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ANATOMY AND PHYSIOLOGY 
They may be considered, together with their res- 
pective physiological functions, under the following 
heads: 1. Nutrition; 2. Sensation; 3. Muscular 
System ; 4. Reproduction. 
Nutrition. — As nutrition consists in the renewal 
of the molecules which constitute an animal body, 
and as that is accomplished by means of the food, 
we have only to consider the principal processes 
which the latter undergoes, in order to obtain an 
indication of the dilferent heads under which this 
department of the subject may most naturally be 
treated. The first of the most important changes to 
which the food is subjected internally, is its conver- 
sion into chyle, previous to which it is incapable of 
affording any nutriment; this process is called diges- 
tion, and takes place in a special organ named the 
alimentary canal . But the chyle is not perfectly 
adapted to its nutritive functions till it has been brought 
into contact with the atmosphere, in order to receive 
a supply of its vital principle, viz. oxygen. There 
being no lungs in insects to which it can be conveyed, 
as to a reservoir, to receive this supply, the necessity 
is provided for by bringing the air into contact with 
it by means of small tubes ramifying through the 
body ; this process is called respiration . In conse- 
quence of the blood being aerated at so many places, 
instead of a particular point, often remote from the 
extremities, there is no necessity for a rapid or general 
circulation of it ; still, however, it is by no means 
stagnant, and its movement is called, although the 
term is rather too strong, the circulation of the blood . 
