382 PHYSIOLOGY 



path taken by any given impulse under 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 process of facilitation and inhibition, which may be due to con- 

 temporaneous or previous events. A conspicuous example of these con- 

 ditions 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 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 t hex- 

 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 th<> 

 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 

 tin- 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 



