THORAX AND ABDOMEN OF THE HORSE 147 



where it ends by dividing into four large vessels, the right and left 

 hypogastric and the right and left external iliac arteries. 



The aorta is not in the median plane, but inclines to the left except 

 at its termination. Lying to the right of the vessel is at first the right 

 crus of the diaphragm and then the caudal vena cava; and, on a level 

 with the first lumbar vertebra, between the aorta and the right crus of 

 the diaphragm, there is the thin-walled cisterna chyli, in which the 

 thoracic duct has its origin, as well as the beginning of the vena azygos. 

 The cisterna must be carefully preserved during the removal of the 

 fibrous tissue and fat from around the aorta where it enters the 

 abdomen. A number of lymph glands (lymphoglandula? lumbales) will 

 be observed arranged in an irregular chain along both sides of the aorta. 

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

 and the left ureter; but the ureter is not usually in actual contact with 

 the artery. 



Dorsally the aorta is related to the bodies of the lumbar vertebrae 

 (ventral longitudinal ligament) and the left psoas minor muscle. 

 Ventrally it is covered by a plexus of sympathetic nerves. 



The branches of the abdominal aorta may be divided into two 

 groups, according as they are paired or single, as follows : — 

 Paired. Single. 



Renal arteries. Coeliac artery. 



Internal spermatic arteries. Cranial mesenteric arter}^ 



Lumbar arteries. Caudal mesenteric artery. 



External iliac arteries. Middle sacral artery. 



Hypogastric arteries. 



The coeliac, cranial mesenteric and caudal mesenteric arteries have 

 already been examined (pages 109 and 124). 



Aa. renales. — The renal arteries are short but large relative to the 

 size of the organs to which they carry blood. They leave the aorta at 

 a right angle about the level of the origin of the cranial mesenteric 

 .jirtery ; but the origin of tlie right artery is nearly always slightly 

 cranial to that of the left. The right artery, further, is longer than the 

 left, and crosses somewhat obliquely the dorsal surface of the caudal 

 vena cava. In the region of the hilus of the kidney each artery divides 

 into a variable number of branches (six to eight), some of which enter 

 the hilus, while the rest disappear into the ventral surface of the kidney. 



Small branches are distributed to the adrenals, ureters, renal lymph 

 glands and the adipose tissue about the kidney. 



Aa. sPERMATlCiE INTERNEE. — Speaking generally, the internal sper- 

 matic arteries leave the aorta about the origin of the caudal mesenteric 



