Bayliös and Starling, lutraventiicular and Aortic TreHBure Curve.«. 427 



found below. This is connected by narrow lead tubing to a tbreeway 

 stopcock, from which on the one hand a tube pi-oceeds to a pres.sure- 

 bottlc, containing 25 7o niagnesium sulphate solution, and on the other 

 to a short piece of lead tube, soldered to a Ijrass nozzle, which fits 

 into a brass stopcock at the end of the heart catheter. By this 

 means, the cai)illary tube can be put into connection, either with the 

 pressure bottle or with the heart cavity. To prepare the instrument 

 for use, the point of the capillary being open, fluid is run in from the 

 pressure-bottle, utitil a small air-space is left at the top, and the 

 point is then sealed by a flame, so that the cavity is closed. The 

 meniscus at the junction of the air and fluid is then focussed by a 

 Zeiss A. microscope objective on to the surface of a photograpliic film 

 attached to a rotating cylinder, a lime-light lantern being used as 

 source of light. Between the microscope lens and the cylinder, there 

 are interposed — 1^*- a shutter for convenience of exposure; and, 

 2"'^- a narrow vertical slit. The image of the slit being focussed on 

 the film by a cylindrical lens, gives a very fine sharp line of light, 

 broken by a dark band where the image of the meniscus falls. One 

 boundary line of the band can be focussed quite sharply by means of 

 the fine adjustment of the microscope. A tuning-fork of 50 vibrations 

 per sec. with a slip of paper projecting from one limb across the slit, 

 serves to register the time. The films found most convenient were 

 Edwards' Isochromatic Instantaneous and w^ere developed by Eikonogen. 



The dimensions of the air-space were the following: 



Length 3-8 mm 



Diameter 03 mm 



therefore volume =0'268 cub. mm. 



It is well at the outset to state that we do not lay any stress 

 on absolute measui'ements of pressure made with this instrument, for 

 two reasons: 



1^*- It is impossible to obtain accurate measmements of tlie 

 dimensions of the air-space, and the capillary is probably very slightly 

 conical in form, and: 



2udiy. \yq i^g^yg Y^Q ujeans of knowing how far the compression 

 of the air takes place adiabatically. If the pressure-bottle is quickly 



