Oil. XXII.] THE ONCOMETER 309 



organ of an animal. Such an instrument is called a plethysmograph. 

 One of these instruments applied to the human arm is shown in the 

 accompanying figure (fig. 269). 



Every time the arm expands with the heart's systole, a little 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. 



A study of the volume pulse shows it to possess the same main 

 characters (for instance, a dicrotic wave on the downstroke) which 

 we have already described in connection with the velocity pulse, and 

 the pressure pulse (see p. 288). 



When the same method in a modified form is applied to such 

 viscera as the kidney or spleen, the instrument is generally called 

 an oncometer. The earliest oncometers were made by Koy. 



Boy's oncometer (figs. 270 and 271) 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 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 recorder by a tube also containing oil. 

 An increase in the volume of the organ squeezes the oil out of the 

 oncometer into the recorder, and so produces a rise of its piston and 

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



These elaborate instruments have now been entirely superseded 

 by air oncometers, and Schafer was the first to employ an air 

 oncometer in his work on the spleen. 



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

 eleventh thoracic nerve on the vessels of the kidney, a tracing is taken 

 simultaneously of the arterial blood-pressure in the carotid, and of 

 the volume of the kidney by the oncometer. On stimulating the 

 nerve rapidly, there is a slight rise of arterial pressure, but a large 

 fall of the recording lever, showing that the kidney has diminished 

 in volume. It is evident that there must be an active contraction of 

 the arterioles of the kidney, causing it to diminish in size, for the 

 blood-pressure tracing (which is taken as a control to be sure the 

 changes are not otherwise produced) shows that there is no failure of 

 the heart's activity to account for it. 



We shall return to the subject of the oncometer in connection 

 with the spleen and the kidney. We may, however, say in passing 

 what a very important experimental method plethysmography is 

 becoming. Since the introduction of air oncometers, the method is 

 remarkably easy to apply, and it is now part of the routine practice 

 of physiologists, when they are investigating the action of a drug, 



