THORAX AND ABDOMEN OF THE HORSE 89 



at the caudal border of the internal oblique muscle and between this 

 and the inguinal ligament. The transverse fascia and the peritoneum 

 enter the canal by its inner opening, traverse the canal, and become 

 the vaginal tunic of the scrotum. 



In addition to the spermatic cord and the external cremaster muscle 

 in the male, the canal also contains the external spermatic and ilio- 

 inguinal (ramus profundus) nerves and the external spermatic artery. 

 In intimate relation to its medial aspect are the external pudendal and 

 caudal epigastric vessels. 



The cavity of the abdomen (Cavum abdominis). — The cavity 

 of the abdomen is separated from that of the thorax by a relatively thin 

 muscular and tendinous partition formed by the diaphragm ; and in 

 order that the topography of the abdominal organs may be properly 

 appreciated, it is necessary to remember that the partition is convex in 

 every direction when viewed from the thoracic side, and that the most 

 cranial part of the convexity occurs about the median plane of the body 

 on a level with the sixth rib and about a horizontal plane slightly below 

 the level of the shoulder joint. From this, the vertex of the convexity, 

 the diaphragm curves laterally towards the costal arch and dorsally 

 towards the sixteenth thoracic vertebra. It therefore follows that a 

 considerable proportion of the abdominal viscera lies under cover of the 

 ribs from the seventh to the last. 



The greatest vertical (dorso-ventral) diameter of the abdominal 

 cavity occurs on a level with the first lumbar vertebra. Opposite the 

 first lumbar vertebra, also, the transverse diameter of the cavity is 

 greatest. 



The bony boundaries of the abdominal cavity are incomplete. Its 

 dorsal wall is formed by the lumbar and sacral vertebrae, with their 

 transverse processes, covered by a thick layer of muscles. Ribs and 

 their cartilages form a girdle round the cranial part of the cavity, while 

 the hip bones fulfil the same purpose at the caudal end. Otherwise the 

 lateral and ventral walls are formed by soft structures only. The 

 ventral wall, much longer than the dorsal, presents a marked downward 

 slope from the pubes to the xiphoid process of the sternum, between 

 which points it is formed by the two straight muscles, the aponeuroses 

 of the external and internal oblique and the transverse muscles, and 

 that yellow elastic tunic which constitutes a specialisation of the deep 

 fascia of the abdominal wall. The oblique and transverse muscles, 

 covered by the rapidly thinning extension of the elastic tunic, compose 

 that part of the lateral wall unprovided with a bony skeleton. 



The muscles of the abdominal wall are lined by fascia that varies 

 in thickness and strength in different regions, and is named in accord- 



