174 HUMAN PHYSIOLOGY 



importance of the brain being in a healthy and well-nourished 

 condition during the training is manifest. 



The power of differentiating various stimuli is dependent 

 on the development of the brain, and becomes more perfect 

 as the animal scale is ascended ; and the complexity of the 

 cerebral action in the higher animals has its basis in the 

 greater number of distinct impressions which have been 

 received and reacted to. Each separate stimulus leaves its 

 mark upon the brain, or as we may say, is sto?*ed in the brain, 

 and each subsequent similar stimulus is sent into these 

 channels, or is associated with the past reactions, and thus 

 the present response is determined, what may be described 

 as an unconsciousness judgment being made. For appro- 

 priate reaction the whole mechanism must be normal a 

 very small injury to any part may completely alter the 

 character of the response to any given stimulus as may be 

 well seen in the insane. 



So far, cerebral action may be considered in a purely 

 material manner as consisting of a series of reflex acts, higher 

 and more complex than those in the spinal cord, in which 

 the result of the stimulus varies in the same way, but to a 

 much greater degree, from its association with more complex 

 past impressions. 



But this cerebral action is generally accompanied by 

 changes in the consciousness of the individual, some of 

 which are termed simply sensations, while others may be 

 described as trains of thought. Essentially, however, a train 

 of thought is nothing more than a train of sensations, each 

 evoked by a stimulus or by a preceding sensation, and, if 

 this be admitted, we may say that it is by sensation alone 

 that we are aware of consciousness, and that, therefore, the 

 two are coterminous. It is impossible to conceive conscious- 

 ness without sensation, or sensation without consciousness. 



Cerebral action frequently goes on without consciousness 

 being implicated ; but so far as we know, consciousness 

 without accompanying cerebral action is unknown, and there 

 is evidence that it is only when the action of the various 

 parts of the cerebrum is co-ordinated that consciousness 

 is possible. In cases of Jacksonian epilepsy, as a result 

 of a small centre of irritation on the surface of the brain, 



