384 PHYSIOLOGY 



include those for the movements of the eye muscles, those concerned in facial 

 expression, and those responsible for the movements of the mouth in mastica- 

 tion and deglutition, and in man, in speech. Important visceral efferent 

 fibres are also contained in the vago-glossopharyngeal nerves, which leave 

 the brain stem at its hindmost part in the region of the medulla oblongata, 

 and influence the condition of the heart and the alimentary canal with 

 its accessory organs. On the other hand, the afferent mechanisms of the brain 

 stem far transcend in importance, i.e. in their influence on the reactions of 

 the animal, those of the spinal cord. Among these afferent mechanisms are 

 those which we have spoken of as ' projicient ' sense organs or organs of 

 foresight, the impulses from which must predominate over all reactions 

 determined by the immediate environment of the animal. Into the medulla 

 oblongata are poured the impulses from the greater part of the alimentary 

 canal and from the heart (the chief factor in the circulation) and the lungs. 

 At the junction of the medulla and pons is the great eighth nerve, really con- 

 sisting of two, one of which, the cochlear nerve, carries impulses from the 

 projicient sense-organ of hearing, while the other, the vestibular nerve, has 

 its terminations in the labyrinth, the sense-organ of equilibration. To the 

 impressions received from this organ all the complex co-ordinating motor 

 mechanisms of the spinal cord have to be subordinated, in order that they 

 may co-operate in the maintenance of the equilibrium of the body as a whole. 

 Into the pons enters the fifth nerve, carrying sensory impressions from the 

 whole of the head, while in the mid- and fore-brain we find the endings of 

 the optic tracts derived from the eyes and carrying visual impressions. From 

 the front of the fore-brain are produced the olfactory lobes. 



At each segment or level in the brain stem the afferent fibres from these 

 various sense-organs enter and join afferent tracts, carrying 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 impressions 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 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- 



