310 ELEMENTS OF PHYSIOLOGY 



much louder sound to awaken him than during later hours. 

 Sleep becomes gradually lighter until awakening occurs. 

 When consciousness is partly awake, the condition is 

 called dreaming. In somnambulism, the sleeper may talk 

 or walk as he dreams. 



568. For a sound nervous system, nothing is so indis- 

 pensable as plenty of sound sleep. It is necessary .for 

 growth and repair of the cells (Fig. 210). Infants sleep 

 almost all the time ; children of four or five, nearly half the 

 time ; those of ten or twelve, ten hours ; most college 



FIG. 209. 



A, auditory center; B, visual center; C, speech center; D, motor cells of speech nerves in the 

 spinal bulb. Sight of dog, bark of dog, word " dog " and sensations from speech muscles are 

 represented by arrows passing into the brain. The centers are connected by association 

 fibers, so that when the child has learned the word "dog" by sound or when written, the 

 spoken or written word calls up the image, the sound of its bark and the muscular 

 memory of how to speak the word " dog. " 



students require eight hours, and this amount is required 

 by most adults until old age is reached. Repair being less 

 in old age, less sleep is required. 



569. The morning should bring a feeling of vigor. If 

 we wake tired and discouraged, there is something amiss. 

 It may be that there was over-exertion the day before and 

 the sleep has not been long enough, or that we have been 

 up too late or have eaten improper things. If the last is 

 the case, there is apt to be a bad taste in the mouth on 

 awakening. By going to bed at the same hour every 

 night, the habit is formed of dropping off promptly and 

 soundly to sleep. A cheerful state of mind with resolute 



