218 



CIKCTJLATION IN THE LOWER ANIMALS 



to the tremendous muscular exertions that the bird is com- 

 pelled to make in the process of flying. Another change 

 is the turning of the aorta to the right instead of to 

 the left. The red corpuscles of this animal are not 

 nucleated, and in this respect the bird is nearer man 

 in the scale of animal life. 



The mammals. — In all the mammals 

 the circulatory system is practically 

 identical with that of man, and the red 

 corpuscles are without nuclei, but differ 

 in shape. 



Significance of these comparisons. — ■ 

 The lessons that these comparisons 

 teach are two. First, they show us 

 that while the apparatus may be differ- 

 ent in construction in the different 

 animals, the purpose of the structures 

 found are identical in all animals, 

 nainely, to distribute food and oxygen 

 and remove wastes. Second, the fact 

 that they differ only in minor details 

 and in adaptation to special conditions 

 of life show us that man bears a very 

 close relation to other animals and dif- 

 fers from them only in the greater per- 

 fection of his organs. It is by compari- 

 sons such as these that men have been 

 enabled to trace the history of the 

 development of living forms on the 

 earth, just as by studying all the different nmsical instru- 

 ments that have ever been produced it is possible to learn 

 the history of instrument making in the arts. In short, 



Fig. 81— Plan of cir- 

 culation in birds and 

 mammals ; c^ right 

 auricle receiving 

 blood from the sys- 

 tem ; 6, left auricle 

 receiving arterial 

 blood from the 

 lungs ; c, right ven- 

 tricle ; c' left ventri- 

 cle ; (7, e, /, systemic 

 arteries, 'veins, and 

 capillaries ; g, pul- 

 monary artery ; k, Ji, 

 pulmonary capilla- 

 ries and vein. 



