NERVE 187 



of command of the common path seen in the spinal reflex 

 action (p. 86) ; (b) by prolongation of the reaction time ; 

 and (c) by a more rapid decrease of the force of muscular 

 contraction. 



The seat of the change is in the cerebral synapses, and, 

 after these have failed to act, the spinal arcs may still 

 be unaftected and spinal reflexes may be produced. 



The cause of the condition is probably primarily the 

 accumulation of the products of activity and the lack of a 

 free supply of oxygen to the brain, and the condition may 

 often be removed by (a) a short rest, or by (6) the substitu- 

 tion of a change of occupation, or by (c) muscular exercise, 

 which increases the flow of blood through the brain, or (d) 

 by sleep. The act of yawning (p. 522) is simply a 

 reflex which increases the flow of blood to the heart and thus 

 on to the brain. It is Nature's eftort to overcome cerebral 

 fatigue. 



As to how the synapses are affected, a consideration of 

 the possible way in which such poisons as chloroform act 

 upon the dendrites and gemmules to abolish definite lines of 

 action suggests that in fatigue the same thing may occur to 

 a lesser deo^ree. 



Continued action also leads to well-marked changes in 



o 



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

 diminish and the nucleus shrivels and becomes poorer in 

 chromatin. 



7. Sleep. 



Fatigue of the cerebral mechanism is closely connected 

 with sleep. As the result of fatigue, external stimuli produce 

 less and less definite effects, and thus the changes, which are 

 the physical basis of consciousness, become less and less 

 marked. At the same time, by artificial means, stimuli are 

 usually so far as possible excluded. Absence of light, of 

 noise, and of tactile and thermal stimuli all conduce to sleep. 

 The purpose of hypnotic drugs is to render the brain less 

 susceptible to external or internal stimuli. 



As sleep advances (1) consciousness fades away, and, as 

 the cerebral activity diminishes, (2) the arterioles throughout 



