NOVEMBEK 16, 1906.] 



SCIENCE. 



615 



In the first place, they arise from all the 

 bodily surfaces which are sensitive to stimu- 

 lation, even from such regions as the eye 

 and the ear, from which an ordinary reflex 

 action affecting the salivary glands is never 

 known to proceed. 



It must be observed that ordinary sali- 

 vary reflexes may originate not only from 

 the cavity of the mouth, but also from the 

 skin and the nasal cavity. In the second 

 place, a conspicuous feature of these re- 

 flexes is that they are in the highest degree 

 inconstant. All stimuli introduced into the 

 mouth of the dog unfailingly give a posi- 

 tive result in reference to the secretion of 

 saliva, but the same objects when presented 

 to the eye, the ear, etc., may be sometimes 

 efficient and sometimes not. In conse- 

 quence of the last-mentioned fact, we have 

 provisionally called the new reflexes ' condi- 

 tioned reflexes, ' and for the sake of distinc- 

 tion we have called the old ones 'uncondi- 

 tioned.' Every conditioned stimulus be- 

 comes totally ineffective on repetition, the 

 explanation being that the reflex action 

 ceases. The shorter the interval between 

 the separate repetitions of the conditioned 

 reflex the more quickly is this reflex obliter- 

 ated. The obliteration of one conditioned 

 reflex does not affect the operation of the 

 others. Spontaneous restoration of the ob- 

 literated conditioned reflexes does not occur 

 until after the lapse of one, two or more 

 hours, but there is a way in which our re- 

 flex may be restored immediately. All that 

 is necessary is to obtain a repetition of the 

 unconditioned reflex— as, for instance, by 

 pouring vinegar into the dog's mouth and 

 then either showing it to him or letting him 

 smell it. The action of the last-mentioned 

 stimuli, which was previously quite obliter- 

 ated, is now restored in its full extent. If 

 for a somewhat long time — such as days or 

 weeks continuously— a certain kind of food 

 is shown to the animal without being given 

 to him to eat, it loses its power of impart- 



ing a stimulus from a distance— that is, its 

 power of acting on the eye, the nose, etc. 



We may, therefore, say that the condi- 

 tioned reflex is in some way dependent on 

 the unconditioned reflex. At the same time 

 we see also the mechanism which is 

 necessary for the production of our condi- 

 tioned reflex. When an object is placed in 

 the mouth, some of its properties exercise 

 an action on the simple reflex apparatus of 

 the salivary glands, and for the production 

 of our conditioned reflex that action must 

 synchronize with the action of other prop- 

 erties of the same object when the last- 

 mentioned action, after influencing other 

 superficial parts of the body that are sensi- 

 tive to such stimuli, arrives in other parts 

 of the central nervous system. Just as the 

 stimulant effects due to certain properties 

 of an object placed in the mouth may be 

 associated as regards time with a number 

 of stimuli arising from other objects, so all 

 these manifold stimuli may by frequent 

 repetition be turned into conditioned stim- 

 uli for the salivary glands. It must be re- 

 membered that in feeding a dog or forcing 

 something into its mouth each separate 

 movement and each variation of a move- 

 ment may by itself represent a conditioned 

 stimulus. If that is the case, and if our 

 hypothesis as to the origin of the condi- 

 tioned reflex is correct, it follows that any 

 natural phenomenon chosen at will may, if 

 required, be converted into a conditioned 

 stimulus. Any ocular stimulus, any de- 

 sired sound, any odor that might be 

 selected, and the stimulation of any por- 

 tion of the skin, either by mechanical means 

 or by the application of heat or cold, have 

 in our hands never failed to stimulate the 

 salivary glands, although they were all of 

 them at one time supposed to be ineffective 

 for such a purpose. This was accom- 

 plished by applying these stimuli simul- 

 taneously with the action of the salivary 

 glands, this action having been evoked by 



