336 GENERAL ZOOLOGY 



head. In the course of the artery leading to the head is a 

 structure (the carotid gland) which temporarily obstructs 

 the flow of blood till the body arteries have become filled. 

 The supply 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 forelimbs, 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. 



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 posterior 

 lymph hearts can be observed externally in the living frog. 



Excretory System. The kidneys (Fig. 173) are a pair of 

 oval, dark-red bodies lying in the dorsal part of the body 

 cavity. The ureters open from them into the cloaca. The 

 urinary bladder is a large, thin-walled sac projecting ven- 

 trally from the cloaca and is a very different organ from 

 the urinary bladder of the perch, which is a dilatation of 

 the ureter. The function of the two organs, is, however, the 

 same, that of receiving the liquid nitrogenous waste from 

 the kidneys. 



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

 (Fig. 174) is built upon the plan seen in the perch, but its 

 appendages are considerably more specialized. The skull 

 is flattened and the cranium articulates with the first verte- 

 bra by two rounded contact surfaces, or condyles. The 



