618 



SCIENCE. 



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



stimulus was either allowed to act for one 

 minute immediately in advance of an un- 

 conditioned stimulus or it was even applied 

 two minutes earlier. Conversely, also, the 

 conditioned stimulus was not brought into 

 action until the unconditioned reflex was 

 at an end. In all these cases the condi- 

 tioned reflex developed itself; but in the 

 cases in which the conditioned stimulus was 

 applied three minutes before the uncondi- 

 tioned one, and was separated from the 

 latter by an interval of two minutes, we 

 obtained a condition which was quite un- 

 expected and extremely peculiar, but was 

 always repeated. "When scratching was ap- 

 plied to a particular spot — for instance, as 

 a conditioned stimulus — after it began to 

 produce an effect it was found that scratch- 

 ing of any other place also produced an 

 effect, just as in the case of cold or heat 

 applied to the skin, new musical sounds, 

 optical stimuli and odors. The unusually 

 copious secretion of saliva, and the ex- 

 tremely expressive movements of the animal 

 attracted our attention. It may appear 

 that this manifestation is of a different 

 kind from those with which we have 

 hitherto been occupied. The fact was that 

 in the earlier experiments at least one coin- 

 cidence of the conditioned stimulus with the 

 unconditioned one was necessary, but on 

 the present occasion manifestations which 

 had never occurred simultaneously with an 

 unconditioned reflex were acting as condi- 

 tioned stimuli. Here an unquestionable 

 point of difference naturally comes to light, 

 but at the same time there is also to be seen 

 another essential property of these mani- 

 festations which they have in common with 

 the former ones— that is, the existence of a 

 very sensitive point in the central nervous 

 system, and in consequence of its position 

 this point becomes the destination of all 

 the important stimuli coming from the ex- 

 ternal world to make impressions on the 



receptive cells of the higher regions of the 

 brain. 



Three characteristic features of this sub- 

 ject make a deep impression upon him who 

 works at it. In the first place, these mani- 

 festations present great facilities for exact 

 investigation, I am here referring to the 

 ease with which they may be repeated, to 

 their character of uniformity under similar 

 conditions of environment, and to the fact 

 that they are capable of further subdivision 

 experimentally. In the second place, it is 

 inevitable that opinions formed on this sub- 

 ject must be objective only. In the third 

 place, the subject involves an unusual 

 abundance of questions. To what depart- 

 ments of physiology does it correspond ? It 

 corresponds partly to what was in former 

 days the physiology of the organs of special 

 sense and partly to the physiology of the 

 central nervous system. 



Up to the present time the physiology of 

 the eye, the ear and other superficial organs 

 which are of importance as recipients of im- 

 pressions has been regarded almost exclu- 

 sively in its subjective aspect; this pre- 

 sented some advantages, but at the same 

 time, of course, limited the range of in- 

 quiry. In the investigation of the condi- 

 tioned stimuli in the higher animals, this 

 limitation is got rid of and a number of im- 

 portant questions in this field of research 

 can be at once examined with the aid of all 

 the immense resources which experiments 

 on animals place in the hand of the physiol- 

 ogist. The investigation of the conditioned 

 reflexes is of very great importance for the 

 physiology of the higher parts of the cen- 

 tral nervous system. Hitherto this depart- 

 ment of physiology has throughout most of 

 its extent availed itself of ideas not its own, 

 ideas borrowed from psychology, but now 

 there is a possibility of its being liberated 

 from such evil influences. The conditioned 

 reflexes lead us to the consideration of the 

 position of animals in nature ; this is a sub- 



