390 GENERAL PHYSIOLOGY 



The best example of the excitation of metabolism by increasing 

 temperature is afforded by the activity of yeast-cells, since in the 

 quantity of carbonic acid that is derived from the decomposition 

 of grape-sugar there is given an excellent measure of the increase of 

 metabolism. The evolution of carbonic acid in a solution of grape- 

 sugar containing yeast is always more active with increasing 

 temperature up to about 30° — 35° C, when it becomes very violent.^ 

 The bubbles of carbonic acid rise in the fermentation-tube as in 

 sparkling champagne. Plant-life likewise affords many clear 

 examples of how with rising temperature within certain limits the 

 vital phenomena, such as cleavage 'of carbonic acid, formation of 

 starch, of proteid, etc., increase in intensity ; it is here found that 

 the temperatures at which the excitation reaches its maximum 

 are very different, not only for the different forms of living 

 substance, but also for the various metabolic processes in the 

 same object. It is observed also in animals that metabolism 

 increases proportionally with the temperature ; and Spallanzani 

 showed for cold-blooded animals, especially for snails, that the 

 consumption of oxygen is thus increased. Whatever may be the 

 details of the metabolism, the law holds good everywhere in the 

 living world, that the intensity of metabolism increases with in- 

 creasing temperature. 



It should be mentioned, however, that there is, apparently, 

 an exception to this general law. This is shown by the behaviour 

 of homothermal (warm-blooded) animals. It is a well-known fact 

 that warm-blooded animals undergo a decrease of metabolism with 

 rising temperature. Man in winter has a much more active meta- 

 bolism than in summer, he consumes most food at the lowest, least 

 at the highest degrees of temperature. Thus far this remark- 

 able paradox has been little explained, and Pfluger ('78), who 

 has studied the subject in detail, arrives at a solution of the 

 apparent contradiction only by the aid of certain hypotheses. 

 As is well known, the peculiarity of warm-blooded animals in con- 

 trast to al] others is the possession of a mechanism in their nervous 

 system that regulates reflexly the temperature of the body and 

 maintains it at a constant height, however great variations the ex- 

 ternal temperature may undergo. The metabolism, which is the 

 source of heat-production in the animal organism, is, however, in 

 the warm-blooded animals, the servant of the heat-regulating 

 mechanism. If the external temperature is low, the metabolism 

 and with it the production of heat are increased reflexly through 

 the nervous system from the skin, in order to compensate for the 

 greater loss of heat by the body ; and, vice versa, if the external 

 temperature is high, the metabolism and with it the production of 

 heat undergo, likewise reflexly, a corresponding depression. The 

 increase of metabolism of the cells in cold and the decrease in 

 ' Cf. von Liebig ('70). 



