250 PRACTICAL ZOOLOGY 



The movement of the blood through capillaries can be observed 

 very easily in the web of the frog's foot. 



Part of the blood of the frog is forced by the heart into the 

 lungs (Fig. 1 43, /g"), where it gets rid of carbon dioxide and re- 

 ceives oxygen. Another part of the blood is carried to the 

 kidneys (Fig. 143, k) and is there relieved of its waste products. 

 The blood returning from these and other organs is carried either 

 through the liver to the right auricle or directly to this auricle, 

 with the exception of that from the lungs, which enters the left 

 auricle. From the auricles the blood is sucked through valves 

 into the ventricle when this muscular portion expands after a 

 contraction. 



There are many spaces in the frog's body filled with a colorless 

 fluid called lymph, which contains colorless corpuscles. Lymph 

 passes from one space into another, enters the blood, and it is 

 always gently flowing over the tissues, thus aiding the blood in 

 the performance of its functions. 



RESPIRATION. By respiration is meant the transfer of oxy- 

 gen from the air to the blood and from the blood to the cells of 

 the body, and of carbon dioxide from the cells to the blood and 

 from the blood to the air. We are accustomed to consider breath- 

 ing as the act of respiration, but this is only external respiration 

 and not so essential as the internal respiration_of the cells. 



In the frog, respiration takes place to a considerable extent 

 through the skin, both in water and in air, but it is carried on 

 principally by the lungs (Fig. 142, L, L'). During inspiration 

 air passes through the nostrils into the mouth cavity. The 

 nostrils are then closed and the air is forced through a slit, the 

 glottis, into a short tube, the larynx, and thence into the lungs. 

 Air is expelled from the lungs (expiration) into the mouth 

 cavity by the contraction of the muscles of the body wall. 



The lungs are pear-shaped sacs with thin, elastic walls. The 

 area of their inner surface is increased by folds which form minute 

 chambers called alveoli. Blood capillaries are numerous in the 

 walls of these alveoli. 



