CIRCULATORY SYSTEM 55 



I 

 that in different species of the same genus (Centropus) the 



same variation occurs tends to throw considerable doubt 

 upon the value of the character, an observation that is fre- 

 quently and unfortunately necessary to make in describing 

 the anatomy of birds. 



The descending aorta gives off three branches, which 

 supply the alimentary canal. These are, in order of origin, 

 the coeliac, the superior, and the inferior mesenteric. These 

 arteries do'^ot, however, supply certain definite regions of 

 the gut. Thus in Bernicla ruhidiceps the left caecum 

 receives its blood both from the coeliac and from the superior 

 mesenteric. The right caecum, however, appears to be 

 supplied by the superior mesenteric alone. The coeliac is 

 mainly concerned with the blood supply of the stomach and 

 the liver. In Porphyrio the two caeca are provided with 

 blood from both coeliac and superior mesenteric. A dissection 

 of Flatalea leucorhodia showed that roughly the coeliac was 

 concerned with the blood supply of the duodenal loop and 

 of the posterior part of the intestine ; the mesenteric, on the 

 other hand, supplied the anterior part of the intestine, ex- 

 cepting the duodenal loop. 



In a few birds {e.g. in Buceros and Haliaetus, Gadow) 

 the coeliac and superior mesenteric arise from a common 

 stem. 



The posterior mesenteric supplies the end of the alimen- 

 tary tract. The aorta also gives off two spermatic arteries, 

 and on either side a crural and an ischiadic. 



"We shall deal briefly with the venous system, since it has hot 

 been up to the present largely used for systematic purposes. 

 This is doubtless due in a great measure to our imperfect 

 knowledge of the variations that occur. As in the lower 

 mammalia, there are two venae cavae superiores. These 

 are form.ed by the union on each side of a jugular, a sub- 

 clavian, and a vertebral. The two jugulars are often of very 

 unequal size ; the right is usually stronger than the left, but 

 the two veins are connected in the neighbourhood of the 

 head by a transverse branch. Sometimes the left jugular 

 may absolutely disappear. Connected with the subclavian 



