258 THE VITAL FUNCTIONS. 



number of minute branches, forming a compli- 

 cated network 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, which has been called the rete mirabile, 

 is evident from the branches afterwards uniting into 

 larger trunks when they have entered the brain, 

 through the substance of which they are then dis- 

 tributed exactly as in other animals, where no such 

 previous subdivision takes place.* 



In the Brady pus tridactylus, 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 col- 

 lateral branches. These curiously subdivided ar- 

 teries are exclusively distributed to the muscles of 

 the limbs ; for all the other arteries of the body 

 branch off in the usual ma.nner. This structure, 

 which was discovered by Sir A. Carlisle,! is not 

 confined to the Sloth, but is met with in other ani- 

 mals, as the Lemm' tardigradus, and the Lemur 



* The rete mirabile is particularly large in the ruminant tribe, and 

 in the hog. It is much developed in the cat, but scarcely percep- 

 tible in the dog. It does not exist in plantigrade and rodent 

 animals, 



t Phil. Trans, for 1800, p. 98, and for 1804, p. 17. Professor 

 Baer has discovered that in the Manati, (Trichechus manatus) and 

 in the Porpus, (Delphinus phocsena) the axillary artery forms a 

 similar plexus, which is continued down the limb. Mem. Imp. 

 Acad. Sc. of St. Petersburg, XI. 204. 



