SLEEP. 315 



distension of the intestines or bladder. The external stimuli which 

 interrupt sleep, are strong sensory impressions, such as sound, light, 

 or mechanical disturbance, as, e. g., shock or shaking ; the cessation 

 of habitual impressions, on the other hand, may also put an end to 

 sleep the miller awakening when his mill stops. 



Violent sensory impressions, mental excitement, and certain sub- 

 stances, such as tea and coffee, retard sleep, but after severe and pro- 

 longed watching, it eventually comes on with greater intensity ; cases, 

 indeed, are related of fatigued soldiers sleeping on the march, or even 

 during a cannonade in battle, and of persons slumbering during the 

 infliction of torture. Cases have occurred in which, after sleep has 

 been postponed for several days, it supervened so profoundly, as to 

 pass into coma and death ; the celebrated French anatomist and sur- 

 geon, Portal, died in this way. 



Sleep is necessary for the maintenance of the functions of animal 

 life, and is common to all animals which possess a nervous system. 

 The suspension of the nutritive functions in the lowest animals, which 

 are destitute of nervous substance, if any such exist, and that noticed, 

 during the night, in the leaves and other parts of plants, is not true 

 sleep. 



Various circumstances modify the amount of sleep required by dif- 

 ferent persons. Thus, age is of great importance; for adults spend, 

 on an average, about one-third of their life in sleep, i. e. about eight 

 hours in the twenty-four; infants pass almost the whole, and children 

 more than half their existence in a state of sleep; whereas by old 

 people, less is required; but in extreme old age, life, as in infants, 

 may be said to be almost a continuous sleep. Temperament also in- 

 fluences the amount of sleep; thus persons of a plethoric or. lymphatic 

 temperament, require more sleep than individuals of a nervous tem- 

 perament. Habit, again, modifies considerably the amount of sleep 

 required by the individual. Pichegru, it is said, only slept four hours 

 out of the twenty-four, during one year's campaign. John Hunter 

 and Frederick the Great required only five hours daily. Lastly, the 

 amount of previous fatigue, whether mental or bodily, of course in- 

 fluences the amount of sleep required. The invasion of sleep is, in 

 some individuals, sudden ; but it is generally gradual. It is marked 

 by heaviness of the eyes, yawning, and an endeavor to obtain an easy 

 position; luminous spots, bright bodies, and indefinite images, are 

 sometimes perceived, showing the gradual decline of the powers of 

 attention. The imagination is, to a certain extent, active when the 

 senses and the reasoning faculties sleep. The functions of sight are 

 first suspended, then those of taste, smell, hearing, and last of all, that 

 of touch. The muscles of the limbs are the first to become relaxed; 

 whilst those of the back are the last over which voluntary control is 

 lost. Sleep is sometimes protracted for twenty-four hours, or more, 

 in succession. The act of awaking is sometimes sudden, but an inter- 

 mediate state generally exists between sleep and waking. Sleep may 

 be heavy or light, the one state gradually passing into the other ; this 

 varies in different individuals. It is most profound and most refresh- 

 ing during the first hours of rest, many persons being then even insen- 



