THE PURIFICATION OF THE BLOOD, ETC. 41 



termed bulbus arteriosus, or arterial bulb, which 

 soon divides into separate branches, one being 

 destined for each leaf of the gills. Here the 

 arterial vessels subdivide into fine capillaries, 

 and these pass into branchial veins, which at 

 last merge into two vessels, and these unite to 

 form the aorta. Into this aorta, then, the blood, 

 purified in the gills or branchiae, is conveyed 

 without first being sent back to the heart, and 

 from this aorta it is distributed through the 

 system. We must not here omit to notice the 

 influence of the nervous system in maintaining 

 the natural heat of the body. It is ascertained 

 by experiments that the quantity of carbonic 

 acid generated in the system and expelled 

 from the lungs, is inadequate to the evolution 

 of caloric, in proportion to the caloric abstracted 

 from the body by the surrounding medium. 

 It is proved that the nerves are the agents by 

 which the due supply of caloric is effected ; but 

 the mode in which this supply is afforded is as 

 yet not satisfactorily demonstrated. Some phy- 

 siologists are disposed to think that the nerves 

 have in themselves a direct specific power of 

 generating heat ; while others consider that the 

 nerves operate indirectly, by the share they 

 take in various processes of the organic economy. 

 Be this as it may, the fact is established. 



It will here be proper to make a few observa- 

 tions on two points, which demonstrate the 

 energy of the vital principle, in reference to 

 temperature-, that of truly warm-blooded animals 

 being especially under consideration. First) 

 b2 



