SLEEP 



179 



On any theory we should expect to find, as in fact we do, 

 that the greatest depth of sleep would be soon reached 

 and passed. There should be a steady gain in the direction 

 of the sweeping away of fatigue substances and a steady 

 improvement in the capacity for reaction, which is what we 

 measure when we induce waking. It does not follow that 



Fig. 24. A schematic diagram of Mosso's plethysmograph for 

 the arms: a, The glass cylinder for the arm, with rubber sleeve and 

 two tubulatures for filling with warm water; s, the spiral spring 

 swinging the test-tube, t. The spring is so calibrated that the level 

 of the liquid in the test-tube above the arm remains unchanged as 

 the tube is filled and emptied. The movements of the tube are re- 

 corded on a drum by the writing point, p. (Howell.) 



the time passed in shallow sleep can be much shortened 

 without detriment. The experiments cited show only 

 that certain mechanisms require less than three hours to 

 regain almost their full irritability; there are many more 

 which may need a longer exemption from the demands of 

 the waking state. 



The vasomotor changes which mark the coming on of 



