1306 



PHYSIOLOGY 



The evolution of heat is not confined to the higher animals, but 

 is common to all living beings. It is very evident, for instance, in the 

 germination of peas or barley. The atmosphere of a bee-hive is often 

 ten degrees above that of the surrounding atmosphere. Whenever 

 we can excite increased activity in an organ, we are able to show, 

 except in the single case of the nerve impulse, that such activity is 

 associated with the evolution of heat. This heat is derived from the 

 chemical changes which proceed in the living cells. Since all chemical 

 processes are quickened by rise of temperature, we should expect to 

 find that the heat produced in. the metabolic processes of organisms 

 would tend in itself to quicken these processes. In most chemical 

 reactions a rise of about 10 C. would increase the velocity of reaction 

 from two and a half to three times, and the same rule is, within the 

 limits of stability of living tissues, found to hold good for them also. 

 The diagram (Fig. 541) shows the influence of temperature on the 

 chemical changes in a lupin seedling as measured by the output of 

 C0 2 per hour per 100 grm. of plant. A marked increase in the rate of 

 chemical decomposition is shown to follow a rise of temperature ; but, 

 about 40 C., the rate of change is at an optimum, and thereafter 

 rapidly declines, owing to the fact that the living tissues are being- 

 killed by the excessive temperature. 



Hence in the animal organism we shall expect to find that the rate 

 of the metabolism is also proportional to the temperature of the animal. 

 This is universally the case whether we are dealing with warm-blooded 

 or cold-blooded animals. In ' cold-blooded ' animals the temperature of 

 the body, and therefore the rate of its metabolism and the amount of 

 its heat production, is proportional to the external temperature 

 (Fig. 542). The following Table gives the average C0 2 output per hour 

 of five lizards placed in a chamber which could be maintained at 

 varying temperature : 



It might be thought that such a reaction in change of tempera- 

 ture would result in a vicious circle. Since the animal is continually 



