THE STRUCTUKE OF THE BRAIN STEM 



383 



cord, of reactions affecting the viscera or the skeletal muscles. On the 

 other hand, both kinds of afferent impressions may pass on up the brain stem 

 to involve higher centres and, mingling with impulses from other afferent 

 nerves or from the projicient sense-organs, may result at some higher level 

 in an efferent discharge, which may include reactions not represented in the 

 cord, or reactions of far greater complexity than are possible in the purely 

 spinal animal. 



In consequence of the endless complex intermingling of afferent im- 

 pulses, any diagrammatic representation of tracts is apt to be misleading, 

 unless it be remembered that at each break or synapse in the chain of neurons 

 there are numerous possibilities of branching discharge, and that in our 

 diagrams we can only give the course of such impulses as, by the frequency 

 of repetition in the average life of the animal, have involved the grouping 

 of a large number of nerve paths of similar function into tracts. The con- 

 stituent elements of these tracts will present similar destinations and possi- 

 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 consist- 

 ing, 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 in our classifications to be 

 guided chiefly by the direction of such 

 impulses in relation to the cerebral Y 

 hemispheres. All tracts going up to \ 

 the cerebral hemispheres may be , 

 involved more or less in the production of 

 nervous matter of these hemispheres as are 

 scious sensation. In the same way there is 

 chains of neurons which carry impulses in a descending direction may be 

 involved in the production of voluntary movement. 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 classification 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 



FIG. 196. 



such changes in the 



associated with con- 



a possibility that the 



