2 RESPIRATORY MECHANISMS 



varies with the activity, being at a minimum during complete 

 rest Tn many organisms, including man, the mechanical 

 respiration and the circulation of the blood are "regulated" 

 so as to correspond to the demand of the moment. 



In the present monograph we will not attempt to deal with 

 all the various aspects of respiration. The processes involved 

 in the internal respiration and their regulation will be taken 

 for granted, and the nervous and chemical mechanisms by 

 which the supply of oxygen to the tissues is regulated will be 

 considered only incidentally. 



What I shall endeavour to illustrate is: 



(1) the enormous differences in the "call for oxygen" shown by 

 different organisms and under varying conditions; 



(2) the variations in the accessibility of oxygen in the different 

 habitats occupied by animals; and the main theme will be 



(3) the adaptation of respiratory mechanisms to these widely varying 

 conditions. 



A description of adaptation can be arranged according to 

 the general zoological system, and I admit that for many 

 purposes such an arrangement is desirable, but as it turns out 

 that within the same systematic group we may have many 

 different types of respiratory organs I have tried to adopt a 

 physiological or oecological system, describing first the respira- 

 tion of animals obtaining their oxygen from water by means 

 of organs of increasing complexity, culminating in the gills of 

 fishes. In the air-breathing animals the development of 

 lungs is followed up to their culmination in mammals and 

 birds. The respiratory functions of the blood in a number 

 of forms are discussed in a separate chapter, and finally the 

 tracheal respiration is discussed, by which oxygen is carried 

 directly to the respiring cells without the intervention of a 

 circulating fluid. 



