THE HIGHER ORGANISMS 



137 



As the perfection of the oxygen absorbing and dis- 

 tributing quality of the blood thus improves, the means 

 of circulating it also improves through more complex 

 adjustments of the heart and vascular system by which 

 freshly aerated blood is continually supplied to the tissues 

 and organs and is prevented from mixing with the ex- 



c. v. r 



. v. I. 



Pio. 49. Diagram of the heart, the branchial arches, and the principal veins 

 in the Teleosts. Ventral view. The heart is represented without the sigmoid 

 flexure; that is, with the auricle posterior, a, Aorta; au, auricle; br.a, branchial 

 arches of the aorta (1-4, numbering from the front); c, carotid; c.v, cardinal 

 veins (right and left); d.a, dorsal arteries; /, jugular veins; d.c, ductus Cuvieri; 

 s,v., sinus venosus; v, ventricle. (Galloway.) 



hausted blood returning from them. Thus there come 

 to exist two distinct circulations: one for the aeration of 

 the blood, the other for the nutrition of the organs 

 and tissues. 



The transformation in the structure of the heart by 

 which this is made possible is not difficult to understand. 



