ANIMAL PHYSIOLOGY 63 



(fig. 22) have the heart consisting of two parts, with mus- 

 cular walls, a single auricle and a single ventricle. The 

 auricle receives the blood pouring from all the tissues of 

 the body through the veins. It contracts and forces the 

 blood into the ventricle. This then contracts and drives it 

 into a short vessel called the ventral aorta, which gives off a 

 branch artery for each gill-arch. The gill-arteries divide 



FIG. 22. Diagram of the circulatory system of a fish; v, ventricle; a, 

 auricle. (After Parker and Haswell.) 



into capillaries in the gills, whence, after aeration, the 

 blood is gathered by another artery and carried to the dor- 

 sal aorta, from which branch arteries distribute it to the 

 capillaries of the general body-tissues. From these it is 

 gathered by the veins and carried back to the auricle to be- 

 gin again. In the course of circulation the blood reaches 

 every part of the body, picking up certain substances here, 

 leaving others there, thus accomplishing the results already 

 pointed out as the objects of the circulation. 



In the circulation of the higher vertebrates the most 

 striking difference from that of the fish is in the structure of 

 the heart, which adapts the circulation to lungs instead of 

 gills, and in the more perfect control and regulation of the 

 action of heart and blood-vessels by the nervous system. 



It may be asked how, since the blood remains in vessels 

 during circulation, the tissue-cells receive anything from it. 

 The blood as such does not reach the tissue-cells. These 



