302 THE ANATOMY OF THE HORSE. 



and reflect it forwards and backwards from the subjacent transversalis. 

 Look for the anterior abdominal artery on its deep face. 



Tiie Anterior Abdominal Artery is one of the terminal branches 

 of the internal thoracic artery. It appears at the side of the ensi- 

 form cartilage, wliere it turns round the 9th costal cartilage behind its 

 tip. It runs backwards along the middle of the superior face of the 

 rectus, giving off lateral branches, and terminating about midway 

 between the sternum and pubis in branches which anastomose with 

 those of the posterior abdominal artery. It is accompanied by a 

 satellite vein. 



The Transversalis Abdominis (Plate 40). This muscle consists 

 of a fleshy band at its origin, and of an aponeurotic tendon over 

 the abdominal floor. In both of these the direction of the fibres is 

 transversely downwards and inwards towards the linea alba. It arises 

 by its fleshy portion from the lower extremities or cartilages of the 

 asternal ribs (last ten), meeting here the origin of the diaphragm ; and 

 from the transverse processes of the lumbar vertebrse. It is inserted 

 by the inner edge of the aponeurotic tendon into the ensiform cartilage 

 and the linea alba. The posterior edge of the tendon is thin and ill- 

 defined. The inner surface of the entire muscle is related to the 

 parietal peritoneum, there being interposed, however, a very thin layer 

 of connective-tissue representing the fascia transversalis of man. Slender 

 branches from the intercostal or asternal vessels run on the peritoneal 

 surface of the muscle. 



Action. — Similar to that of the oblique muscles. 



Directions. — The abdominal cavity will be exposed by the removal of 

 the transverse muscle and its peritoneal lining. If only one side of the 

 abdominal wall has been dissected the other side niaj' now be used for 

 the better display of things not satisfactorily made out in the first ; and 

 particularly, a portion of the abdominal wall in front of Poupart's liga- 

 ment should be turned back in its entire thickness, so as to expose its 

 peritoneal aspect and the internal abdominal ring. When the latter has 

 been examined the inguinal canal may be laid open in order to expose 

 the spermatic cord in its entire length. 



The Internal Abdominal Ring (Plate 44) is the abdominal opening 

 of the inguinal canal. As seen from the abdominal side, its posterior 

 or outer edge is prominent, and corresponds to the edge of the muscular 

 part of the internal oblique ; while the opposite boundary of the ring is 

 flattened over the sublumbar muscles covered by the continuation of 

 Poupart's ligament. 



The student can now see the direct continuity between the peri- 

 toneum and the tunica vaginalis, the latter membrane passing directly 

 into the inguinal canal, and forming a well-defined edge on the 

 posterior and outer side of the entrance. It is by this opening that a 



