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Nervous System and Sense Organs 211 



When these are recorded, the tracing is known as an electroencephalo- 

 gram which is really a recording of the so-called "brain waves" which 

 originate from the activity of the neurons of the cerebral cortex. These 

 waves are of varying frequency and amplitude, with the alpha waves 

 occurring at a rate of about 10 per second, and having the greatest 

 amplitude. The beta waves which occur at the rate of from 14 to 60 

 per second are superimposed on the alpha waves; the delta waves are 

 those that occur at a rate slower than 5 per second. The form and 

 frequency of these waves varies with the activity of the individual. 

 Various mental disturbances can be detected by abnormalities of the 

 patterns; thus they are of some value in psychosomatic medicine. 



Sleep. — Periodically, in normal individuals, there is a suspension 

 of consciousness which we term sleep. Even though the exact cause 

 and nature of sleep are as yet unknown, certain facts do throw some 

 light on the problem. For example, it is realized that the depth of 

 sleep may vary. It deepens toward the end of the first hour after , 

 which it lessens sharply at first and then more slowly until waking 

 occurs. Dreams usually occur during the periods of light sleep while 

 deep sleep is dreamless. 



During sleep all the senses are not responsive to the same degree. 

 The unconsciousness is greatest for smell and least for hearing, touch, 

 and pain. Blood pressure, heart rate, pulse rate, muscle tone, and 

 respiration are all decreased. On the other hand, the secretion of 

 gastric juice and the rate of digestion are little changed. 



The theories as to the causes of sleep are many. Some maintain 

 that the hypothalamus is intimately concerned with the sleeping and 

 waking phenomena. Others believe that the accumulation of waste 

 products in the blood results in sleep. 



It is known, however, that periodic sleep is absolutely essential. 

 The amount needed varies with the age and condition of the individual. 

 At birth, a child sleeps nearly twenty-four hours except for the brief 

 feeding periods. The amount of sleep needed gradually decreases to 

 where an adult may need only eight hours per night. Each individual 

 must determine his own needs. 



THE SENSE ORGANS 



Although the central nervous system controls the activity of the 

 body, it would be unable to function properly without receiving informa- 

 tion from the outer environment. This information is relayed to the 



