THE VASOMOTOR NERVES. 



609 



the contrary, will suck water from the recorder into the plethysmograph. 

 In the author's laboratory a modification that has been found most conve- 

 nient is represented in Fig. 250. To avoid escape of water at the upper end 

 of the tube and at the same time to prevent compression of the veins of the 

 arm a very thin rubber glove with long gauntlet is used. The gauntlet 

 is strengthened by cuffs of dam tubing, as shown in the illustration, and all 

 are reflected over the end of the plethysmograph. The outer cuff (3) may 

 be omitted. The hand is inserted into the cylinder and is held in place by 

 flexing the fingers through the rings. The plethysmograph being suspended 

 freely from the ceiling, any movement of the arm will move the instrument as 

 a whole without disturbing the position of the arm in the instrument. By 

 means of rings of hard rubber (D, E) , one fitting around the rim of the plethys- 

 mograph and the other adapted more or less closely to the size of the forearm, 

 the reflected portion of the gauntlet and cuff is held in place and prevented 

 from giving way readily to any rise of pressure in the plethysmograph. The 



Fig. 249. 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 ; 

 8, 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. 



ulterior of the latter is connected, as shown in Fig. 249, to a test tube swung 

 by a spiral spring (Bowditch's recorder) . The spring is so adjusted by trial that 

 it suiks and rises exactly in proportion to the inflow or outflow of water. By 

 this means the level of the water in the tube is kept constant, and since the posi- 

 tion of this level determines the pressure upon the outside of the arm in the 

 plethysmograph this pressure is also kept constant independently of the 

 changes in volume of the arm. The level should be set in the beginning 

 so as to make a slight positive pressure on the arm sufficient to flatten 

 the thin glove to the skin and thus drive out the air between the two. 

 When the apparatus is conveniently arranged, with slings to support the 

 elbow, observations may be made upon the changes in volume of the arm 

 during long periods. The results so obtained are referred to under several 

 headings. With the form of recorder described the plethysmograph gives 

 usually only the slow changes hi volume of the arm, due to a greater or less 

 amount of blood. By using a more sensitive recorder and making the con- 



39 



