INTRODUCTION. Xlll 



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 internal 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 hiber- 

 nation. 



The respiratory and sanguiferous systems being thus 

 circumscribed in their extent, and the nervous and mus- 

 cular 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 proportion to the degree of temperature of the 

 surrounding atmosphere ; and even under the most favour- 

 able circumstances is extremely slow. Many Serpents are 

 more than a fortnight after taking their food before the 

 undigested remains are voided, during which period they 

 usually fast. If the temperature in which they are placed 

 be very low, it is greatly retarded, and during hibernation 

 it ceases altogether ; for I have known a Tortoise which 

 had fed largely upon grass immediately before it became 

 torpid, retain the grass unchanged in the stomach during 

 the whole of the winter, so that on opening the body after 

 its death, which took place immediately on its awaking in 

 the spring, and before it had any access to food, the sto- 

 mach was found filled with a large quantity of grass 

 wholly undigested. 



