AND THE LOWER ANIMALS. 39 



central matter. Next, it becomes S-shaped, twists 

 upon itself, is constricted at one point, and widened 

 at another, acquires dense and muscular walls, and is 

 eventually converted into the large apparatus with 

 four separate chambers, which occupies nearly the 

 entire central portion of the chest. 



In the embryo as in the adult, the heart is placed 

 between the organs for the purification of the blood 

 on the one hand, and those to which the pure blood is 

 supplied on the other. In the fully-formed mammal, 

 which leads an independent existence and breathes 

 the surrounding air, the purifying organs are the 

 lungs; but in the embryo, plunged in liquid and 

 dependent on the mother for its nutrition, the func- 

 tion of respiration, or rather its equivalent, is per- 

 formed by the envelopes of the egg. It is on this 

 account that the circulatory apparatus is so different 

 in the two stages. 



In the adult each half of the heart is composed of 

 two separate chambers, and comes in contact with one 

 kind of blood (venous or arterial) only. The right 

 auricle receives the venous Hood which has flowed from 

 all parts of the body and requires purification, and 

 forces it into the right ventricle, which then propels it 

 through a large artery to the lungs. Having been 

 purified in the latter, the blood (now arterial) flows 

 into the left auricle, and then from this into the left 

 ventricle, from which a large vessel called aorta arises, 

 whose numerous ramifications convey the vital fluid to 

 the several organs of the body. 



In the embryo the lungs are inactive, and the blood- 

 vessels which supply them are quite rudimentary. 

 To compensate for this absence of true respiration, a 



