1156 SLEEP. [Book iv. 



changes in the organism itself ; but we saw in treating of 

 hibernation, that intrinsic changes prepared the Avay for the 

 action of external cold. It has been urged that during sleep 

 the brain is anaemic, and though observations have 3 r ielded con- 

 flicting results, the evidence 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 primary cause. Nor can the view which 

 regards sleep as the result of a change in the mechanical ar- 

 rangements of the cranial circulation, such as either a retarda- 

 tion or acceleration of the venous outflow, be considered as 

 satisfactory. The essence of the condition is rather to be sought 

 in purely molecular changes, though we cannot, however, at 

 present make any definite statements concerning the nature of 

 these molecular changes. 



The phenomena of sleep shew very clearly to how large an 

 extent an apparent automatism is the ultimate outcome of the 

 effects of antecedent stimulation. When we wish to go to sleep 

 we withdraw our automatic brain as much as possible from the 

 influence of all extrinsic stimuli. We lie down in order to 

 relieve the skeletal muscles and indeed the heart too from the 

 labour entailed by the erect posture; we put off the tight gar- 

 ments which continually spur the skin; we empty the bladder 

 to avoid the stimulus of its distension; and we choose for sleep 

 the night and a quiet place, drawing the curtains, in order 

 that our eyes may be withdrawn from light and our ears from 

 sounds. In this connection may be quoted the interesting case 

 which is recorded of a lad whose sensory tie with the external 

 world was, from a complicated anaesthesia, limited to that 

 afforded by a single eye and a single ear; the lad could be 

 sent to sleep at will, by closing the eye and stopping the ear. 



§ 721. The cycle of the day is however manifested in many 

 other ways than by the alternation of sleeping and waking, 

 with all the indirect effects of these two conditions. There is 

 a diurnal curve of temperature, apparently independent of all 

 immediate circumstances, the hereditary impress of a long and 

 ancient sequence of days and nights. Even the pulse, so sen- 

 sitive to all bodily changes, shews, running through all the 

 immediate effects of the changes of the minute and the hour, 

 the working of a diurnal influence which cannot be accounted 

 for by waking and sleeping, by working and resting, by meals 

 and abstinence between meals. And the same may be said 

 concerning the rhythm of respiration, and the products of pul- 

 monary, cutaneous and urinary excretion. There seems to be 

 a daily curve of bodily metabolism, which is not the product 

 of the day's events. Within the day we have the narrower 

 rhythm of the respiratory centre with the accompanying rise 



