LANDOIS' GAS-SPHYGMOSCOPE. 135 



It is very convenient to write the curve upon a plate of glass fixed 

 to a tuning-fork kept in vibration. Every part of the curve shows 

 little elevations (whose rate of vibration is known beforehand). All 

 that is required is to count the number of vibrations in order to 

 ascertain the duration of any part of the curve. 



Fig. 54 was taken in this way from (A) the carotid, (B) the radial, 

 and (C) the posterior tibial arteries of a healthy student. The results 

 are : 



Carotid. Radial. 



1-2, ...7. ..7. -.8 



13, ... 17 ... 16 ... 19 



14, . . . 23'5 . . . 22-5 . . .28 



15, 56 39 49 



This method has also been used for the registration of other physiological 

 processes e.g., contraction of muscle. 



Landois' Gas-SphygniOSCOpe. A superficially placed artery communicates its 

 movements to the overlying skin, and also to any freely movable body in contact 

 with the skin. In this instrument (Fig. 55) a thin layer of air over the pulsating 

 artery, a, is enclosed by means of a thin piece of metal, which is so adjusted that 

 its concave side forms a tunnel of air over the artery. The narrow space between 

 the metallic wall, b, and the skin, a, is filled with ordinary gas, one end of the 

 metal shield being connected by means of a tube, g, with the gas-supply, while to 

 the other end there is attached by means of a short piece of caoutchouc, x, q, a 

 bent glass-tube, t, with a very small aperture which acts as a gas-burner. The 

 gas is allowed to flow through the apparatus at a low pressure, and is so regulated 

 that the flame, v, is only a few millimetres in height. The flame rises isochron- 

 ously with every pulse-beat, and the dicrotic beat in the normal pulse is quite 

 observable. 



Fig. 55. fo 



Landois' gas-sphygmoscope a, skin over artery ; 6, metal plate ; p, g, gas ; 

 x, q, caoutchouc tube attaching glass gas-burner, t to b. 



Czermak photographed a beam of light set in motion by the movements of the 

 pulse. 



Haemautography. Expose a large artery of an animal, and divide it so that 

 the stream of blood issuing from it strikes against a piece of paper drawn in front 



