228 DISTRIBUTION OF ARTERIES. 
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 
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