392 COMPARATIVE PHYSIOLOGY. 



Wood ; that in the case of muscle the oxygen that is used is first 

 laid hy, banked as it were against a time of need, in the form of 

 intra-molecular oxygen, which is again set free in the form of 

 carbon dioxide, but by what series of changes we are quite un- 

 able to say. Though our knowledge of the respiratory processes 

 of muscle is greater than for any other tissue, there seems to 

 be no reason to believe that they are essentially difi^erent else- 

 where. The advantages of this banking of oxygen are, of 

 course, obvious ; were it otherwise, the life of every cell must 

 be at the mercy of the slightest interruption of the flow of 

 blood, the entrance of air, etc. Even as it is, the need of a 

 constant supply of oxygen in warm-blooded animals is much 

 greater than in cold-blooded creatures, which can long endure 

 almost entire cessation of both respiration and circulation, 

 owing to the comparatively slow rate of speed of the vital 

 machinery. 



If one were to rely on mere appearances he might suppose 

 that in the more active condition of certain organs .there was 

 less chemical interchange (respiration) between the blood and 

 the tissues than in the resting stage, or, properly speaking, 

 more tranquil stage, for it must be borne in mind that a living 

 cell is never wholly at rest ; its molecular changes are cease- 

 less. It happens, e. g., that when certain glands (salivary) are 

 secreting actively, the blood flowing from them is less venous 

 in appearance than when not functionally active. This is not 

 because less oxygen is used or less abstracted from the blood, 

 but because of the greatly increased speed of the blood-flow, so 

 that the total supply to draw from is so much larger that, 

 though more oxygen is actually used, it is not so much missed, 

 nor do the greater additions of carbon dioxide so rapidly pol- 

 lute this rapid stream. 



It is thus seen that throughout the animal kingdom respira- 

 tion is fundamentally the same process. It is in every case 

 finally a consumption of oxygen and production of carbonic 

 anhydride by the individual cell, whether that be an Amoeba 

 or an element of man's brain. These are, however, but the 

 beginning and end of a very complicated biological history of 

 by far the greater part of which nothing is yet known ; and it 

 must be admitted that diffusion or any physical explanation 

 carries us but a little way on toward the understanding of it. 



