216 CIRCULATIOI?^ IN THE LOWER ANIMALS 



and the blood. These capillaries ultimately return the 

 blood to a dorsal artery which branches and carries the 

 blood to all parts of the body, where the division of the 

 branches into capillaries permits the gi^'ing up of food and 

 the removal of the waste of the tissues. These capillaries 

 are now collected into veins, and the blood is returned by 

 these tubes to the auricle of the heart to repeat this cycle 

 indefinitely. As in man the food of the digestive tract is 

 absorbed by capillaries which unite to form the portal vein, 

 and this absorbed food is carried to the liver by this vein. 

 Here the portal A^ein produces capillaries, and these, after 

 branching throughout the liver, unite again into the hepatic 

 vein and are carried directly to the auricle by this tube. 



In this circulation the single auricle and ventricle cor- 

 respond to the right side of the human heart, and the heart 

 receives and expels only the venous blood. The friction 

 in the gill capillaries retards the flow so greatly that the 

 circulation in the arteries of the fish is very sluggish. 



The red corpuscles of the fish are nucleated, and it also 

 possesses a lymphatic system. 



The frog. — In the frog the blood is aerated in the lungs, 

 and the heart has two auricles and one ventricle. The 

 right auricle, as in the fish, receives blood from the veins 

 which bring it from all parts of the body to this division of 

 the heart. The left auricle receives aerated blood from 

 the lungs. It is to be noted that the frog has advanced 

 over the fish in having the aerated blood returned to the 

 heart before pumping it over the body, and as we would 

 expect, this arrangement allows a little faster flow through 

 the arteries. Since, however, there is only one ventricle, 

 the two kinds of blood found in the auricles must become 

 mixed in this chamber, and the arteries in consequence are 



