170 



FOUNDATIONS OF BIOLOGY 



huge outpocketing from the alimentary canal into the 

 anterior portion of the coelom, which constitutes the 

 lungs. This entails, in turn,.a complex respiratory mechanism 

 so that the air within the lungs may be changed at frequent 



intervals. As a matter of fact 

 one ordinarily thinks of the move- 

 ments involved in the renewal of 

 the air in lungs as respiration, but 

 from what has been said it is 

 clear that neither the respiratory 

 movements involved in inhala- 

 tion and exhalation, nor the inter- 

 change of gases between blood and 

 air through the lung membrane is 

 respiration proper. The essential 

 feature of respiration takes place 

 throughout the body when the 

 blood trades its oxygen supply for 

 carbon dioxide with the tissue cells. Thus respiration in 

 the final analysis is the securing of energy from food. 



C. CIRCULATION IN THE HIGHER VERTEBRATES 



But to return to the blood vascular system, which neces- 

 sarily undergoes far-reaching modifications as a result of the 

 substitution of lungs for gills. In the first place the series of 

 paired branchial arteries, which formerly supplied the gills, 

 no longer break up into capillaries, but instead lead directly 

 into the dorsal aorta, and accordingly are termed AORTIC 

 ARCHES. Thus Fishes bequeath, as it were, to higher forms a 

 series of pairs of aortic arches which, though they are no 

 longer of use in their former capacity, appear in the develop- 

 mental stages. Some disappear at that time and others are 

 modified and diverted to various uses in the adult. (Fig. 95.) 



FIG. 94 Diagram of a verti- 

 cal section through the head 

 region of Fish (above) and Reptile 

 or Bird (below) to show the paths 

 of the respiratory currents (a) and 

 food (6). See Fig. 87. 



