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CHAPTER III. 



Sleep, 



" that knits up the ravelled sleeve of care, 

 The birth of each day's life, sore labour's bath, 

 Balm of hurt minds, great nature's second course, 

 Chief nourisher in life's feast." 



THE essence of nearly all that has been written 

 on the subject of sleep, from the time of Hip- 

 pocrates to the present day, is contained in the 

 above lines of Shakespeare. But the rationale 

 of the process has never yet been explained 

 in a philosophical manner. I have already 

 shown that the power of the stomach to digest, 

 of the muscles to contract, of the brain to think, 

 and of the nerves to feel, is directly in pro- 

 portion to the rapidity with which their compo- 

 sition is renewed, ceteris paribus, but that the 

 animal heat obtained by respiration, and the or- 

 ganic particles of arterial blood by which the 

 solids are nourished, are still more rapidly ex- 

 pended by the action of the brain and voluntary 

 muscles, than they are renewed in short, that 

 the cause of force is always expended in pro- 



