NERVE IMPULSES AND REFLEX ACTION 325 



while those going over the vagus nerves, direct from the brain, 

 produce a slowing effect. 



DISEASES OF THE NERVOUS SYSTEM 



If there is trouble in. the brain it is liable to affect the whole 

 life of the individual and especially his intelligence. 



Idiocy. Sometimes a person is born with the brain only 

 partially developed, and even as he grows, it never becomes 

 as large as it should. The skull, too, is usually small and 

 peculiarly shaped. With an abnormally small brain there 

 is sure to be found imperfect intelligence, and such a person 

 is called an idiot. Idiocy is thus a lack of normal intelligence, 

 due usually to the failure of the brain to reach its full size. 

 Size alone may not determine the degree of intelligence in an 

 individual. Frequently abnormal conditions in other organs 

 (especially the thyroid) may produce defective mentality. 



Insanity. On the other hand, a person may have a well 

 developed brain, but something may occur to interfere with 

 its proper functioning. Sometimes, for example, an abscess 

 grows inside the skull and presses on the brain; or after cer- 

 tain accidents, a bit of bone may press in upon it. In all such 

 cases the mental powers of the person are thrown out of 

 balance; he may imagine all sorts of strange things, e.g. that 

 he is another person, or he may become violent and dangerous. 

 Indeed, it is never possible to tell what he may do or think, 

 and we call a person whose mind is so affected, insane. In- 

 sanity is very different from idiocy, for it is due to the de- 

 rangement of brain functions which were originally normal. 

 Insanity varies from a very mild type in which the person is 

 perfectly sane on almost all subjects, but cannot think clearly 

 on some one topic, to that in which the person is violent, and 

 his thinking powers are completely upset. 



Inasmuch as insanity is due, sometimes, to some pressure 

 on the brain, it is occasionally possible to cure it by a surgical 

 operation; in many cases, however, there is no cure. When a 



