xii INTRODUCTION. 



mals ; and in these, also, this peculiarity arises from the 

 small quantity of respiration which they enjoy ; but the phe- 

 nomenon which in the reptiles is produced by the trans- 

 mission 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 pre- 

 sented to the blood ; for in the latter, although the whole of 

 the blood is sent through the hrancliia, 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 con- 

 trary, where the heart is double, and the circulation conse- 

 quently complete, combined with atmospheric respiration, the 

 animal heat is considerable ; and in birds it is even higher 

 still, from the increased quantity of respiration produced by 

 numerous air sacs and cells pervading all parts of the body, 

 and communicating with the lungs. 



From the structure of the Reptilia, it follows that they 

 possess but little power of sustaining muscular action ; and 

 although capable in many instances of moving rapidly for a 

 short time, it is only when they are supplied with that degree 

 of temperature externally which is denied them from an in- 

 ternal source, that they are capable of exerting any energy 

 at all ; and if deprived of external heat to a certain extent, 

 they pass into a state of torpidity or hibernation. 



The respiratory and sanguiferous systems being thus cir- 

 cumscribed in their extent, and the nervous and muscular 

 powers in a corresponding degree limited, we are prepared 

 to find that the other functions of the body are no less tardy 

 in their operation. Digestion is carried on only in propor- 

 tion to the degree of temperature of the surrounding atmo- 



