94 



classes of animals, and double in birds and mam* 

 miferous animals alone ; that is to say, it is, in 

 the two former, appropriated either directly to 

 the body in general, and only indirectly to the 

 respiratory organs, or directly to the respiratory 

 organs, and only indirectly to the body in gene- 

 ral ; while, in the two latter, one compartment 

 of it is appropriated directly to the body in gene- 

 ral, and the other equally directly to the organs 

 of respiration. The heart of fishes is situated 

 between their gills, or respiratory organs, and 

 therefore in most fishes, close to the head. It is 

 in general very small in proportion to the size of 

 the body, not exceeding, upon an average, 1 -550th 

 part of the weight of the animal ; whereas, in 

 reptiles it is upon an average 1 -250th ; in mam- 

 miferous animals 1 -150th ; and in birds, not less 

 than l-85th of this weight. These differences 

 are precisely such as the less or greater energy 

 of the circulating powers in each of these tribes 

 requires. The heart of fishes does not, like that 

 of most of the animals of which we have hitherto 

 been speaking, receive its blood from the respira- 

 tory organ, and send it directly into the arteries 

 of the rest of the body ; but it receives its blood 

 from the large veins coming up on the lower part 

 of the body, and directly transmits it by vessels, 

 called the branchial arches, to the gills, whence 

 it is conveyed by other vessels the union of 

 which, along the back of the animal, constitutes 

 the main artery of the body to all the rest of 

 the system. The single heart of fishes is accord- 



