228 DISTRIBUTION OF AKTERIES. 



branch, is neither the carotid nor the subclavian, both of 

 which are subordinate branches given-off from it; but it is 

 the trunk which distributes the blood to the muscles of the 

 breast, and which in Man is a subordinate branch of the sub- 

 clavian artery (the mammary). The descending aorta is seen 

 to lose itself almost entirely in supplying the viscera of the 

 trunk ; so that the branches into which it divides at last for 

 the supply of the legs, are very small. These limbs, in birds, 

 are usually required only for the support of the body at times 

 of rest, and are seldom much concerned in locomotion ; so that 

 they possess little muscular power, and require but a small 

 supply of blood. 



261. It is very interesting to trace such differences in the 

 arrangement of the vascular system, corresponding with vari- 

 ations in the general plan of structure, yet not exhibiting 

 any actual departure from the general type. Thus, there is 

 probably not a single large artery in Man, to which a corre- 

 sponding branch might not be found in the Bird ; on the 

 other hand, there is perhaps not a single large artery in the 

 Bird, to which there is not an analogous branch in Man. 

 The chief difference consists in the relative sizes of the seve- 

 ral trunks ; and these correspond closely with the amount of 

 tissue they have respectively to supply. Here, then, we have 

 one example, out of many that might be adverted-to, of that 

 Unity of Design which we see everywhere prevalent through- 

 out nature ; manifesting itself in the close conformity of a 

 great number of apparently-different structures to one general 

 plan, whilst there is, at the same time, an almost infinite 

 variety in the details. 



262. There is a very interesting peculiarity in the distribu- 

 tion of the arteries, by which the due circulation of blood in 

 their branches is provided for, even though there should be 

 an obstruction in the main trunk. The branches which are 

 given-off from it at different points, have frequent communica- 

 tions or anastomoses with each other ; so that blood may pass 

 from an upper part of a main artery into the lower, by means 

 of these lateral communications, even though its flow through 

 the trunk itself should be completely stopped. 



263. These anastomoses are very numerous in the arteries 

 of the limbs, and particularly about the joints ; and it is well 

 that they are so ; for, by relying on the maintenance of the 



