416 



SCIENCE 



[N. S. Vol. XLI. No. 1055 



course of time (ten days or two weeks) the 

 application of the ice to the same cutaneous 

 area would evoke a flow of saliva without 

 the formality of feeding. The application 

 of ice to other parts of the skin was also 

 effectual, apparently because the sensa- 

 tions of cold concerned in the result were 

 not local, but more or less generalized. In 

 another series of experiments a note of a 

 certain pitch always accompanied the ta- 

 king of food, and this stimulus, too, after 

 sufficient repetition, could evoke the flow 

 of saliva, while a note of distinctly different 

 pitch was ineffectual. The reactions thus 

 acquired were soon lost with disuse, al- 

 though it is possible that if the "training" 

 had been continued over much longer pe- 

 riods of time the reactions might have 

 become more firmly fixed; it is even con- 

 ceivable that they may take place in the ab- 

 sence of consciousness ; that is to say, with- 

 out the participation of cerebral centers; 

 but these are questions which, so far as I 

 am aware, experiment has not yet answered. 

 Finally, they are more or less capricious; 

 not infrequently the acquired response to 

 the stimulus does not occur, thus contrast- 

 ing with the response to gustatory stimu- 

 lation, which seldom fails. 



In what way is this type of reaction ac- 

 quired? The phenomenon of reenforce- 

 ment (of the knee jerk, for example) shows 

 that activity of any one part of the nerv- 

 ous system causes the irradiation over the 

 entire brain and cord of some exciting influ- 

 ence which, though itself minimal or even 

 subminimal, yet adds itself to any other 

 stimulus that may enter about the same 

 time. Pawlow's work seems to show, more- 

 over, that, when two nerve centers are 

 habitually active at the same time, there is 

 beaten out a path of conduction between 

 the two, the two become "associated" so 

 that activity of the one is liable to excite 

 activity of the other. When, for example, 



the knee jerk is reenforced by stimulus of 

 sound, not only does such an irradiation 

 from auditory centers pass to all parts of 

 the nervous system, the sacral motor centers 

 included, but one also irradiates from the 

 sacral centers to all parts of the nervous 

 system, the auditory centers included; and 

 just as when there are two lights in a room 

 the path between these lights is the most 

 intensely illuminated portion of the room, 

 so in the case in question the path between 

 the two centers is most strongly in the ex- 

 cited state. If now this same combination 

 of activity be repeated over and over again, 

 this path becomes more irritable and con- 

 ductive by use until we arrive at the con- 

 dition shown in the above experiments of 

 Pawlow where activity of one center can, of 

 itself, excite activity of the other. It would 

 indeed be interesting to know whether, just 

 as clapping the piece of ice on the skin 

 evoked a secretion of saliva, so the dog ex- 

 perienced a sensation of cold every time he 

 ate. 



The path of conduction or association 

 thus established is presumably through the 

 gray matter, perhaps with the help of the 

 short neurones of the border zones.^" Our 

 present knowledge of the anatomy of the 

 nervous system is inadequate to give a satis- 

 factory idea of the mechanism involved in 

 the development of this new path of con- 

 duction; but it is inconceivable that the 

 anatomical basis of the physiological con- 

 nection between the centers in question 

 should be the same as that pictured in the 

 typical reflex are of the text-books. Apart 

 from the improbability of the development 

 of new neurones, the observed facts of the 

 capriciousness of the reaction and the ease 

 with which, once acquired, it is lost by dis- 

 use determine as the logical course its pro- 

 visional classification in a group of its own. 



1° One thinks of the ' ' neuropile ' ' of some Ms- 

 tologiats as a possible tissue in which this path is 

 blazed. 



