204 THE VITAL FUNCTIONS. 



rangemcnt wliich, on the well known principles of hydrau- 

 lics, must greatly check the velocity of the hlood conducted 

 through them. That such is the real purpose of this struc- 

 ture is evident from the hranchcs afterwards uniting into 

 larger trunks when they have entered the brain, through tiic 

 substance of which they are then distributed exactly as in 

 other animals, where no sucli previous subdivision takes 

 place. 



In the Bradijpxis tridacii/lus^ or great American Sloth, 

 an animal remarkable for the slowness of its movements, a 

 plan somewhat analogous 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 extremities, 

 suddenly divide into a great number of cylindric vessels of 

 equal size, communicating in various places by collateral 

 branches. These curiously subdivided arteries are exclu- 

 sively 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 discovered by Sir A. Carlisle,* is 

 not confined to the Sloth, but is met with in other animals, 

 as the Lcinxir tardigradus, and the Lemur lo?'is, which re- 

 semble the sloth in the extreme sluggishness of their move- 

 ments. It is extremely probable, therefore, that this pecu- 

 liarity in the muscular power results from, or is at least in 

 some way connected with this remarkable structure in the 

 arteries. In the Lion, and some other beasts of prey, a simi- 

 lar construction is adopted in the arteries of the head, pro- 

 bably with a view to confer a power of more permanent 

 contraction 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 im- 

 mense power of the ventricle, or prime mover in the circu- 

 lation of the blood, we have but to reflect on the numerous 

 obstacles impeding its passage through the arterial system. 

 There is, first, the natural elasticity of the coats of the ar- 



• Pliil. Tr.ins. for 1800, p. 98, and for 1804, p. 17. 



