Xll INTRODUCTION. 



nature, consisting of a portion of the impure blood just 

 returned from thence, with that which has been aerated 

 in the lungs. It is clear that the blood is by this me- 

 chanism but partially changed by the action of oxygen ; 

 in other words, that the quantity of respiration, speaking 

 with reference to the physiological meaning of the term, is 

 comparatively small. Hence arises the circumstance that 

 these animals have what is called cold blood ; for as it is 

 from respiration that the blood derives its heat, and the 

 temperature of the body is thereby sustained in animals 

 which have more perfect respiration, it follows that where 

 this function is but imperfectly performed, the animal 

 heat, muscular force, and all the other functions dependent 

 upon respiration, will be diminished. In the fishes the 

 same effect is produced by different means. These are 

 likewise cold-blooded animals ; and in these, also, this pecu- 

 liarity arises from the small quantity of respiration which 

 they enjoy ; but the phenomenon which in the reptiles is 

 produced by the transmission of a part only of the blood 

 through the respiratory organ at each contraction of the 

 heart, is in the fishes effected by the different medium 

 through which the oxygen is presented to the blood ; for 

 in the latter, although the whole of the blood is sent 

 through the hranchi^ or gills, the quantity of oxygen, 

 held in solution in the water which bathes them, is so 

 small as to effect but a partial change in the condition of 

 the blood. In one case the circulation is complete, but 

 the respiration is aquatic, and therefore imperfect ; in the 

 other, the application of the air to the blood is immediate, 

 but the circulation is incomplete. In the Mammalia, on 

 the contrary, where the heart is double, and the circula- 

 tion consequently complete, combined with atmospheric 

 respiration, the animal heat is considerable; and in birds 



