448 NERVOUS SYSTEM. 



to which we have become accustomed will fail to disturb our 

 natural rest. Those who have been long habituated to the 

 endless noise of a crowded city frequently find difficulty in 

 sleeping in the oppressive stillness of the country. "We must 

 have sleep ; and this demand is so imperious, that we soon 

 accommodate ourselves to the most unfavorable surrounding 

 conditions. It is remarkable, also, that prolonged exposure 

 to intense cold induces excessive somnolence, and if this be 

 not resisted, the sleep passes into stupor, the power of resist- 

 ance to cold becomes rapidly diminished, and death is the 

 inevitable result. Intense heat often produces drowsiness, 

 but, as is w T ell known, is not favorable to natural sleep. We 

 generally sleep less in summer than in winter, though in 

 summer, perhaps, we are less capable of protracted mental 

 and physical exertion. 



Sleep is preceded by an indescribable feeling of drowsi- 

 ness, an indisposition to mental or^physical exertion, and a 

 general relaxation of the muscular system. It then requires 

 a decided eifort to keep awake ; and if we yield to the sopo- 

 rific tendency, the voluntary muscles cease to act, the lids 

 are closed, we cease to appreciate the ordinary impressions 

 of sound, and we sometimes pass into a dreamless condition, 

 in which we lose all knowledge of existence. We say some- 

 times, because the mind is not generally inactive during 

 what we may regard as normal sleep. We may have dreams 

 which are not due, as far as can be ascertained, to impres- 

 sions from the external world received during sleep. Ideas 

 in the form of dreams may be generated in the brain from 

 impressions previously received while awake, or trains of 

 thought may be gradually extended from the moments im- 

 mediately preceding sleep into the insensible condition. 

 During the nine years that we have been almost unremit- 

 tingly engaged in the preparation of this work, we have 

 frequently labored during sleep for an entire night to no 

 purpose, it is true upon difficult questions to which we had 

 devoted a great deal of thought. 



