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 lorisy 

 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 this remarkable 

 structure in the arteries ; or is at least in some 

 way connected with it. 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 

 contraction in the muscles of the jaws for hold- 

 ing a strong animal, such as a buffalo, and car- 

 rying 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. 



