1 62 SCIENCE FROM AN EASY CHAIR 



the nerves of the organs of the senses complex changes 

 in the brain, no longer do so. They not only fail to 

 excite consciousness and to leave their mark on the 

 memory, but they do not produce even a simple uncon- 

 scious response. Yet if they are of a sufficient degree 

 of violence (varying according to the depth of the sleep), 

 they do reach the brain, and thus " awake " the sleeper. 

 Corresponding to the absence of receptive activity of the 

 brain in sleep is the absence of outgoing impulses from 

 that organ ; there is no such control of the muscles as 

 in the waking state, the head nods, the eyelids droop, 

 and the muscular action by which the erect posture 

 is maintained is in abeyance, although in a greatly 

 lessened degree some amount of muscular tone is un- 

 consciously retained. 



The passage from the waking state to that of deep 

 sleep is not sudden but graduated, and so is the process 

 of awakening. In the intermediate condition, either 

 before or after deep sleep (often only a minute or two 

 in duration) the brain can still receive, more or less 

 confusedly, impressions from the exterior through the 

 organs of sense, and it is in this way that " dreams " are 

 set going, and may be afterwards either forgotten or 

 remembered. In full sleep the mind is a blank. As a 

 rule healthy sleep becomes gradually more complete in 

 the first hour, and then very slowly less profound. But 

 there are not any sufficient observations on the " quality " 

 of sleep after short or long duration. In sleep it is not 

 only the brain which is at rest: the whole body shares 

 in the condition. The pulse and breathing are slower, 

 the digestive organs and the bladder are more or less at 

 rest. Both the intake of oxygen into the lungs and the 

 expiration of carbonic acid are lessened. The chemical 

 changes within the body are lessened though still proceed- 

 ing, and as a consequence the temperature is lowered. 



