. 



358 APPENDIX. 



ders, and then forms a curve downwards, called the arch 

 of the aorta (see figure,) and descends along the front of 

 the spinal column, behind the heart, as far as the lower 

 part of the trunk, where it divides into two great branches, 

 forming the iliac and femoral arteries ; still descending, it 

 forms the tibial arteries, and spreads over the feet. 



From the arch of the aorta are given off the vessels 

 which supply the head and arms, as the carotids and 

 axillary branches ; also the subclavians. The subcla- 

 vian and carotid arteries of the right side commonly 

 arise together from the aorta in man, by a common trunk, 

 but its arrangement varies much in the other mammalia. 

 Thus in the elephant, the two carotids arise by a common 

 trunk, the two subclavians separately. In some of the 

 whales, all four are separate. In the bat, the subclavian 

 and carotid of the left side arise from a common trunk, 

 like those of the right ; and in such ruminating animals as 

 have long necks, all four arteries come off from the aorta 

 together, by a large trunk, which first gives off the sub- 

 clavians on each side, and then divides into the carotids. 

 " All these varieties" says Dr. Carpenter, occasionally 

 present themselves in man a fact of no small interest. 



Besides the branches already mentioned as given off 

 by the aorta, there are others of no less importance, 

 which originate from the same source, as it descends to- 

 ward the extremities. Among these are the cozliac, 

 which supplies the liver, spleen, and stomach ; the 

 renal, which goes to the kidneys ; and the mesenteric, to 

 the intestines. 



Comparison between the Arteries of Man and Birds. 

 It is both curious and interesting to trace the similari- 

 ty between the structure of our own species and that of 

 other animals. In the mammalia as a class, we should 

 not expect much difference in the general structure of 

 the viscera, or of the heart and blood-vessels, and it is 

 true that little difference exists. But in the birds, the 

 external forms and habits of which are so widely removed 

 from the above-mentioned class, that we could not sus- 

 pect any coincidence of internal structure. And yet, 

 physiologists inform us, " that there is probably not a 



