DISTRIBUTION OF BLOOD-VESSELS. 257 



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. 



Wlien 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 cir- 

 cuitous paths, or by subdividing 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 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 extraordinary pro- 

 vision is made to obviate this danger. The arteries 

 which supply the brain, on their entrance into the 

 basis of the skull, suddenly divide into a great 



VOL. II. s 



