STIMULI AND THEIR ACTIONS 301 



warmth are, therefore, not called out directly by the temperature, 

 but by stimuli that come from the central nervous system. But 

 the paradox is not thus removed, it is merely deferred. The excita- 

 tion of the central nervous system, which affords the stimuli, is 

 brought about along the path of the temperature-nerves from the 

 surface of the body, the skin, by cooling or warming ; and thus the 

 question remains still open, how increasing temperature can pro- 

 duce a depression, and falling temperature an increase of excitation 

 in the central nervous system. In order to answer this question 

 in harmony with the general law of temperature, and also to set 

 aside the apparent paradox, Pfliiger (78) formed the following 

 very plausible hypothesis. He supposes " that the central organ 

 of the sense of temperature contains two substances as substrata of 

 two different specific energies : the excitation of one of these sub- 

 stances manifests itself in consciousness as the feeling of warmth, the 

 excitation of the other as the feeling of cold. One would then have 

 to suppose further that the two substances are related in such a 

 way that the excitation of one is decreased, when that of the other 

 is increased, and vice versa." As a matter of fact, such conditions 

 are frequently recognised in the central nervous system. Upon 

 this supposition it is clear that with rising external temperature the 

 heat-centre must be excited and the cold-centre be depressed, while 

 with falling temperature the heat-centre must be depressed and 

 the cold-centre excited. If, therefore, the cold-centre is connected 

 with the nerve-trunks that influence metabolism, depression of it 

 by increased external temperature must have as a result a 

 depression of the metabolism, and vice versa. Thus the law of 

 temperature would preserve its general validity. The idea is, 

 however, only hypothetical. 



The augmentation of vital phenomena by increase of temperature 

 is also evident in form-changes, where in general it is clearly ex- 

 pressed, especially in organisms that are undergoing development 

 and in cells whose living substance is growing and reproducing. 

 Thus, plant seeds begin to germinate at a certain temperature ; 

 Indian com at approximately 9° C, seeds of the date at approxi- 

 mately IS' C.^ From these points on, with increasing tempera- 

 ture, growth increases constantly up to about 30° — 40° C. Numerous 

 observations have been made upon Bacteria which have shown the 

 same relation. The hay-bacillus, according to the investigations 

 of Brefeld, begins to grow at 6° C, and with rising temperature 

 increases constantly and more rapidly up to 30° C. The bacillus 

 of tuberculosis, as Koch has shown, begins to grow first at 28° C, 

 and reproduces most rapidly at 37°- — 38° C. The fact that this 

 bacillus begins its growth at so high a temperature is due to its 

 parasitic manner of life in the tissues of warm-blooded animals, 

 with whose body-temperature the optimum of its growth coincides. 



1 Cf. Sachs ('82). 



