SLEEP. 69 



sniisliine, and retire to tlieir dens, holes, or burrows 

 witli tlie sh.ides of night. In other words, d;iy is 

 the most convenient time for action, and night 

 is tlie most convenient time for repose and rest. 



Now, whatever part of our bodies is exercised at 

 any moment is thereb}'" to some extent used up 

 and rendered less fit for similar exercise in future. 

 This is true even with the apparently untiring 

 heart itself. At each pulsation it uses up a por- 

 tion of its strength ; but in the very slight inter- 

 val between the pulsations it rests ; and during 

 that brief moment of rest it rebuilds and restores 

 itself against its next effort. The interspace of 

 repose is there indeed exceedingly short and 

 almost imperceptible, yet in that fleeting fraction 

 of a second the tissues of the heart find time 

 enough to extract from the circulating fluid a 

 store of nourishment sufficient to replace them on 

 a sound working basis. But in most parts of our 

 bodies the intervals of work and restoration are 

 far more protracted. If we take a long walk, or 

 climb a steep hill, or pull a boat against stream 

 for half an hour, we are conscious of a marked 

 feeling of fatigue in our legs or arms, as the case 

 may be ; we have in so far unbuilt tlie material of 

 our bodies, and we need rej)ose to set them right 

 Jigain. But this repose does not merely mean, as 

 we might at first sight imagine, a cessation from 

 work ; it means also an actual rebuilding of the 



