CIRCULATION 



175 



subtractions into the pulmonary system of the higher Verte- 

 brates, including Man. (Figs. 121, C; 123.) 



The vascular system is, in truth, a highly efficient apparatus. 

 Day in and day out throughout life the human heart, beating 



PRIMITIVE 



FISH 



AMPHIBIAN 



REPTILE 



BIRD 



MAMMAL 



9' 9 



9' 



Fig. 122. — Diagram to show the transformation of the six pairs of primitive 

 gill arteries (aortic arches) in the ascending series of Vertebrates, a, dorsal 

 aorta; b, ventral artery from heart; c, internal carotids; d, external carotids; 

 e, e', right and left aortic arches;/, pulmonary arteries; g, g', subclavian arteries 

 to fore limbs. 



rhythmically at an average rate of 70 times per minute, does about 

 300,000 foot-pounds of work. This power is expended in moving 

 the weight of the blood, in imparting to it the velocity of its mo- 

 tion, and in maintaining pressure in the aorta and pulmonary 

 artery. In its circulation through the body of a man, the blood 



