THE REGULATION OF THE CIRCULATION 273 



that the vessels there become less conspicuous in sleep 

 than they are in waking. Quite aside from these rare 

 observations we have some indirect evidence in regard 

 to the brain circulation which is highly suggestive. It 

 throws light not only upon the local problem but upon 

 the general principles of vasomotor operations. 



FIG. 66. A schematic diagram of Mosso's plethysmograph for the 

 arms: a, the glass cylinder for the arm, with rubber sleeve and two tubu- 

 latures 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 recorded on a drum 

 by the writing point, p. (Howell.) 





It can be shown that when there is a marked change 

 in mental activity there is a vasomotor change detectable 

 in the extremities. Commonly the hand is the part 

 under scrutiny. The instrument used is called the 

 plethysmograph. It is simple in theory though quite 

 troublesome to apply successfully. The plethysmo- 



18 



