348 PHILOSOPHY OF ZOOLOGY. 



constitution of the white blood of the inferior animals. In 

 nearly all the experiments which have been performed on 

 the blood, that fluid has been obtained from man, or the 

 more common domestic quadrupeds. 



We come now to consider the changes produced on the 

 blood by the aerating organs. 



We have already stated the necessity of a constant sup- 

 ply of atmospheric air to the continuance of the life of or- 

 ganised beings. We are here to consider this supply in 

 connection with the circulation of the vital fluid. 



The aerating organs of animals may be divided into two 

 kinds, Lungs (Pulmones), and Gills (Branchiae), both des- 

 tined to accomplish the same end. The lungs are suited 

 for bringing free air into contact with the blood, and there- 

 fore belong to those animals which have their residence on 

 the land. The gills are calculated to separate air from wa- 

 ter, with which it is always united, and bring it in contact 

 with the blood, and belong therefore to those animals which 

 reside in the sea or in fresh water. It is to be observed, 

 however, that many animals which reside in the water, breathe 

 by means of lungs, and are obliged, at intervals, to come 

 to the surface to respire, such as whales; but there are 

 no animals which reside on the land and are furnished 

 with gills which are obliged to return to the water .to re- 

 spire. 



Whether the aerating organs be lungs or gills, it appears 

 to be the object of nature in their construction to expose a 

 large surface to the contact of the air. This object is ac- 

 complished by their division into numerous cells and leaf- 

 like processes, or by their extension on the walls of cavities, 

 or the surface of pectinated ridges. The blood brought 

 to these organs by the pulmonic vessels, is there distribu- 

 ted by their terminating branches. Although still retained 

 in vessels, it can nevertheless br easily acted upon by the 



