72 



COMPARISON OF COLD AND WARM-BLOODED ANIMALS. 



and vital forces, and of a definite temperature, and the exercise of the intellect, 

 involve more decided and constant changes in their elements. 



The circulatory apparatus filled with blood is the great laboratory in which these 

 physical and chemical changes of the elements are carried on and the results dis- 

 tributed to every living molecule of matter. According, then, to the perfection of 

 an animal, and the rapidity of these changes, Avill be the dependence of the organs 

 and apparatus upon the circulatory fluid. It follows as a necessary consequence 

 that the deprivation of this fluid will prove fatal in a length of time inversely 

 proportional to the development and perfection of an animal. 



The question now presents itself: Why is the life of cold-blooded animals so 

 sluggish, and all the physical and chemical changes of their elements so tardy, and 

 their temperature and intellect so low ? Can Nature be said to be uniform in her 

 operations when all the phenomena of life are so dissimilar in these two classes of 

 animals ? 



A consideration of the important diflerences existing between the structure and 

 functions of their respiratory and circulatory systems may serve to settle definitely 

 this question, as will be seen in the course of this chapter. 



The principal or only heart in many fishes, has but one auricle and one ventri- 

 cle, and is traversed by venous blood alone, and corresponds with the right heart 

 of the higher vertebrata. 



Although the circulatory apparatus is more highly developed in the reptiles, still 

 a mixture of venous and arterial blood always takes place in the ventricle. 



As our experiments have been confined almost exclusiv*ely to the higher orders 

 of cold-blooded animals, we shall consider briefly the circulatory and respiratory 

 systems of the Ophidia, Sauria, and Chelonia. 



The ventricle of the hearty in these higher orders is generally divided by an 

 imperfect septum, which, in the heart of the alligator, is very strong and almost 

 complete. Just at the outlet of the ventricle, however, we find a communication 

 established between the two, and thus the venous and arterial blood are mixed 

 together, and the similarity to the heart of the rest of the reptiles, and the foetus 

 of birds and mammals, is preserved. The venous blood from all parts of the body 

 is returned to the right auricle of the heart through the venae cavsB, the terminations 

 of which are guarded by strong valves. The left auricle is appropriated exclusively 

 to the lungs, from which it receives the aerated blood through the pulmonary veins. 

 From the single ventricle two sets of vessels are sent off, the pulmonary and the 

 aortic. The pulmonary artery divides into two branches, one for each lung. The 

 aorta, immediately after its origin, divides into two trunks, which, winding back- 

 wards, join and form a large vessel, the branches of which distribute the blood to 

 all parts of the system. The contraction of the right auricle forces the venous blood 

 into the ventricle, whilst the contraction of the left auricle transmits the aerated 

 blood from the lungs into the same common cavity. The contraction of the ventri- 

 cle distributes a portion of the mixed blood into the lungs through the pulmonary 

 artery, and the remainder to all parts of the body through the aorta and its 

 branches. 



