THE GRAPHIC METHOD 



11 



to be recorded; and (2) that while it is not necessary to have the drum move 

 at a uniform speed, if the time record is made simultaneously with the physio- 



FIG. 8. Curves of blood pressure in the left ventricle (V) and in the aorta (.4) of the dog, after 

 Hiirthle. S, the time-record in iJ 5 ths of a second. To be read from left to right. 



logical record, much more satisfactory results will be obtained if it does move 

 both uniformly and steadily. 



C. RECORDING BY AIR-TRANSMISSION 



The method of air-transmission for the registration of physiological events, 

 first introduced into physiology by Buisson (1861) and later brought to 

 perfection by Marey, has also found wide application. The principles of this 

 method may be understood from the following: 



When two thin-walled rubber bulbs are connected with each other by means 

 of a rubber tube having fairly rigid walls, and pressure is exerted on the one, 



FIG. 9. Recording tambour of Marey, actual size, a, metallic case ; 6, thin India-rubber mem- 

 brane; c, thin disk of aluminium supporting the lever d (a small portion of which only is 

 represented); e, screw for placing support of lever vertically over c; /, metallic tube com- 

 municating with cavity of tambour for attachment to an India-rubber tube. 



the other will of course be dilated. Now if a writing lever be connected with 

 one of the bulbs it can be made to record any such variations in pressure taking 

 place in the other. The apparatus necessary for registration by air-transmission 



