THE STRUCTURE OF THE BRAIN STEM 



433 



impulses on from the spinal cord impulses originally derived from the 

 muscles and skin of the trunk and limbs. At each level there may 

 be an immediate ' reflection ' back to the cord, so that the spinal 

 afferent impressions may co-operate with the cranial afferent impres- 

 sions in the production, through the spinal 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 reaction's 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 impulses, any dia- 

 grammatic 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 constituent elements of these tracts 

 will present similar destinations and possibilities 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. 193), 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 

 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 pro- 

 duction of voluntary movement. It is therefore usual to classify these 

 twosets 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 



28 



FIG. 197. 



