236 RELATION OF RESPIRATI6N WITH CIRCULATION. 



high intellectual endowments, if their exercise were ren- 

 dered null and void by other incompatible conditions. 

 Dr. B. It is on this mutual dependence of the func- 

 tions and the aid they reciprocally give and receive, that 

 are founded the laws which regulate the combinations 

 of organs in the animal economy laws, says Cuvier, 

 " which have their origin in a necessity equal to that of 

 mathematical or metaphysical laws." For if the condi- 

 tion of one organ or set of organs be modified in a man- 

 ner incompatible with a corresponding modification in 

 the other organs, it is very evident that being could not 

 exist. If we take one of the functions separately, and 

 assuming it as a standard of comparison, observe the re- 

 lations of the others with it, we shall see a little more 

 distinctly how they are all made to conform to it. 



Emily. But you do not mean certainly that any one 

 can regulate the conditions of the others, because the 

 influence would then have a beginning and an end. 



Dr. B. Very true, but what I have said is not in- 

 compatible with the idea, that the conditions of the 

 organs instead of being regulated by a particular one, 

 are governed by each other. 



Emily. Well, then, to begin your comparison which 

 organ shall you take for the common standard ? Sup- 

 pose they are the organs of respiration. 



Dr. B. It is almost immaterial which we take, but as 

 respiration is a function o( the first importance, it will 

 answer our purpose as well as any other. In the first 

 place then let us see its relations with circulation. In 

 the higher animals where respiration is performed by 

 lungs, the circulation is provided with a double heart by 

 which the blood is impelled to the body and lungs. Of 

 the three classes of animals in which this kind ol respira- 

 tion exists, the Birds possess it in the greatest quantity 

 and activity. Their inspirations are made more frequent- 

 ly, and their organs have greater extent comparatively, 

 than in the mammiferous animals. The circulation has 

 a correspondent degree of activity ; in no other animals 



