DISTRIBUTION OF BLOOD-VESSELS. 283 



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 confirmation 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 nourished, preserve that 

 equality of strength, which their function rigidly 

 demands. 



When a large quantity of blood is wanted in 

 any particular 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 sub- 

 dividing them, before they reach their destination, 

 into a great number of smaller arteries. 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 required by that 

 organ for the due performance of its functions. 

 Accordingly we find that all the arteries which 

 ^0 to the brain are very tortuous in their course ; 



