258 FROGS, TOADS, AND NEWTS [CH. VIII 



are known to dispose of harmful bacteria that have gained 

 entrance into the body. It is from some of these colour- 

 less corpuscles that a ferment, fibrin-ferment, is set free 

 which converts a soluble proteid in the plasma into solid, 

 insoluble fibrin, and causes the blood to clot on wounded 

 surfaces, and so staunch the bleeding. The blood is con- 

 tained in a system of well-defined vessels heart, arteries, 

 capillaries and veins; there are no extensive, irregular, 

 vascular spaces such as occur in molluscs and arthropods. 

 With this system the lymphatic vessels are in communica- 

 tion in a few places, and it is said that on the ventral 

 surface of the kidney there is direct connexion between 

 the ccelomic space and the cavities of the tributaries of the 

 renal vein. 



The fact that the blood is kept in motion may be 

 easily verified by examining the web of the foot of the 

 adult, or the tail or external gills of the tadpole, under 

 the microscope. The heart, which is the force-pump for 

 maintaining the flow, lies in the anterior part of the body, 

 and is protected from external shock and pressure by the 

 sternum and ventral portions of the shoulder-girdle. It 

 consists of, (1) a dorsally situated sinus venosus, into which 

 the blood from the greater part of the body is poured by 

 three great veins, the two anterior and the single posterior 

 vena cava; (2) the right and left auricles, forming the 

 wider anterior part of the heart ; the right is the larger, 

 and receives the blood from the sinus venosus ; into the left 

 arterial blood is poured by the pulmonary vein ; the two 

 auricles are completely separated by a thin partition ; 

 (3) a single muscular ventricle, bluntly conical in shape, 



