THE FROG AND VERTEBRATES IN GENERAL 193 



cells, or corpuscles. Although blood plasma is composed of 

 about ninety parts of water and ten parts of solid materials in 

 solution, it is in reality a highly complex medium and marvelously 

 fitted for the transportation of various essential materials. In 

 addition, it possesses an intricate arrangement which results, when 

 necessary, in clotting, or coagulation — an important process 

 which controls bleeding as will be noted later. Since the plasma 

 continually receives and gives off an almost infinite variety of 

 materials in its relations with the cells, the exact chemical composi- 

 tion varies continuously and, therefore, an exact analysis can never 

 be obtained. 



The blood corpuscles comprise two main types of living cells, the 

 red corpuscles and the white corpuscles, or leucocytes. The 

 red corpuscles are so-called because they contain the complex red 

 pigment, hemoglobin, which has a strong affinity for oxygen, and 

 is necessary for its transportation from the skin or lungs to the cells 

 of the body. They may be described as tiny, biconcave disks with 

 a central oval-shaped nucleus. In the higher Vertebrates, the 

 nucleus is lacking in the mature red cell. 



There are several varieties of white corpuscles which show con- 

 siderable variation in size, form, character of nucleus, and func- 

 tion. In general, however, these cells are amoeboid in character. 

 There is no definite cell wall, the shape is variable, and they move 

 and capture food by temporary pseudopodia-like projections. 

 They function primarily in the control of infection through the 

 actual ingestion (phagocytosis) of the invading organisms. ( W. 

 f. 7, C.) 



Heart. The heart of the Frog consists of three chambers, 

 namely, a ventricle and two auricles. Thus in its structure it 

 may be said to occupy a middle position between the two-cham- 

 bered fish heart and the four-chambered heart of the higher Ver- 

 tebrates. The thick-walled, conical-shaped ventricle lies pos- 

 terior. This is the main pumping part and, by its rhythmical con- 

 tractions, drives the blood through the arteries to all parts of the 

 body. Anterior to the ventricle there are two thinner-walled 

 cavities, the auricles, one to the right and one to the left. Open- 

 ing into the right auricle, on the dorsal surface of the heart, is a large 

 triangular sac, the sinus venosus, which receives blood from all 

 parts of the body through three veins ; from the anterior regions 

 through the right and left anterior vena cava, and from the 



