432 PHYSIOLOGY 



any given set of circumstances is determined by the varying resistance 

 at the synapses which intervene between the terminations of the 

 afferent fibres conveying the impulse and the next relay of neurons. 

 These resistances in their turn are altered by the processes of facilita- 

 tion and inhibition, which may be due to contemporaneous or previous 

 events. A conspicuous example of these conditions is afforded by 

 the phenomena of simultaneous and successive spinal induction. 



The uniformity of afferent and efferent mechanisms disappears 

 when we include the brain stem with the spinal cord. The main 

 efferent channel of impulses is still through the spinal cord, since 

 here are found the efferent mechanisms for all the skeletal muscles 

 of the trunk and limbs, the chief servants of the central nervous 

 system in the daily events of life. Other efferent channels are added, 

 which acquire special importance with the growth of the upper brain or 

 cerebral hemispheres. These mechanisms include those for the move- 

 ments of the eye muscles, those concerned in facial expression, and those 

 responsible for the movements of the mouth in mastication 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 

 consisting of two, one of which, the cochlear nerve, carries impulses 

 from the projicient sense-organ of hearing, while the other, the vesti- 

 bular 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 



