58 AN INTRODUCTION TO ZOOLOGY 



acid gas from the blood constitutes respiration and the exchange of 

 the two gases is termed the respiratory exchange. 



Some authorities maintain that the mere osmotic interchange of 

 gases just outlined is not sufficient to account for the total amount of 

 gases taken up and given off, and suggest that the alveolar epithelium 

 also plays a part in it by actually absorbing the gas from the one 

 side and passing it through to the other. 



The lungs are the principal centre of respiration, and in the 

 higher animals the only places where it occurs. In the frog, 

 however, this respiration, i.e. pulmonary, is supplemented by two 

 other kinds. The small blood-vessels in the skin are much larger 

 than in other animals and come near to the surface, which is always 

 moist, and so allow of a cutaneous respiration. Again, the vessels 

 in the wall of the pharynx and buccal cavity give off curious sac-like 

 diverticula, which lie between the epithelial cells lining those regions. 

 This brings the blood into contact with the air, and a buccal or 

 pharyngeal respiration occurs. These two last forms of respiration 

 are peculiar to the Amphibia, the class to which frogs belong, among 

 the Vertebrates, and are not equally important in all members of 

 that class. 



Respiration is the exchange of carbonic acid gas in the 

 blood for oxygen in the air, and is carried out mainly by osmosis in 

 the very vascular lungs. The exchange is aided by a force-pump 

 action of the floor of the mouth, and in Rana is supplemented by 

 similar exchanges in the skin and lining of the mouth and 

 pharynx. 



Circulatory System. 



The circulatory or vascular system of a vertebrate consists 

 of a series of tubes by means of which the blood is carried round the 

 body. The central point in this system is the heart, an elaborate 

 structure which pumps the blood into the blood-vessels, and by 

 means of series of valves causes it to keep circulating in the same 

 direction. Its position in the pericardium and the main vessels 

 connected with it have already been noted. The large trunks con- 

 veying blood away from the heart are termed the arteries, and these 

 break up into smaller and smaller branches as they get farther from 

 the heart. The smallest of these are termed arterioles, and they 

 penetrate into all the tissues of the body. In the tissues they break 

 up into an interlacing network of very minute vessels with extremely 

 thin walls, the capillaries, which unite again to form somewhat 

 larger vessels, the venules. These in their turn unite to form larger 

 and larger trunks carrying the blood back to the heart, these are the 

 veins. 



