284 THE VITAL FUNCTIONS. 



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 ex- 

 traordinary provision 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 number of mi- 

 nute branches, forming a complicated net-work 

 of vessels ; an arrangement which, on the well 

 known principles of hydraulics, must greatly 

 check the velocity of the blood conducted 

 through them. That such is the real purpose 

 of this structure is evident from the branches 

 afterwards uniting into larger trunks when they 

 have entered the brain, through the substance of 

 which they are then distributed exactly as in 

 other animals, where no such previous sub- 

 division takes place. 



In the Brady pus tridactylus, or great Ame- 

 rican Sloth, an animal remarkable for the slow- 

 ness of its movements, a plan somewhat ana- 

 logous to the former is adopted in the structure 

 of the arteries of the limbs. These arteries, at 

 their entrance into both the upper and lower ex- 

 tremities, suddenly divide into a great number 

 of cylindric vessels of equal size, communicating 



