CENTRAL NERVOUS SYSTEM. 225 



movement of the operated eye toward the median plane (the right), and i<> 

 the position in which it would be held by the clastic tissues alone. This 

 could be explained only through a relaxation or inhibition of the external 

 rectus muscle, as a consequence of the cortical stimulation. Further experi- 

 ments support the explanation, and also show that the cells, the activity of 

 which is thus inhibited, must lie below the cerebral cortex, for the inhibition 

 follows when the fibre-bundles below the cortex arc directly stimulated, the 

 cortex having been first removed. 



The general bearing of these results is of the greatest importance. As 

 has been pointed out by Hughlings-Jackson, 1 damage of any sort to a portion 

 of the nervous system may, in the simplest case, decrease the activity of the 

 group of neurones controlled by the damaged part by cutting off the stimulat- 

 ing impulses from them ; or, on the other hand — and this is often overlooked — 

 it may permit them to become abnormally active by the stoppage of some 

 impulses exerting an inhibitory control. Further, whether impulses from a 

 given set of cells shall prove stimulating or inhibitory depends on the other 

 impulses affecting the receiving cell group, and on the time relations between 

 these several sets. This consideration serves to indicate the complex rela- 

 tions which may underlie the manifestations of disease in the central nervous 

 system. 



As to the mechanism for these inhibitory reactions, it can be safely said 

 that for the most part the effects are not dependent on the existence of a 

 special class of inhibitory nerves, and the most we can think of structurally 

 is a different but not necessarily constant dendritic pathway for the ccllu- 

 lipetal impulses causing inhibition. 



C. Reactions Involving the Encephalon. 



On attempting to distinguish between a voluntary and a reflex act from the 

 physiological standpoint we find the chief difference to be that the voluntary 

 act is not predictable, because, according to the capabilities of the animal, it 

 may be more variable in form than is the reflex response, and also because, 

 instead of necessarily occurring within a short interval after the stimulus, as 

 does the reflex, the voluntary response may be delayed even for years. 



Reflexes have been illustrated by the reactions from a portion of the spinal 

 cord. It is to be remembered, however, that any of the sensory cranial or 

 spinal nerves can serve as a pathway for the afferent impulses, and any of the 

 groups of efferent cells situated in the ventral horns or their liomologues in 

 the brain stem, can carry the efferent impulses needed. Further, it musi be 

 remembered that it is these same afferent cells which always furnish the first 

 set of impulses, and the efferent cells controlling the muscles and glands 

 which furnish the last set of impulses in both reflex ami voluntary reactions. 

 The processes then which distinguish the two forms of reactions must take 

 place in the central cells. We turn, therefore, t<» the nervous connections <>f 

 the encephalon with the cord, since it is by means of these connections that 



1 Hughlings-JacksoD : Lancet, 1898, vol. i. 



Vol. II.— 15 



