xiv 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 circulatio7i. 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 hmgs, 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 laminfe, called gills, {branchice,) 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 trachea, or air-vessels, by which 

 the air is conveyed through the body in elastic canals ; and in these 



