60G RAYMOND PEARL. 



supposed to exist in the cortical centres, for example, of a 

 mammal. 



2. The Neuro-muscular Mechanism. — In the negative 

 reaction to mechanical stimuli the anterior end of the body 

 is turned sharply away from the source of the stimulation, 

 while in the positive reaction it is equally sharply turned 

 towards the source. These relations immediately suggest the 

 following questions : — Is the negative reaction the result of a 

 crossed impulse, which, originating at the point stimulated, 

 crosses over to the other side of the body and causes the 

 contraction of the longitudinal muscles on that side, thus 

 producing the turning away from the stimulus ? What is the 

 course of the nerve impulse which produces the positive 

 reaction ? What sets of muscles are concerned in the pro- 

 duction of each reaction ? 



The discussion of the negative reaction may be taken up 

 first. If the nervous impulse producing this reaction 

 crosses the body to produce a contraction on the side 

 opposite from the stimulus, the experiment cited in the 

 section above shows that this crossing cannot occur entirely 

 in the brain, but must also occur in some part of the body 

 posterior to the brain ; or at any rate, be capable of so doing 

 in a quite normal fashion immediately after removal of the 

 brain. In this experiment where the body has been cut 

 in two behind the brain, the posterior piece performs the 

 negative reaction in a quite normal Avay immediately after 

 the operation. This experiment may be carried farther, 

 and the animal cut in two transversely in places nearer 

 and nearer to the posterior end of the body. In all of these 

 cases, until the piece becomes too small to show definite 

 movements of any sort, the negative reaction niay be obtained 

 by strong unilateral stimulation. This shows conclusively, 

 then, that if the negative reaction is to be considered a 

 crossed reflex, there must be all along the body a series of 

 cross-commissures which are at all times ready to bring about 

 in co-ordinated perfection a result with which they have never 

 previously had anything to do. This conclusion seems in- 



