THE GREEN FROG 331 



of mixed blood having become exhausted, the head receives 

 only aerated blood. The impure blood from the organs of the 

 body-cavity and from the hind legs returns either through 

 the liver or the kidneys ; while that from the head, the fore 

 limbs, and from the skin and muscles generally, is poured 

 directly into the right auricle. 



The frog breathes with the mouth closed. By depressing 

 the tongue, air is drawn into the mouth-cavity through the 

 nostrils. When the tongue is raised the nostrils close by 

 valves and the air is forced into the lungs. Considerable 

 exchange of gases takes place through the soft, moist skin, 

 which is well supplied with blood-vessels (Fig. 165, 28). 



The lymphatic system is well-developed in the frog, and 

 the lymph is assisted in its circulation by two pairs of 

 lymph-hearts, one at the posterior end of the body, and one 

 in the region of the shoulders. The pulsations of the pos- 

 terior lymphs-hearts can be observed externally in the living 

 frog. 



The kidneys (Fig. 165, 29) are a pair of oval, dark-red 

 bodies lying in the dorsal part of the body-cavity. The ureters 

 (Fig. 165, 30) open from them into the cloaca (Fig. 165, 31). 

 The urinary bladder (Fig. 165, 32) is a large, thin-walled sac 

 projecting ventrally from the cloaca, and is a very different 

 organ from the urinary bladder of the perch, which is a dila- 

 tation of the ureter. The function of the two organs is, how- 

 ever, the same, that of receiving the liquid nitrogenous waste 

 from the kidneys. 



The Skeletal System. In general, the skeleton of the frog 

 is built upon the plan seen in the perch, but its appendages 

 are considerably more specialized. The skull (Fig. 166, 1) 

 is flattened and the cranium articulates with the first verte- 

 bra by two surfaces, or condoles. The vertebral column consists 

 of nine vertebrae, terminated by a long bone called the urostyle 

 (tail-bone) (Fig. 166, 3). 



