DISTRIBUTION OF BLOOD VESSELS. 203 



aorta, and it proceeds in a more direct course from the heart 

 than the artery of the left arm, which has its origin in com- 

 mon with the artery of that side of the head. Hence it has 

 been inferred that the right arm is originally better supplied 

 with nourishment than the left. It may be alleged, in con- 

 firmation of this view, that in birds, where any inequality 

 in the actions of the two wings would have disturbed the 

 regularity of flight, the aorta, when it has arrived at the 

 centre of the chest, divides with perfect equality into two 

 branches, so that both wings receive precisely the same 

 quantity of blood; and the muscles, being thus equally nou- 

 rished, preserve that equality of strength, which their func- 

 tion rigidly demands. 



When a large quantity of blood is wanted in any particu- 

 lar organ, and yet the force with which it would arrive, if 

 sent immediately by large arteries, might injure the texture 

 of that organ, contrivances are adopted for diminishing its 

 impetus, either by making the arteries pursue very winding 

 and circuitous paths, or by subdividing them, before they 

 reach their destination, into a great number of smaller arte- 

 ries. The delicate texture of the brain, for instance, would 

 be greatly injured by the blood being impelled with any 

 considerable force against the sides of the vessels which are 

 distributed to it; and yet a very large supply of blood is re- 

 quired by that organ for the due performance of its func- 

 tions. Accordingly we find that all the arteries which go 

 to the brain are very tortuous in their course; every flexure 

 tending considerably to diminish the force of the current of 

 blood. 



In animals that graze, and keep their heads for a long 

 time in a dependent position, the danger from an excessive 

 impetus in the blood flowing towards the head is much 

 greater than in other animals; and we find that an extraor- 

 dinary provision is made to obviate this danger. The arte- 

 ries which supply the brain, on their entrance into the basis 

 of the skull, suddenly divide into a great number of minute 

 branches, forming a complicated net- work of vessels, an ar- 



