THE STRUCTURE OF THE BRAIN STEM 385 



bilities of interruption, i.e. of reactions involving the motor mechanisms at 

 the different levels in the brain stem. It is thus much more difficult in the 

 brain stem than in the spinal cord to describe a ' way in ' and a ' way out ' 

 In a chain consisting, say, of six neurons, a, b, c, d, e, f (Fig. 196), though 

 a is certainly afferent and/efferent, it must always be more or less a question 

 of words whether we regard neurons c and d as afferent or efferent in character 

 It is usual m our classifications to be guided chiefly by the direction of such 

 impulses in relation to the cerebral hemispheres. All tracts going up to 

 the cerebral hemispheres may be involved more or less in the production of 

 such changes in the nervous matter of these hemispheres as are associated 

 with conscious sensation. In the same way there 

 is a possibility that the chains of neurons which 

 carry impulses in a descending direction may be 

 involved in the production of voluntary move- 

 ment. It is therefore usual to classify these two 

 sets of tracts as ascending and descending, or as 

 afferent and efferent. If we adopt such a classifi- 

 cation it must be with a distinct reservation that 

 tracts which apparently are going 

 downwards may play a greater part 

 in the determination of sensation than 

 in the determination of movement, 

 and that there may, and indeed must, \ 

 be a reverberation of impulses through 

 these ascending and descending tracts, 

 so that it must be difficult to dissociate the various elements in the extremely 

 complex neural events which are involved, say, in the simplest kind of 

 conscious sensation. 



As we trace out the evolution of the brain we find an ever- increasing 

 subordination of the lower to the higher centres, so that in man himself 

 many reactions which in the lower animals are carried out by the spinal 

 cord alone, involve the educated co-operation of the cerebral hemispheres. 

 With this increased control there is a corresponding increase in the develop- 

 ment of long paths. In the brain of a fish, for instance, the cerebral hemi- 

 spheres are connected only with the fore-brain ; a little higher up there are 

 connections between the hemispheres and the mid-brain as well. The chief 

 long tracts are those which run between the thalamus, the mid-brain or the 

 hind-brain, and the spinal cord. With the huge development of the cerebral 

 hemispheres in man there is also development of long paths, the pyramidal 

 tracts, from the hemispheres down to all the motor mechanisms of the cord, 

 and of tracts which connect all parts of the cortex with the- grey matter of 

 the pons and indirectly with the cerebellum. The tracts which in the lower 

 animals were of supreme importance in determining subordination of lower 

 to higher centres, of immediate reactions to those determined by the organs 

 of foresight, dwindle therefore in importance. Those tracts, such as the 

 thalamo-spinal, tecto-spinal, vestibulo-spinal, which form the main mass 



13 



FIG. 196. 



