36 GENERAL ZOOLOGY 



out taking in air from the outside, frogs can call under water. In 

 the male the volume of sound is increased by the vocal sacs, 

 which are merely extensions of the floor of the mouth at each 

 angle of the jaw, capable of dilation when filled with air and 

 which act as resonators. They are lacking in the female. 



The frog has a well-developed blood-vascular system, con- 

 sisting of a heart, arteries, veins, and capillaries, through which 

 the blood is circulated. The heart is a pump which furnishes the 

 motive power, by means of which the fluid blood is propelled 

 through the system. The blood is forced from the heart through 

 arteries which eventually break up into thin-walled capillaries 

 from which the blood is returned to the heart by veins. There 

 are two vascular circuits in the frog: (1) a pulmonary circuit 

 connecting the heart and the lungs, and (2) a general systemic 

 circuit connecting the heart with all other parts of the body. 

 The blood itself is made up of a fluid plasma and several different 

 kinds of cellular corpuscles suspended in the plasma. It func- 

 tions as a vehicle for the conveyance to the cells of the body of 

 nutritive material, absorbed from the alimentary canal, and of 

 oxygen, absorbed from the lungs, skin, and buccal cavity. It 

 also absorbs from the tissues generally, products of metabolism, 

 including carbon dioxide, which are excreted through the kidneys, 

 lungs, and skin. 



The lymphatic system is related to the blood circulatory system 

 both structurally and functionally. It consists of large and 

 small spaces, between and within body tissues, all connected, and 

 drained into the blood system by means of four lymph hearts. 

 Lymph also enters the blood stream through ciliated openings in 

 the ventral surface of the kidneys which lead from the coelomic 

 cavity to the renal veins. The subcutaneous lymph spaces are 

 large and abundant, as a result of which the skin seems to be 

 loosely attached to the underlying muscles. A large lymph 

 space within the body is known as the cisterna magna which 

 occupies most of the space above the dorsal side of the body 

 cavity. The anterior pair of lymph hearts pumps lymph into 

 the vertebral veins and the posterior pair pumps it into the 

 transverse iliac veins. The general direction of flow of the lymph 

 is from the tissues toward the veins, which in turn lead toward 

 the heart. The lymph supplements the action of the blood in 

 absorbing excretory products from the tissues. 



