X1V INTRODUCTION. 
the entrance of food, the other for the exit of matters unfit for the 
purposes of life; others have only a single opening, destined to this 
double use; and a few which are found in water, absorb their nourish- 
ment in the manner of vegetables, with this difference, that the canals 
which run from their numerous mouths, end in a common cavity. 
The solid matters introduced into the digestive cavity, or stomach, 
are converted by an internal process, first, into a pulpy mass, named 
chyme, and afterwards, into a semi-fluid substance, denominated chyle, 
which is finally taken up, or absorbed, by appropriate vessels, and 
conveyed to the great centre of circulation, the heart. 
The movement communicated by the action of the heart to the 
internal fluids, now mixed with other animal liquids, and termed 
blood, by which they are impelled through the body, is known by the 
name of circulation. The vessels which conduct the blood or chyle 
to the heart, are called veins; those which conduct it from the heart 
to the other parts of the body, are called arteries; and the alternate 
dilatation and contraction of this important organ, is the mechanism 
by which this object is accomplished. In certain classes of animals, in 
which the circulation is simple, the venous blood terminates in a kind 
of reservoir, or appendage to the heart, named auricle. A muscular 
apparatus, attached to this sinus, propels the blood, which it receives 
through an orifice, into the cavity of the heart. The ventricle, com- 
posed of thicker and stronger muscular walls, is furnished with move 
able valves, which prevent the blood from returning into the auricle, 
while it is impelled by the contraction of the ventricle, into the artery. 
This arrangement varies much, both in the mechanism and in the 
number of auricles and cavities in the heart, in different classes, and 
even in families of the same class of animals. 
The liquid, prepared by the process of digestion, requiring to be 
submitted to the action of the atmosphere, or water containing air, to 
absorb the oxygen and deprive it of certain principles, the function by 
which this is accomplished is called respiration. The organ which 
performs this service is the lungs, through which the blood is forced 
by the action of the heart. In animals doomed by their organization 
to live constantly in water, respiration is effected by means of mem- 
branous lamine, called gills, (branchie,) which separate the air from 
the water, as it passes over their multiplied surface. 
Among animals which appear to have no true circulation, there 
exists another mode of respiration, by trachee, or air-vessels, by which 
the air is conveyed through the body in elastic canals; and in these 
