SLEEP. 743 



result of the disease; and these do not involve the action of any special centres or 

 nerves. 



As regards the muscular atrophy itself, if the nervous stimulus he progressively de- 

 stroyed, the muscular tissue will necessarily undergo progressive degeneration and 

 atrophy. 



"With the above considerations, we leave the trophic cells and nerves to the patholo- 

 gist ; and we can only admit the existence of centres and nerves specially and directly in- 

 fluencing the nutrition of the muscular system, when it has been demonstrated that there 

 are lesions of particular structures in the nervous system, which produce phenomena that 

 cannot he explained by our knowledge of the action of ordinary motor and sensory nerves 

 and of the vaso-motor system. 



We have thus endeavored to represent what is actually known concerning the sym- 

 pathetic system, but it is evident that we have much to learn with regard to its physi- 

 ology. The great sympathetic ganglia may have functions of which we have no definite 

 idea ; and we are better prepared to advance our knowledge in this direction, by admit- 

 ting our ignorance, than by attempting to supply the deficiencies in our positive infor- 

 mation by theories unsupported by facts. 



Sleep. 



When we remember that about one-third of our existence is passed in sleep, and that, 

 at this time, voluntary motion, sensation, the special senses, and various of the functions 

 of the organism, are greatly modified, the importance of a physiological study of this 

 condition is sufficiently apparent. The subject of sleep is most appropriately considered 

 in connection with the nervous system, for the reason that the most important modifica- 

 tions in function are observed in the cerebro-spinal axis and nerves. Repose is as neces- 

 sary to the nutrition of the muscular system as proper exercise ; but repose of the mus- 

 cles relieves the fatigue due to exercise, without sleep. It is true that, after violent and 

 prolonged exertion, there is frequently a desire for sleep, but simple repose will often 

 restore the muscular power. After the most violent effort, a renewal of muscular vigor 

 is most easily and completely effected by rest without sleep, a fact familiar to all who are 

 accustomed to athletic exercises. The glands engaged in the production of the true secre- 

 tions need certain intervals of repose ; but this does not necessarily involve sleep. After 

 prolonged and severe mental exertion, however, or after long-continued muscular effort 

 which involves an excessive expenditure of the so-called nerve-force, sleep becomes an 

 imperative necessity. If the nervous system be not abnormally excited by effort, sleep 

 follows moderate exertion as a natural consequence, and it is the only physiological means 

 of complete restoration ; but the two most important muscular acts, viz., those con- 

 cerned in circulation and respiration, are never completely arrested, sleeping or waking, 

 although they undergo certain modifications. 



In infancy and youth, when the organism is in process of development, sleep is more 

 important than in adult life or old age. The infant does little but sleep, eat. :m<l <!! 

 In adult life, under perfectly physiological conditions, we require about eight hours of 

 sleep ; some persons need less, but very few require more. In old age, unless after 

 extraordinary exertion, less sleep is required than in adult life. Each individual learns 

 by experience how much sleep is necessary for perfect health, and there is nothing which 

 more completely incapacitates one for mental or muscular effort, especially the former, 

 than loss of rest. 



Sleeplessness is one of the most important of the predisposing causes of certain forms 

 of brain-disease, a fact which is well recognized by practical physicians. ( >ne ot the 

 most refined and exquisite methods of torture is long-continued deprivation of sleep; and 

 persons have been known to sleep when subjected to acutely painful iinpn. ssions. Severe 

 muscular effort, even, may be continued during sleep. In forced marches, rcirinieiits have 



