314 SPECIAL PHYSIOLOGY. 



action of certain voluntary muscles, as, for' example, of those which 

 roll the eyeballs upwards, is increased. The iris is contracted. The 

 various excretions are less abundant ; but the quantity of phosphoric 

 acid separated by the kidneys, is said to be somewhat increased. The 

 animal heat is also lowered, hence the sensation of cold which is often 

 felt on awaking. The quantity of blood in the vessels of the brain, 

 and the rapidity of its circulation, are both much diminished. (Dur- 

 ham.) This observation is contrary to the old opinion, that sleep was 

 the result of a turgescence of the vessels of the brain ; but it is con- 

 firmed by the state of the retina or expansion of the optic nerve, which 

 has been found, by aid of the ophthalmoscope, hereafter described, to 

 be paler, and less vascular during sleep. (Hughlings Jackson.) It has 

 been suggested, that the bloodvesssels of the choroid plexuses in the 

 ventricles of the brain, may become more turgid during sleep, and, by 

 a sort of erection, may act as diverticula for the blood in the cranium, 

 whilst the cerebral vessels are proportionally emptied. The less full 

 state of the vessels of the brain-substance has been called its nutritive 

 circulation, and the more full condition, its functional circulation ; the 

 vessels of both the choroid plexuses and the brain, may be understood 

 to be governed by the state of the vasi-motor nerves. 



Reflex movements still occur during sleep, for the excitability of the 

 afferent and motor nerves, and of those parts of the nervous centres 

 which are not necessary for conscious sensation, but which govern the 

 reflex movements, still remains. The periods of remission and rest of 

 vegetative life, therefore, do not coincide with those of animal life ; 

 moreover, they present very great variations in different organs. 

 Thus the heart's substance and its nervous ganglia, must take their 

 rest in the intervals between its pulsations ; the respiratory muscles 

 and nervous centres, between the inspirations ; and the secreting 

 glands, during the abeyance of their function. 



The causes of sleep are mental and bodily fatigue, long-continued 

 exertion of the senses, diminution or absence of external impressions, 

 as silence and darkness, monotonous continuance of sensory impres- 

 sions, as, for example, the humming of bees, also heavy meals, spiri- 

 tuous drinks, certain narcotic agents, and cold. The recumbent pos- 

 ture also induces sleep, not only through habit, but also by increasing 

 the tendency of the blood to the cranium, thus probably causing tem- 

 porary fulness of the vessels of the choroid plexus, and so diminishing 

 the quantity of blood in the gray matter of the cerebrum. 



As regards the internal physiological conditions of sleep, but little 

 is known. 



The act of awaking, or the cessation of the temporary suspension of 

 the cerebral functions, is either spontaneous, when the nervous parts 

 have recovered from their fatigue, or else it is caused by internal or 

 external stimuli; and this latter is one of the principal points of dis- 

 tinction between sleep and coma, or that absolute state of insensibility 

 from which a person cannot be aroused. Amongst the internal stimuli 

 which interrupt sleep, may be mentioned, very vivid dreams, pain, or 

 sudden disturbances in muscles connected with organic life, as, for 

 example, those of respiration, or the uneasy sensations produced by 



