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. Ifthe 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. 
