FORCE OF THE HEART. 285 



in various places by collateral branches. These 

 curiously subdivided arteries are exclusively 

 distributed to the muscles of the limbs ; for all 

 the other arteries of the body branch off in the 

 usual manner. This structure, which was dis- 

 covered by Sir A. Carlisle,* is not confined to 

 the Sloth, but is met with in other animals, as 

 the Lemur tardigradus, and the Lemur loris, 

 which resemble the sloth in the extreme slug- 

 gishness of their movements. It is extremely 

 probable, therefore, that this peculiarity in the 

 muscular power results from, or is at least in 

 some way connected with this remarkable struc- 

 ture in the arteries. In the Lion, and some other 

 beasts of prey, a similar construction is adopted 

 in the arteries of the head, probably with a view 

 to confer a power of more permanent contrac- 

 tion in the muscles of the jaws for holding a 

 strong animal, such as a buffalo, and carrying it 

 to a distance. 



That we may form an adequate conception 

 of the immense power of the ventricle, or prime 

 mover in the circulation of the blood, we have 

 but to reflect on the numerous obstacles im- 

 peding its passage through the arterial system. 

 There is, first, the natural elasticity of the 

 coats of the arteries, which must be overcome 

 before any blood can enter them. Secondly, 



* Phil. Trans, for 1800, p. 98, and for 1804, p. 17. 



