692 SLEEP. [BOOK iv. 



very substance in its slow molecular travail can gather head for 

 explosions only after long pauses of rest. And such few and distant 

 beats as do occur are amply sufficient to meet the needs of the 

 feeble metabolism of the several tissues. The sleep of every day 

 differs from the sleep of winter-cold chiefly because the slackening 

 of molecular activities is due in the former not to extrinsic but to 

 intrinsic causes, not to changes in the medium, but to exhaustion 

 of the subject, and because the phenomena are largely confined to 

 the cerebral hemispheres. It is true that the whole body shares in 

 the condition. The pulse and breathing are slower, the intestine 

 and other internal muscular mechanisms are more or less at rest, 

 the secreting organs are less active, some apparently being wholly 

 quiescent, and the sleeper on waking rubs his eyes to bring back 

 to his conjunctiva its needed moisture. Indeed the whole meta- 

 bolism and the dependent temperature of the body are lowered ; 

 but we cannot say at present how far these are the indirect results 

 of the condition 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 the Cheyne-Stokes 

 rhythm of respiration (see p. 362) is frequently observed. During 

 sleep the pupil is contracted, 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 eye-balls 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 

 executing during sleep movements, often irregular and unsymme- 

 trical and unaccompanied by changes in the pupils. 



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

 which culminate in this inactivity of the cerebral structures. It 

 has been urged that during sleep the brain is ansemic ; but even if 

 this anaemia is a constant accompaniment of sleep, it must, like the 

 vascular condition of a gland or any other active organ, be regarded 

 as an effect, or at least as a subsidiary event, rather than as a 

 primary cause. Nor can the view which regards sleep as the result 

 of a shifting of the mechanical arrangements of the cranial circu- 

 lation be considered as satisfactory. The explanation of the con- 

 dition is rather to be sought in purely molecular changes ; and the 

 analogy between the systole and diastole of the heart, and the 

 waking and sleeping of the brain, may be profitably pushed to a 

 very considerable extent. The sleeping brain in many respects 

 closely resembles a quiescent but still living ventricle. Both are 

 as far as outward manifestations are concerned at rest, but both 

 may be awakened to activity by an adequately powerful stimulus. 

 Both, though quiescent, are irritable, in both the quiescence will 



