CEEEBRUM 189 



of faulty brain action, rather than that his actions are a result 

 of the modified ideas. 



2. Time of Cerebral Action. The cerebral mechanism takes a 

 very appreciable time to act, and the time varies with the 

 complexity of the action and with the condition of the 

 nervous apparatus. 



Of the time between the presentation of a tlash of light to 

 the eye or a. touch to the skin and a signal made when it is 

 perceived by the person acted upon, part is occupied by the passage 

 of the nerve impulses up and down the nerves and in the latent 

 period of muscle action, but a varying period remains, repre- 

 senting the time occupied in the cerebral action. (Practical 

 Physiology.) 



Prolonged action of the nerve centres soon leads to a pro- 

 longation of the reaction time, and the same thing is produced 

 by the action of alcohol, chloroform, and other poisons. 



3. Fatigue of Cerebral Mechanism. This naturally leads to 

 the consideration of fatigue of the cerebral mechanism. The 

 way in which, as a result of poisons, the definite co-velationship 

 of certain sets of neurons with certain other sets is probably 

 abolished by the generalised expansion of the gemmules of the 

 dendrites has been already dealt with. In all probability the 

 same thing occurs in fatigue, and by interrupting the definite 

 chain of action allows rest and recovery to supervene. But 

 continued action may further lead to well-marked changes in 

 the cell protoplasm of the neurons. The Nissl's granules 

 diminish and the nucleus shrivels and becomes poorer 

 in chromatin. 



In all reflex action, whether spinal or cerebral, it is the 

 central part of the mechanism which first becomes fatigued. 

 If, by reflex excitation of the central nervous system of 

 a frog, muscular movements are caused for some time, the 

 stimulation ultimately fails to act ; but, if the nerves going to 

 the muscles are stimulated, the muscles at once respond, 

 showing that the central mechanism has given out before the 

 peripheral structures. . 



Fatigue of the central nervous system is manifest both 

 upon the receiving and reacting mechanism ; upon the 



