258 THE BIOLOGY OF THE FROG 



CHAPTER XV 



THE CIRCULATORY SYSTEM 



The principal functions of the circulatory system are to 

 carry food material and oxygen to all parts of the body, and 

 to remove the carbon dioxide and other waste products of 

 tissue metabolism to the organs where they are eliminated. 

 These functions are discharged by means of two fluids, the 

 blood and the lymph. We shall describe the blood first. 



The Blood. — The blood of the frog consists of a fluid, the 

 plasma, in which there occur numerous free cells, or corpus- 

 cles. The corpuscles are of three kinds : red corpuscles 

 {erythrocytes), white corpuscles {leucocytes), and spindle cells. 

 The red corpuscles are elliptical in outline and have an oval 

 nucleus in the center. When seen on edge they appear 

 flattened and slightly bulging in the center where the nucleus 

 lies. The nearly transparent cytoplasm of the cell contains 

 a large percentage of hemoglobin, which gives it a yellowish 

 color. Blood appears red only when a layer of considerable 

 thickness is seen. 



The white corpuscles, unlike the red ones, vary exceed- 

 ingly both in size and form. The outline of the white cor- 

 puscle undergoes changes much like those of an Amceba. 

 The changes are slow, but they may easily be shown by 

 making outline sketches of the corpuscle at intervals of one 

 or more minutes. The cytoplasm of the white corpuscles is 

 nearly colorless, but it often contains granules of various sizes 

 and staining reactions. The form of the nucleus varies 



