174 ANIMAL BIOLOGY 



lower Vertebrates. The modifications of this which occur in higher 

 forms are related chiefly to changes in the respiratory mechanism 

 necessitated by abandoning an aquatic for a terrestrial mode of 

 life, with consequent dependence on the free oxygen of the atmos- 

 phere instead of that dissolved in the water. 



B. Circulation in the Higher Verterrates 



We may now note some of the far-reaching changes that the 

 blood vascular system undergoes 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 developmental stages. Some disappear at that time and 

 others are modified and diverted to various uses in the adult. 

 (Fig. 122.) 



For our purpose it is sufficient to emphasize that in Man's 

 body one aortic arch continues to carry blood directly from the 

 heart to the dorsal aorta, while parts of another deliver blood 

 from the heart to the lungs and back again to the heart. Thus 

 there is established a second current of blood through the heart, 

 which necessitates a median partition in both the auricle and ven- 

 tricle in order to keep the two currents separate. 



In this way a four-chambered heart arises which consists of right 

 and left auricles and ventricles. The right auricle receives blood 

 from the venous system of the body and passes it through the 

 tricuspid valve into the right ventricle to be pumped through 

 the pulmonary artery to the lungs. After traversing the capil- 

 laries of the lungs, the blood is returned by the pulmonary vein 

 to the left auricle, thence through the mitral valve into the 

 left ventricle, which forces it into the aorta and so on its way 

 about the body as a whole. To all intents and purposes, the higher 

 Vertebrates have two hearts which act in unison — a right, or 

 pulmonary, heart receiving non-aerated blood from the entire 

 body and pumping it to the lungs, and a left, or systemic, heart 

 receiving aerated blood from the lungs and delivering it to the 

 body as a whole. Thus the blood vessels of the primitive aquatic 

 respiratory apparatus are transformed by gradual additions and 



