THE FROG AND VERTEBRATES IN GENERAL 191 



The respiratory, or breathing, movements in the Frog are differ- 

 ent from those found in Man or other higher Vertebrates, but the 

 end result is the same in either case, namely, the supplying of the 

 specialized lung tissues with fresh air so that the respiratory ex- 

 change can take place. The main feature of breathing in Man 

 is based on changes in the size of the thoracic cavity in which the 

 lungs lie. The thoracic cavity is enlarged by muscular action 

 which pulls the ribs up and out with a synchronous lowering of the 

 diaphragm. Air, impelled by the atmospheric pressure of 15 pounds 

 per square inch, rushes through the external openings, pharynx, 

 trachea, and then into the alveoli of the lungs where the respira- 

 tory interchange takes place. It is expelled by a contraction of 

 the thoracic cavity, which results from the lowering of the ribs 

 and the elevation of the diaphragm. (W. fs. 118, 119.) 



The normal air capacity of the human lung is approximately 

 3500 cc. (about 230 cu. inches). By effort this capacity can be 

 increased to about 4000 cc. It is impossible completely to empty 

 the lungs by expiration, some 1500 cc. of residual air remaining 

 at all times. At each inspiration and expiration approximately 

 500 cc. of tidal air is moved. 



Functional. Frequent attention has been called to the fact 

 that the essential feature of respiration, namely, the continuous 

 interchange of gases between the protoplasm of an organism and 

 its environment, takes place in every one of its living cells. It 

 has also been observed in the higher Invertebrates that both a 

 specialized respiratory mechanism and a system of transportation 

 are necessary features of cell respiration. These basic facts re- 

 mained unaltered in the Frog and other Vertebrate animals. We 

 have just noted the structural features of the lungs, and in the next 

 section, consideration will be given to the vascular system as the 

 great transporting medium to and from the cells, not only of the 

 respiratory gases, but of the numerous types of materials which 

 are necessary for the life-processes of the organism. 



It may be well to call attention at this point to one characteris- 

 tic of the Vertebrates which is of fundamental importance in trans- 

 porting to the cells the oxygen necessary for respiration, namely, 

 the red corpuscles which contain the remarkable compound hemo- 

 globin. This complex substance has the ability to form an un- 

 stable compound (oxyhemoglobin) with oxygen in the lungs 

 where the oxygen is abundant, and it is in this form that a great 



