1552 SLEEP. [BOOK iv. 



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 a rise and fall of the 

 respiratory movements, resembling if not identical with the Cheyne- 

 Stokes rhythm of respiration ( 375), 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 

 executing during sleep movements, often irregular and unsymme- 

 trical and unaccompanied by changes in the pupils. The contrac- 

 tion of the pupils is worthy of notice, since it shews that the 

 condition 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 

 ( 540) are probably real ; the chief difference appears to be that 

 in the latter the diminished activity is due to an extrinsic cause, 

 cold, and in the former to intrinsic causes, to changes in the 

 organism itself; but we saw in treating of hibernation, that 

 intrinsic changes prepared the way for the action of external 

 cold. It has been urged that during sleep the brain is anaemic, 

 and though observations have yielded conflicting results, the evi- 

 dence seems to be in favour of this view ; 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 pri- 

 mary cause. Nor can the view which regards sleep as the result of 

 a change in the mechanical arrangements of the cranial circulation, 

 such as either a retardation or acceleration of the venous outflow, 

 be considered as satisfactory. The essence of the condition 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 

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

 may be awakened to activity by an adequately powerful stimulus. 



