THE FROG : VISCERA AND VASCULAR SYSTEM 61 



fluid, known as lymph, which is produced by exudation 

 through the capillary walls. This fluid is gathered by small 

 lymphatic vessels into the big lymph sacs already mentioned, 

 whence it is pumped back into the veins by two pairs of 

 small contractile sacs known as lymph hearts. One pair 

 of these lies below the scapulas and opens into the sub- 

 scapular veins ; the other lies at the end of the urostyle and 

 opens into the femoral veins. 



Sinus venosus 

 ^ Right auricle : 

 Venrricle^. 



Truncus arteriosus, 



I , 



Lungs 



"J- . -•-- V 



Left auricle 

 r 

 Ventricle <• 



Truncus arteriosus 



Head 



Sinus venosus 



Fig. 32. — A diagram of the circulation of the blood in the frog. \ 

 Thick lines indicate venous blood, narrow lines arterial. 



The respiratory organs of the frog are the lungs, the 

 skin, and the mucous membrane of the mouth. 

 Respiration. The lungs communicate with the pharynx by 

 way of the glottis, which leads into a short, 

 wide windpipe consisting of the larynx or voice organ only, 

 without the long trachea, or windpipe proper, which is 

 found in animals with necks. The walls of this cavity are 

 supported by a pair of fiat arytenoid cartilages and a very 



