344 



PHYSIOLOGY 



CHAP. 



curves by Traube and Hering. Lastly (1875), Mosso found with 

 his plethysmograph (Fig. 156) that the volume of the human fore- 

 arm exhibits the same long and irregular oscillations, which can 

 only be interpreted as the effects of a peculiar rhythm of alternate 

 contractions and dilatations of the vessels of the limb. Von Basch 



FIG. 156. Mosso's plethysmograph, for recording slow variations in volume of vessels of fore- 

 arm (diagram). A, B, glass cylinder to receive the forearm, closed by rubber band ; this 

 rests on the board E, which is suspended from the ceiling by a wire ; C, opening closed with a 

 cork, through which the cylinder can be tilled with lukewarm water ; D, opening through 

 which a thermometer is passed, showing temperature of water ; F, G, tube through which the 

 cylinder containing the forearm communicates with the small cylinder M, which floats above 

 the level of the fluid a, b, contained in large vessel P ; N, lead weight carrying the pen to write 

 on moving cylinder of kymograph, which counterpoises M, with which it is connected by two 

 silk threads passing over the double pulley L ; H, J, burette that can be raised or lowered in 

 adding or changing the water in the float. The instrument works as follows : when the vessels 

 of the forearm contract, an amount of water corresponding with the diminution in volume is 

 aspirated from the float M to the cylinder A, B ; this raises the float and depresses the counter- 

 poise N, which records the diminution of volume on the revolving cylinder. When, on the con- 

 trary, the vessels of the forearm dilate, a quantity of water in the cylinder A, B is driven out 

 into M, so that it sinks, and X is raised, recording the increase of volume. To avoid positive 

 or negative pressure above the forearm immersed in the cylinder A, B, care must be taken 

 that its upper level is at the same level a, b as the water contained in the receiver P where M 

 is floating. 



(1876) confirmed Mosso's facts with a weighing plethysmograph, 

 which gives more relative values than Mosso's apparatus. From 

 all these observations it appears probable that the autochthonous 

 rhythm of the vessels is common to many other vascular tracts 

 that have not yet been fully examined (Figs. 157 and 158). 



It must not, however, be assumed that rhythmical activity is 

 continuous and constant in all vascular regions : in most cases, 

 indeed, the microscopic study of transparent vascular tracts shows 



