ADDITIONAL TOPICS IN HUMAN BIOLOGY 161 



Not only in youth, but throughout life, must the student, the 

 business man, or the laborer, at the end of a day's employment, find 

 relaxation in other forms of activity. If he fails to do this, not only 

 will he become weary of his work, but he will also finally come to lose 

 the power of enjoying the pleasures he has been neglecting. In the 

 later years of his hfe, the great naturahst, Charles Darwin, wrote as 

 follows : " My mind seems to have become a kind of machine for 

 grinding general laws out of large collections of facts. . . . If I were 

 to hve my life again, I would have made a rule to read some pwetry 

 and Usten to some music at least once every week ; for perhaps the 

 parts of my brain now atrophied would thus have been kept alive 

 through use. The loss of these tastes is a loss of happiness, and 

 may possibly be injurious to the intellect, and more probably to the 

 moral character, by enfeebling the emotional part of our nature." 



241. Rest. — Experiments with animals show a striking differ- 

 ence in the appearance of nerve cells before and after vigorous exer- 

 cise. In the nerve cells of a bird that has been fljdng all day, the 

 protoplasm has a distinctly granular appearance, which is not seen in 

 the cells before exercise. Tired nerve cells can be restored by rest 

 alone. In childhood and youth an abundance of sleep is absolutely 

 essential for healthy development. Late hours of evening enter- 

 tairmient or of study should never be allowed to keep growing boys 

 or girls from having at least nine hours of sleep. 



242. Effect of alcohol on the nervous system. — " The effect of 

 alcohol appears to be, as it were, to shave off the nervous system, 

 layer by layer, attacking first the highest developed faculties and 

 leaving the lowest to the last, so that we find that a man's judgment 

 may be lessened, though at the same time some lower faculties, such 

 as the imagination and emotions, may appear to be more active than 

 before. . . . Thus you find that after a man has taken alcohol his 

 judgment may be diminished, but he may become more loquacious 

 and more jolly than before. Then after a while his faculties become 

 duU; he gets stupid and drowsy. . . . Later on it affects the motor 

 centers, probably the cerebellum, so that the man is no longer able 

 to walk, and reels whenever he makes the attempt. At this time, 



