i/6 HUMAN PHYSIOLOGY 



the eye or a touch to the skin and a signal made by the 

 person acted upon when it is perceived, part is occupied 

 by the passage of the nerve impulses up and down the 

 nerves and in the latent period of muscle action, but some- 

 thing over T Vth of a second remains, representing the time 

 occupied in the cerebral action. (Practical Physiology, 

 Chap. XII.) 



If the observation be complicated by requiring a dis- 

 crimination to be made between different stimuli, the reaction 

 time is longer, and, the more unaccustomed the differentia- 

 tion, the longer will the reaction time be. Thus in one 

 accustomed to deal with figures, the discrimination of a 

 series of these is more rapidly carried out than in one 

 unaccustomed to do so. Prolonged action of the nerve 

 centres soon leads to a prolongation of the reaction time, 

 and the same thing is produced by the action of alcohol, 

 chloroform, and other poisons. This fatigue of the nerve 

 mechanism is the physical basis of that state of the con- 

 sciousness which is called loss of attention. 



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-relationship of certain sets of neurons with certain other 

 sets is 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 leads to well- 

 marked changes in the cell protoplasm of the neurons. 

 The Nissl's granules diminish and the nucleus shrivels and 

 becomes poorer in chroinatin. 



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

 central part of the mechanism which first becomes fatigued. 

 If, by direct 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. 



