheath is formed by the aponeurosis 

 of the transverse muscle and the deeper layer of the aponeurosis of the internal 

 oblique muscle. Caudal to this the transverse aponeurosis alone forms the 

 sheath to a point about midway between the umbilicus and the pubis where, 

 the aponeurosis passing ventral to the rectus, the deep sheath becomes deficient. 



Where the sheaths of the two recti muscles meet in the middle line a strong 

 cord is formed. This is known as the linea alba, and stretches from the xiphoid 

 process of the sternum to the symphysis pubis. Composed of interlacing and 

 decussating fibres derived from the aponeuroses of the oblique and transverse 

 muscles, it is broadest in the xiphoid region, gradually narrowing towards the 

 pubis. At a point on the linea alba, generally about two-thirds of the distance 

 from the pubis to the xiphoid, is a scar, the umbilicus, marking the point of 

 attachment of the umbilical cord of embryonic life. 



