Maech 19, 1915] 



SCIENCE 



415 



and that there are all gradations between 

 this and the highest development of the 

 human will. Perhaps they are right; we 

 will not argue the point; but we neverthe- 

 less retain our two categories of reflex and 

 volitional actions, because so long as this 

 conception of volition is a pure hypothesis 

 it is unwise to forget that what are sub- 

 jectively different may be objectively 

 different as well. 



The same principle of definition should 

 be applied to the case under discussion. A 

 reflex from which consciousness is entirely 

 absent and one in which consciousness 

 seems subjectively to play a causal role 

 may, from the objective standpoint, be one 

 and the same thing ; and yet so long as this 

 is only one of two opposing tenable hypoth- 

 eses, it would seem to be the sensible thing 

 to make a distinction between them. 



One of the world's most eminent phys- 

 iologists does indeed make such a distinc- 

 tion. I refer to Pawlow's differentiation 

 between the unconditioned and the condi- 

 tioned reflex. I can hardly think that 

 Pawlow's very striking experiments upon 

 which he bases this distinction are unknown 

 to physiologists generally and all will agree 

 that he is a man whose opinion should com- 

 mand attention ; yet I find no notice what- 

 ever of this matter in the three admirable 

 text-books of physiology which are most 

 widely used to-day in England and Amer- 

 ica. Because of this and in view of the 

 fact that this address is to a semi-popular 

 audience I shall go into this matter at 

 somewhat greater length than if I were 

 speaking to specialists in neurology. 



The distinction between the conditioned 

 and the unconditioned reflex is well illus- 

 trated by the excitation of salivary secretion 

 through the nervous system. When the 

 taste endings are excited by food in the 

 mouth, a purely reflex flow of saliva results. 



The work of Miller^ seems to establish the 

 existence of definite bulbar centers for this 

 reaction, the gustatory fibers of the lingual 

 and the glossopharyngeal nerves serving as 

 the afferent neurones. On the other hand, 

 the mouth may water ' ' at the very sight of 

 food." Here the afferent stimulus comes 

 through the optic nerve, but it differs from 

 that through the gustatory fibers in the fact 

 that the reaction is secured only in a con- 

 scious animal. It is also more capricious in 

 its occurrence; the whole setting of the 

 nervous system must be right to have it 

 occur at all; the subject must be hungry, 

 the food must be appetizing, it is more apt 

 to occur at the accustomed hour for meals. 

 In short, a certain state of consciousness 

 must exist to insure effective connection be- 

 tween the afferent optic neurones and the 

 secretory efferent neurones. In the uncon- 

 ditioned reflex the nervous processes con- 

 cerned in consciousness are in no way in- 

 volved; it will take place in a decerebrate 

 animal and may occur under anesthesia; 

 it is a rare thing that the application of the 

 proper stimulus fails to elicit it, although, 

 like any other reflex, it may be inhibited, 

 as in the old rice test. Above all, it is not 

 easily lost by disuse, perhaps never perma- 

 nently lost except by some actual atrophy 

 of the neurones involved. 



In a remarkable series of experiments 

 Pawlow" actually developed in animals 

 conditioned reflexes which could by no pos- 

 sibility have formed part of the previous 

 life of the animal or of its ancestors. Every 

 time a dog was fed, a piece of ice was ap- 

 plied to a certain part of its skin. In the 



s F. E. Miller, Quar. Jour. Experimental Physi- 

 ology, 1913, VI., 57. 



9 Pawlow, I. P., Huxley Lecture for 1906, Brit- 

 ish Medical Journal, 1906, Vol. II., p. 871; Lancet, 

 1906, Vol. 171, p. 911; Science, 1906, N. S., 

 XXIV., p. 613; see also Pawlow's articles on the 

 same subject, Ergebnisse der Physiologie, 1904, 

 III., 1, p. 177; ib., 1911, XL, p. 345. 



