n6 A MANUAL OF PHYSIOLOGY 



about 15 miles in twenty-four hours, and would require five 

 years to go round the world. 



The Volume-pulse. When the pulse-wave reaches a part it 

 distends its arteries, increases its volume, and gives rise 

 to what may be called the volume-pulse. This may be 

 readily recorded by means of a plethysmograph, an instru- 

 ment consisting essentially of a chamber with rigid walls 

 which enclose the organ, the intervening space being filled 

 up with liquid (Fig. 39). The movements of the liquid are 

 transmitted either through a tube filled with air to a record- 

 ing tambour, or directly to a piston or float acting upon a 

 writing lever. Special names have been given to plethys- 



FIG. 39. PLETHYSMOGRAPH FOR ARM. 



F, float attached by A to a lever which records variations of level of the water in B, 

 and therefore variations in the volume of the arm in the glass vessel C. Or the 

 plethysmograph may be connected to a recording tambour. The tubulure at the 

 upper part of C is closed when the tracing is being taken. 



mographs adapted to particular organs ; for example, Roy's 

 oncometer for the kidney. The method has been successfully 

 applied to the investigation of circulatory changes in man, 

 a finger, a hand or an entire limb being enclosed in the 

 plethysmograph. With a fairly sensitive arrangement, every 

 beat of the heart is represented on the tracing by a primary 

 elevation and a dicrotic wave. The general appearance of 

 the curve is very similar to that of an ordinary pulse-tracing, 

 though there are some differences of detail, especially in 

 the time relations. A volume-pulse has been actually ob- 

 served not only in limbs and portions of limbs, but also (in 

 animals) in the spleen, kidney and brain, and other organs, 

 and in the orbit. In the soft tissues of the mouth and 



