WARM-BLOODED CIRCULATION. 277 



auricles, but its ventricle is divided, by two par- 

 titions, into three chambers : each of the par- 

 titions is perforated to allow of a free communi- 

 cation between the chambers ; and the passages 

 are so adjusted as to determine the current of 

 aerated blood, returning from the lungs, into 

 those arteries, more especially, which supply the 

 head and the muscles of the limbs ; while the 

 vitiated blood is made again to circulate through 

 the arteries of the viscera.* 



It is in warm-blooded animals that the two 

 offices of the circulation are most efficiently per- 

 formed ; for the whole of the blood passes 

 alternately through the greater and the lesser 

 circulations, and a complete apparatus is pro- 



* It would appear, from this arrangement of the vessels, that 

 the brain, or central organ of the nervous system, requires, 

 more than any other part, a supply of oxygenated blood for the 

 due performance of its functions. The curious provision which 

 is made for sending this partial supply of blood of a particular 

 quality in the larger kinds of reptiles, such as the Crocodile, 

 has been pointed out by many anatomists ; but has been lately 

 investigated more particularly by M. Martin St. Ange. (See 

 the Report of G. St. Hilaire, Revue Medicale, for April, 1833). 

 It is found that in these animals, as well as in the Chelonia, a 

 partial respiratory system is provided for by the admission, 

 through two canals opening externally, of aerated water into 

 the cavity of the abdomen, where it may act upon the blood 

 which is circulating in the vessels. Traces of canals of this 

 description are also met with in some of the higher classes of 

 vertebrated animals, as, for instance, among the Mammalia, in 

 the Monotremata and the Marsupicdia. 



