1168 



PHYSIOLOGY 



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

 mon to all living beings. It is very evident, for instance, in the germina- 

 tion 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 tempera- 

 ture, we should expect to find that the heat produced in the metabolic pro- 

 cesses 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. 543) 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 decomposi- 

 tion 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 produc- 

 tion, is proportional to the external temperature (Fig. 544). 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 temperature would 

 result in a vicious circle. Since the animal is continually producing heat and 

 thus raising its temperature above that of its surroundings, one might expect 

 to find that the higher the external temperature the greater would be the 

 difference between this and the temperature of the animal, until finally the 

 latter would rise to such a height that the animal would die of heat-stroke, 

 its tissues being destroyed by the actual temperature attained. A certain 



