auricle of the heart, thus completing the 

 circuit. 



The single circulation of the gill-breathing 

 vertebrates — which collects and distributes 

 oxygen in one and the same circuit — suffices 

 to supply the relatively modest requirements 

 of these cold-blooded creatures. However, the 

 fact that the heart must force the blood 

 against the high resistance of at least two net- 

 works of capillaries, before the blood returns 

 to the heart, has tended to limit the effi- 

 ciency of such a system. 



In the higher (lung-breathing) vertebrates, 

 the single circulation has been replaced by a 

 double circulation. The birds, mammals, and 

 many of the reptiles have developed a re- 

 spiratory (pulmonary) circuit that is quite 

 separate from the general (systemic) circuit 

 of the blood. Such forms possess a four- 

 chambered heart. In man, for example, the 

 unaerated blood is collected by the right 

 auricle and pumped through the lung capil- 

 laries by the right ventricle. Then the aerated 

 blood returns from the pulmonary circuit to 

 the left auricle and is pumped by the left 

 ventricle out into the systemic circuit. Thus 

 the four-chambered heart (Fig. 17-8) has sepa- 

 rate ventricles for pumping blood through 

 the pulmonary and systemic circuits, and 

 such a heart is able to maintain a more rapid 

 circulation. The blood first collects oxygen, 

 in passing through the pulmonary capillaries, 

 and then it returns to the heart for a new 

 impetus, before passing to the capillaries of 

 the systemic circuit, where the oxygen is dis- 

 tributed. 



The three-chambered heart of the Am- 

 phibia (Fig. 17-8) and the incomplete four- 

 chambered heart of some of the reptiles rep- 

 resent transitional developments, more or less 

 intermediate between the lower and higher 

 vertebrates. In the amphibian circulation, 

 the single ventricle pumps blood through 

 both the pulmonary and systematic circuits. 

 The unaerated and aerated kinds of blood 

 are received by the right and left auricles 

 respectively, and sent to the ventricle. To a 

 large extent the ventricle keeps the two types 



The Circulatory System - 331 



of blood separate, sending most of the un- 

 aerated through the pulmonary, and most of 

 the aerated through the systemic circuits. 

 However, there is some mixing; and conse- 

 quently this kind of system is called an in- 

 complete double circulation. 



MAJOR ARTERIES AND VEINS OF THE 

 HUMAN BODY 



In the circulation of man, the main arteries 

 and veins of the systemic circuit and some of 

 the vessels of the pulmonary circuit are pre- 

 sented in Figure 17-10. This figure shows the 

 postcaval and precaval veins entering the 

 right auricle, and the pulmonary artery 

 emerging from the right ventricle. Also, the 

 figure shows the pulmonary veins returning 

 to the left auricle, although this chamber 

 lies in front of the pulmonary veins. The 

 ventral aorta, and the aortic arch— which 

 loops around the pulmonary artery to reach 

 the dorsal aorta— can be seen clearly, as is 

 also true of the lower part of the dorsal 

 aorta. This large artery passes through the 

 diaphragm — near the lower margin of the 

 heart — and continues downward in the ab- 

 dominal cavity, sending branches to all the 

 lower parts of the body. 



The aorta serves as the trunk line of the 

 systemic circulation; and the major branches 

 of the aorta display essentially the same ar- 

 rangement in all Mammalia. The innomi- 

 nate artery (Fig. 17-10), or first large branch, 

 forks almost immediately into (1) the right 

 subclavian, which supplies the right shoulder 

 and arm regions; and (2) the right common 

 carotid, which supplies the right side of the 

 head as a whole. The second and third large 

 branches of the aorta are, respectively, the 

 left common carotid and left subclavian. 

 These arteries supply: (1) the left side of the 

 head; (2) the left shoulder and arm. 



In the thorax (dorsal to the heart) the 

 aorta gives off a number of small intercostal 

 arteries (not shown in Fig. 17-10), which sup- 

 ply the thoracic body wall. In the abdomen, 

 the aorta sends branches to the digestive 



