72 DISSECTION OF THE DOG 



however, are of moment. (1) The aortic hiatus (hiatus aorticus) between the 

 vertebral column and the middle of the three arches formed by the tendons 

 of the pars lumbalis. Through this pass the aorta, the vena azygos, and the 

 thoracic duct. (2) The caval foramen (foramen venae cavse) in the right part 

 of the tendinous centre, for the passage of the caudal vena cava. (3) The 

 oesophageal hiatus (hiatus oesophageus) between the right and left muscular 

 fibres of the pars lumbalis. This transmits the oesophagus and the oesophageal 

 nerve-trunks. 



Aorta. — The last part of the aorta, namely that which is found in the 

 abdomen, has now to be examined. The abdominal aorta continues the 

 thoracic aorta from the aortic hiatus of the diaphragm to the seventh lumbar 

 vertebra, where it ends by dividing into four large vessels, the two hypogastric 

 and the two external iliac arteries. 



The abdominal aorta is not precisely in the median plane, but inclines a 

 little towards the left except at its termination. Lying to the right is the vena 

 cava ; and between the aorta on the one side, and the right lumbar origin of 

 the diaphragm and the sublumbar muscles on the other, is the delicate-walled 

 cisterna chyli in which the thoracic duct has its origin. The cisterna and the 

 duct must be carefully preserved during the removal of the fibrous tissue and 

 fat from around the aorta at its entrance into the abdomen. 



To the left of the aorta are the left adrenal gland, the left kidney, and the 

 left ureter ; but the kidney and ureter are generally not in actual contact with 

 the artery. 



Ventrally the aorta is covered by a plexus of sympathetic nerves ; while 

 dorsally it is in contact Avith the tendinous origin of the diaphragm and the 

 psoas muscles. 



The branches of the abdominal aorta may be divided into two groups, 

 according as they are paired or single, as follows : 



Paired. Single. 



Phrenic arteries. Cceliac artery. 



Phrenico-abdominal arteries. Cranial mesenteric artery. 



Renal arteries. Caudal mesenteric artery. 



Internal spermatic arteries. Middle sacral artery. 

 Lumbar arteries. 

 Deep circumflex iliac arteries. 



A. threnica. — The phrenic artery, destined for the diaphragm, generally 

 arises with the phrenico-abdominal artery, and runs ventralwards, a little to 

 the side of the hiatus cesophageus. to the tendinous centre. 



A. phrenico-abdominalis. — The phrenico-abdominal artery leaves the 

 aorta beyond the origin of the cranial mesenteric artery, and runs in a lateral 

 direction across the psoas muscles, at the lateral margin of which it meets the 



