308 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXI. 



of the fluid in the plethysmograph is expelled and raises the lever. 

 Variations in volume due to respiration are also seen in the tracing. 

 An air plethysmograph connected to a sensitive recorder gives equally 

 good results. 



The same instrument in a modified form applied to such organs 

 as the spleen and kidney is generally called an oncometer, and the 

 recording part of the apparatus, the oncograph. These instruments 

 we owe to Eoy, and the next two figures represent respectively 

 sections of the kidney oncometer and oncograph. 



An oncometer consists of a metal capsule, of shape suitable to 

 enclose the organ: its two halves are jointed together, and fit 

 accurately except at one opening which is left for the vessels of the 

 organ. A delicate membrane is attached to the rim of each half, the 



FIG. 301. Plethysmograph. By means of this apparatus, the alteration in volume of the arm, E, which 

 is enclosed in a glass tube, A, filled with fluid, the opening through which it passes being firmly 

 closed by a thick gutta.percha band.F, is communicated to the lever, D, and registered by a recording 

 apparatus. The fluid in A communicates with that in B, the upper limit of which is above that in 

 A. The chief alterations in volume are due to alteration in the blood contained in the arm. When 

 the volume is increased, fluid passes out of the glass cylinder, and the lever, D, also is raised, and 

 when a decrease takes place the fluid returns again from B to A. It will therefore be evident that 

 the apparatus is capable of recording alterations of the volume of blood in the arm. 



space between which and the metal is filled "with warm oil. The 

 tube from the oncometer is connected to the oil-containing cavity of 

 the oncograph by a tube also containing oil. An increase in the 

 volume of the organ squeezes the oil out of the oncometer into the 

 oncograph, and so produces a rise of the oncograph piston and lever ; 

 a contraction of the organ produces a fall of the lever. 



Very good results are obtained by using saline solution instead of 

 oil; and Schafer has shown in connection with the spleen that a 

 spleen box of simple shape covered with a glass plate, made air-tight 

 with vaseline, and communicating by a tube with a Marey's tambour, 

 gives a far more delicate record of the splenic alterations of volume 

 than the oil oncometer. 



If now we are investigating the action of the anterior root of 



