206 



BIOLOGY 



leu. 



leu 



FlG. 101. A SINGLE CAPILLARY 



Showing the corpuscles being forced through its 

 walls. 



re, red corpuscles ; 

 leu, leucocytes; 



leu, a leucocyte that is forcing its way through the 

 walls of the capillary into the surrounding tissues. 



the heart, which acts as a pump. In the frog's heart there are 

 three chambers and the circulation is as follows: The blood 

 which enters the heart from the body, which is impure blood, 



is received first into the 

 venus sinus (Fig. 92 B, 

 vs), and from here it en- 

 ters the right auricle; 

 Fig. 102 ra. At the 

 same time pure blood 

 enters from the lungs 

 and skin, and is received 

 in the left auricle. Now 

 the two auricles con- 

 tract and force the 

 blood into the single 

 ventricle v, through the 

 openings indicated by 

 the arrows in Figure 



102. The ventricle thus receives both pure and impure blood, 

 the pure blood being poured into its left side and the impure 

 blood into its right side. These two kinds of blood are partly 

 mixed, except for a fraction of a second, when they are sep- 

 arate from each other. They are kept from mixing too quickly 

 by several muscular bands stretching from the walls of the 

 heart. But almost at the same instant that the ventricle is 

 filled it contracts, and its contained blood is forced into the 

 large artery, the bulbus arteriosus. This artery, as will be seen 

 from Figure 102 6a, opens on the right side of the ventricle and 

 consequently will receive first the blood which entered the ven- 

 tricle from the right auricle, which is impure blood. Thus impure 

 blood passes first into the arteries, to be followed by mixed blood 

 and finally by the purer blood that comes from the left side of 

 the ventricle, and hence from the left auricle. With each con- 

 traction of the heart there enters the arteries first a little impure 

 blood, then a little mixed blood, and finally a little pure blood. 



