SPECIAL PHYSIOLOGY. 55 



by numerous successive irritations which weaken it, and by 

 certain toxical or medicinal agents as hydrocyanic (prussic) 

 acid, bromide of potassium, atropine, etc. 



C. ENCEPHALON. 



General Functions of the cerebral or encephalic Centres 

 properly so called. By generalizing the expression reflex 

 phenomena we can apply it to the phenomena which occur 

 between the spinal cord and encephalon ; in fact, the brain 

 does not appear to communicate directly with any portion of 

 the periphery, and can only perceive that which goes on in 

 the spinal cord ; thus in the brain infinite reflex actions occur 

 between the numerous centres that are united by numerous 

 commissures ; and, in those phenomena which are considered 

 voluntary, the brain reacts upon the spinal cord and thence 

 outwards, in accordance with that series of actions which 

 constitute the perception or ego. 



Sensations. The brain is then the seat of the interior 

 phenomenon of perception, under the influence of an exter- 

 nal agent whose action is transmitted to it by means of the 

 peripheral nerves and by the spinal cord. Indeed perception 

 is not produced during sleep, at which time the brain is at 

 rest : but in speaking of the brain we should include, in the 

 view of sensations, the whole encephalic mass and not merely 

 its superficial layers, as a large number of acts attributed to 

 perception seem to take place at the protuberance (see 

 before, p. 50) ; so also a portion of the hemispheres and cere- 

 bellum can be removed without thereby causing the loss 

 of sensation. 



The phenomena of perception are divided into those which 

 give us precise information of external objects, such as spe- 

 cial sensations, which we shall refer to under the head of 

 organs of special sense ; and those called general sensations, 

 which warn us only of those modifications that our organs 

 undergo, without giving us precise information of the nature 

 of the agents producing these modifications ; pain is the spe- 

 cial type of this latter kind of sensations. Intermediate be- 

 tween these two kinds of sensations have been placed those 

 called subjective and objective. 



The general or subjective sensations can also present two 

 phases : in the first, the sensation (pain, for instance) is per- 

 fectly localized, as the sensation of a burn upon the skin ; 

 in the second form, on the contrary, the sensation is vague 

 and difficult to localize ; as the general malaise that marks the 



