Chap, hi.] THE PHASES OF LIFE. 1155 



(§ 470) and of breathing afford, so far as the skeletal muscles 

 are concerned, almost the only token that the central nervous 

 system is alive. But we cannot enter here into the psychology 

 of sleep and dreams. 



Though the phenomena of sleep are largely confined to the 

 central nervous system and especially to the cerebral hemi- 

 pheres, the whole body shares in the condition. The pulse and 

 breathing are slower, the intestine, the bladder, and other in- 

 ternal muscular mechanisms are more or less at rest, and the 

 secreting organs are less active, some apparently being wholly 

 quiescent ; the secretion of mucus attending a nasal catarrh is 

 largely diminished during slumber, and the sleeper on waking 

 rubs his eyes to bring back to his conjunctiva its needed moist- 

 ure. The output of carbonic acid, and the intake of oxygen, 

 especially the former, is lessened ; the urine is less abundant, 

 and the urea falls. Indeed the whole metabolism and the de- 

 pendent temperature of the body are lowered ; but we cannot 

 say at present how far these are the indirect results of the con- 

 dition of the nervous system, or how far they indicate a partial 

 slumbering of the several tissues. 



Thoracic respiration is said to become more prominent than 

 diaphragmatic respiration during sleep, and a rise and fall of 

 the respiratory movements, resembling if not identical with the 

 Cheyne-Stokes rhythm of respiration (§ 305), is frequently 

 observed. During sleep the pupil is constricted, during deep 

 sleep exceedingly so ; and dilation, often unaccompanied by 

 any visible movements of the limbs or body, takes place when 

 any sensitive surface is stimulated ; on awaking also the pupils 

 dilate. The eyeballs have been generally described as being 

 during sleep directed upwards and converging, or according to 

 some authors, diverging ; but others maintain that in true sleep 

 the visual axes are parallel and directed to the far distance. 

 The eyes of children have been described as continually execut- 

 ing during sleep movements, often irregular and unsymmetrical 

 and unaccompanied by changes in the pupils. The contraction 

 of the pupils is worthy of notice, since it shews that the condi- 

 tion of sleep is not merely the simple and direct result of the 

 falling away of afferent impulses ; when the eyes are closed in 

 slumber the pupils ought, since the retina is then quiescent, to 

 dilate ; that they are constricted, the more so the deeper the 

 sleep, shews that important actions in the brain, probably in 

 the middle portions of the brain, are taking place. 



We are not at present in a position to trace out the events 

 which culminate in this inactivity of the cerebral structures. 

 The analogies between ordinary sleep and winter sleep or 

 hibernation are probably real ; the chief difference appears to 

 be that in the latter the diminished activity is due to an ex- 

 trinsic cause, cold, and in the former to intrinsic causes, to 



