136 



ANIMAL BIOLOGY 



passes backward in the trunk, where it is united again to its 



fellow. (Colored Fig. 2.) 



Both of the pulmonary veins, returning to the heart with pure 



blood from the lungs, empty into the left auricle. Veins with the 



impure blood from the body empty into the right auricle. Both 



the auricles empty into the ventri- 

 cles, but the pure and impure blood 

 are prevented from thoroughly mix- 

 ing by ridges on the inside of the 

 ventricle. Only in an animal with 

 a four-chambered heart does pure 

 blood from the lungs pass unmixed 

 and pure to all parts of the body, 





fEM-y 



Fig. 257. — Plan of Frog's 

 Circulation. 



Venous system is black; the arterial, 



white. A U, auricles; V, ventricle; 



L, lung; Z.//', liver. Aorta has one FlG. 258. — FROG'S BLOOD (magnified 2500 



branch to right, another to left, which areas). Red cells oval, nucleated, and 



reunite below. Right branch only ]arger than human blood cells. Nuclei of 



persists in birds, left branch in beasts twQ whUe ce]Js yisible near centen ( p ea . 



and man. , , „ 



body.) 



and only such animals are warm-blooded. The purer {i.e. the more 

 oxygenated) the blood, the greater the oxidation and warmth. 



The red corpuscles in a frog's blood are oval and larger than those 

 of man. Are all of them nucleated? (Fig. 258.) The flow of blood in 

 the web of a frog's foot is a striking and interesting sight. It may 

 be easily shown by wrapping a small frog in a wet cloth and laying 

 it with one foot extended upon a glass slip on the stage of a 

 miscroscope. 



