614 



SCIENCE. 



[N. S. Vol. XXIV. No. 620. 



only the precise connection which exists be- 

 tween the given natural phenomenon and 

 the responsive faculty of the living organ- 

 ism with respect to this phenomenon— or, 

 in other words, to ascertain completely how 

 the given living object maintains itself in 

 constant relation with its environment. 

 The question is simply whether this law is 

 now applicable to the examination of the 

 higher functions of the higher quadrupeds. 

 I and my colleagues in the laboratory be- 

 gan this work some years ago, and we have 

 recently devoted ourselves to it almost com- 

 pletely. All our experiments were made 

 on dogs. The only response of the animals 

 to external impressions was a physiolog- 

 ically unimportant process— namely, the ex- 

 cretion of saliva. The experimenter always 

 used perfectly normal animals, the meaning 

 of this expression being that the animals 

 were not subjected to any abnormal in- 

 fluence during the experiments. By means 

 of a systematic procedure easy of manipu- 

 lation it was possible to obtain an exact 

 observation of the work of the salivary 

 glands at any desired time. 



It is already well known that there is 

 always a flow of saliva in the dog when 

 something to eat is given to it or when 

 anything is forcibly introduced into its 

 mouth. In these circumstances the escap- 

 ing saliva varies both in quality and 

 quantity very closely in accordance with 

 the nature of the substances thus brought 

 into the dog 's mouth. Here we have before 

 us a well-known physiological process — 

 namely, reflex action. It is the response 

 of the animal to external influences, a re- 

 sponse which is accomplished by the aid of 

 the nervous system. The force exerted 

 from without is transformed into a nervous 

 impression, which is transmitted by a cir- 

 cuitous route from the peripheral extremity 

 of the centripetal nerve through the centrip- 

 etal nerve, the central nervous system, and 

 the centrifugal nerve, ultimately arriving 



at the particular organ concerned and ex- 

 citing its activity. This response is specific 

 and permanent. Its specificity is a mani- 

 festation of a close and peculiar action of 

 the external phenomena to physiological 

 action, and is founded on the specific sensi- 

 bility of the peripheral nerve-endings in the 

 given nervous chain. These specific reflex 

 actions are constant under normal vital con- 

 ditions, or, to speak more properly, during 

 the absence of abnormal vital conditions. 



The responses of the salivary glands to 

 external influences are, however, not ex- 

 hausted by the above-mentioned ordinary 

 reflex actions. We all know that the sali- 

 vary glands begin to secrete, not only when 

 the stimulus of appropriate substances is 

 impressed on the interior surface of the 

 mouth, but that they also often begin to 

 secrete when other receptive surfaces, in- 

 cluding the eye and the ear, are similarly 

 stimulated. The actions last mentioned are, 

 however, generally considered apart from 

 physiology and receive the name of psy- 

 chical stimuli. We will take another course, 

 and will endeavor to restore to physiology 

 what properly belongs to it. These excep- 

 tional manifestations unquestionably have 

 much in common with ordinary reflex ac- 

 tion. Every time that there is a flow of 

 saliva attributable to this cause, the occur- 

 rence of some special stimulus among the 

 external influences may be recognized. On 

 very careful exercise of his attention, the 

 observer perceives that the number of spon- 

 taneous flows of saliva forms a rapidly 

 diminishing series, and it is in the highest 

 degree probable that those extremely infre- 

 quent flows of saliva for which no partic- 

 ular cause is at first sight apparent are in 

 reality the result of some stimulus invisible 

 to the eye of the observer. From this it 

 follows that the centripetal paths are al- 

 v/ays stimulated primarily and the centrif- 

 ugal paths secondarily, of course, with the 

 interposition of the central nervous system. 



