vni BLOOD-STKEAM : MOVEMENT IN VESSELS 279 



Frangois-Franck (1876) made some useful modifications in the 

 details of Buisson's apparatus, giving the apparatus the form of 

 Fig. 115. The flask placed in front of the rubber tube, which 

 joins the plethysmograph with the writing tambour, cuts out the 

 oscillations of the fluid along the vertical tube. The method and 

 instrument afterwards used by Mosso (1880) for recording the 



FIG. 116. A', Plethysmogram of forearm in fasting state,; A, the same, in same individual, after 



a meal. (Mosso.) 



pulsatile changes in the volume of the forearm, to which he gave 

 the name of hydrosphygrnograph, is very similar. 



Among the various results of more or less importance obtained 

 by the plethysmographic method, we can here only refer to the 

 form of the curves depending on the pulsatile oscillations in 

 volume of the part explored, i.e. to vascular plethysmograms and 

 their interpretation. 



FIG. 117. V, Plethysmogram of hand taken with apparatus of Fig. 115, the oscillations of thf- 

 column of fluid being suppressed by the bulb interposed between the exploring apparatus and 

 the recording tambour. (Fr.-Franck.) 



Vascular plethysmograms are very similar to sphygmograms, 

 and exhibit the same principal features, including dicrotism, as 

 shown in Figs. 116 and 117. Still they cannot be identical since, 

 as we have seen, sphygmograms are obtained with an apparatus 

 which by means of a tense spring exerts pressure on the artery 

 investigated, depressing its lumen to a greater or less extent ; while 

 plethysmograms, on the contrary, depend solely on the alterations 

 in volume of the forearm or hand, all external pressure being as 

 far as possible excluded. Since the flow of blood in the veins is 

 continuous and uniform, it is clear that the changes of volume in 



