THE PHASES OF LIFE. 861 



Dot 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 con- 

 junctiva its needed moisture. Indeed the whole metabolism and the depend- 

 ent 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. 



817. Thoracic respiration is said to become more prominent than dia- 

 phragmatic respiration during sleep, and the Cheyne-Stokes rhythm of 

 respiration (see p. 385) is frequently observed. During sleep the pupil is 

 contracted, during, deep sleep exceedingly so ; and dilatation, often unaccom- 

 panied 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 upward 

 and converging or, according to some authors, diverging ; but others main- 

 tain that in true sleep the visual axes are parallel and directed to the far 

 distance. The eyes of children have been described as continally executing 

 during sleep movements, often irregular and unsymmetrical and unaccom- 

 panied by changes in the pupils. 



818. 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. anaemic ; 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 

 circulation be considered as satisfactory. The explanation of the condition 

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

 the systole and the diastole of the heart, and the waking and sleeping of the 

 brain, may be profitably pushed to a very considerable extent. The sleep- 

 ing brain in many respects closely resembles a quiescent but still living ven- 

 tricle. 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 ultimately 

 give place to activity, and in both an appropriate stimulus applied at the 

 right time will determine the change from rest to action. Just as a single 

 prick will under certain circumstances awaken a ventricle, which for some 

 seconds has been motionless, into a rhythmic activity of many beats, so a 

 loud noise will start a man from sleep into a long day's wakefulness. And 

 just as in the heart the cardiac irritability is lowest at the beginning of the 

 diastole and increases onward till a beat bursts out, so is sleep deepest at its 

 commencement after the day's labor ; thence onward slighter and slighter 

 stimuli are needed to wake the sleeper. For, judging of the depth of ordi- 

 nary nocturnal sleep by the intensity of the noise required to wake the 

 sleeper, it may be concluded that, increasing very rapidly at first, it reaches 

 its maximum within the first hour ; from thence it diminishes, at first rapidly, 

 but afterwards more slowly. 



819. We cannot, however, at present make any definite statements 

 concerning the nature of trie molecular changes which determine this rhythmic 

 rise and fall of cerebral irritability. The fact that the products of proto- 

 plasmic activity when they accumulate within the protoplasm appear to 

 become in the end an obstruction to that activity, has suggested the idea that 



