FACTORS INFLUENCING RATE OF METABOLISM 41 



passing to the respiratory centers influences the rate of 

 breathing movements. 



At high environmental temperatures, above about 37 

 to 40 deg. C, many homoiothermic and poikilothermic 

 animals do not flourish (Eubner, 1924a). The reasons 

 for this are not altogether clear, but they are, of course, 

 quite different from those related to the slow metabolic 

 rates which limit the activities of most poikilotherms at 

 low temperatures. 



Respiration. — There are two types of respiration: ex- 

 ternal, which is concerned with the bringing of oxygen 

 into the body of an animal and with the elimination of 

 waste gases ; and internal, which supplies oxygen to tis- 

 sue cells and permits such cells to eliminate their waste 

 products. The oxygen consumed is perhaps the best in- 

 dex of the metabolic rate of an animal and is, of course, 

 the chief agent, through oxidation, for producing animal 

 heat and maintaining body temperature. Judged by 

 oxygen consumed and carbon dioxide eliminated, there 

 are considerable variations in the metabolism of individ- 

 uals and of species. There are also many influences 

 which temporarily affect respiratory movements and 

 the exchange of gases for respiratory processes. The 

 oxygen-carrying capacity of the blood of various marine 

 fishes is apparently related to the degree of activity 

 which each species normally shows. These fishes are 

 each adapted to a certain metabolic level and the limiting 

 factor to some degree is the rate at which oxygen can 

 be supplied to the tissues. If two bullheads are put in 

 an aquarium together they will show higher rates of 

 exchange for respiratory gases than if they are left by 

 themselves. The presence of one individual apparently 

 influences the rate of respiration in the other. The oxy- 

 gen consumption of different species of salamanders 



