22 PROCEEDINGS OF THE AMERICAN ACADEMY 



II. 



CONTRIBUTIONS FROM THE PHYSIOLOGICAL LABORATORY OF 

 THE HARVARD MEDICAL SCHOOL. 



A NEW FORM OF PLETHYSMOGRAPH. 

 By H. P. Bowditch, M.D. 



Presented May 14, 1879. 



A problem which frequently presents itself to physiologists is that 

 of measuring the changes in the size of organs, either hollow or solid, 

 which are produced by variations in the conditions to which they are 

 subjected. The simplest way of doing this is to fill the organ with 

 fluid, if it is hollow, or to place it in a closed vessel containing fluid, 

 if it is solid, and to allow the fluid thus contained within or surround- 

 ing the organ to communicate with a small glass tube, in which its rise 

 and fall furnishes a measure of the changing size of the organ under 

 observation. It is evident, however, that this rise or fall of the 

 fluid changes the pressure to which the organ is subjected, and that this 

 change of pressure, by affecting the size of the organ, introduces an 

 error into the observation. 



The Plethysmograph is au instrument devised to meet this difficulty. 

 Its essential part is a contrivance by which the fluid is allowed to flow 

 freely to and frtmi the organ to be measured without changing its 

 absolute level in the receptacle into which it flows, while at the same 

 time a record is made of the volume of the fluid thus displaced. 



The problem was successfully solved by Mosso* who supported the 

 receptacle for the fluid coining from the organ to be measured by 

 letting it float in a liquid the specific gravity of which was so adjusted 

 that any rise of fluid in the receptacle was counterbalanced by the 

 sinking of the receptacle in the liquid in which it floated. Von 

 Baschf accomplished the same object by suspending the receptacle for 

 the fluid from one arm of a balance counterpoised in such a way that 

 the weight of the fluid entering the receptacle caused the latter to 

 sink by an amount precisely equal to the rise of the fluid within it. 



* Arbeitcn aus der pliys. Anstalt zu Leipzig, 1874, p. 156. 

 t Wiener medicinische Jahrbiicher, 1870, IV. 



