526 ELEMENTARY PHYSIOLOGY 



organ of vision (p. 421) consists of certain nerve-cells 

 lying in a particular part of the posterior surface of the 

 cerebral hemisphere, and that the central end-organ of 

 hearing (p. 386) consists of other nerve-cells lying else- 

 where on "the cerebral surface, it will still leave us com- 

 pletely in the dark as to what goes on in the cerebral 

 hemispheres when we think and when we will. 



There is no doubt that a molecular change in some 

 part of the cerebral substance is an indispensable ante- 

 cedent to every phenomenon of consciousness. And it 

 is possible that the progress of investigation may enable 

 us to map out the brain according to the psychical relations 

 of its difierent parts. But supposing we get so far as to 

 be able to prove that the irritation of a particular frag- 

 ment of cerebral substance gives rise to a particular state 

 of consciousness, the reason of the connection between 

 the molecular disturbance and the psychical phenomenon 

 appears to be out of the reach, not only of our means of 

 investigation, but even of our powers of conception. 



Reflex Actions of the Brain. — Even while the 

 cerebral hemispheres are entire, and in full possession of 

 their powers, the brain gives rise to actions which are as 

 completely reflex as those of the spinal cord. 



When the eyelids wink at a flash of light, or a threatened 

 blow, a reflex action takes place, in which the afferent 

 nerves are the optic, the efferent the facial. When a bad 

 smell causes a grimace, there is a reflex action through 

 the same motor nerve, while the olfactory nerves constitute 

 the afferent channels. In these cases, therefore, reflex 

 action must be effected through the brain, all the nerves 

 involved being cerebral. 



When the whole body starts at a loud noise, the 

 afferent auditory nerve gives rise to an impulse which 

 passes to the medulla oblongata, and thence affects the 

 great majority of the motor nerves of the body. 



It may be said that these are mere mechanical actions, 

 and have nothing to do with the operations which we 



