THEORIES OF SLEEP. 461 



mation, to assume that sleep is due simply to want of oxy- 

 gen, and it is more in accordance with well-established 

 physiological facts to attribute it to a necessity for the gen- 

 eral regeneration of the nervous tissue, though into this, 

 the necessity for oxygen may enter as one element in the 

 physiological repair. 



During sleep, nearly all of the functions, except those 

 directly under the control of the sympathetic nervous sys- 

 tem, are diminished in activity. The circulation is slower, 

 and the pulsations of the heart are less frequent, as well as 

 the respiratory movements. These points have already been 

 considered under the heads of circulation and respiration. 

 AVe have but little positive information with regard to the 

 relative activity of the processes of digestion, absorption, 

 and secretion, during sleep. The drowsiness which many 

 persons experience after a full meal is probably due to a de- 

 termination of blood to the alimentary canal, and a conse- 

 quent diminution in the supply to the brain. 



