ELEMENTARY EXPERIMENTAL PHYSIOLOGY 19 



The drum is covered with white glazed paper, the surface of which 

 is then blackened by a thin layer of soot, obtained by revolving the 

 drum through either the luminous part of a broad gas flame or the 

 smoke of burning turpentine or camphor. The writing point of 

 the lever, as the drum revolves, rubs off the layer of soot and leaves 

 a white magnified image of the movement of the muscle or heart or 

 whatever change is being recorded. 



The white paper is of the same width and longer than the surface of 

 the drum, and the under-surface of the overlap is gummed. The paper 

 must be laid evenly and without wrinkles round the drum, the gum is 

 then moistened and the paper fastened. The layer of soot obtained 

 from the gas flame should be dark brown in colour, and care must be 

 taken to revolve the drum sufficiently rapidly through the flame to 

 prevent scorching or burning of the paper. The film of soot from 

 camphor is less firmly attached to the paper, and must not be made 

 too thick, otherwise the writing point does not, without undue friction, 

 rub off enough of it to leave a distinct tracing. In recording it must be 

 so arranged that the tracing does not come at the overlap, for the joint 

 in the paper is liable to make the point of the lever jump. Further, 

 it is very important that the drum should be made to revolve away 

 from and not towards the writing point, in other words, the tracing as 

 it is taken should pass from the writing point, not towards but away 

 from the lever. When the tracing is finished, the paper is cut through 

 at the overlap and the details of the experiment written on it. The 

 tracing is preserved by drawing it once through a varnishing solution l 

 and pinning it up to dry. 



This graphic method, as we shall see, introduces several errors, but 

 such accuracy as it has must depend upon the drum remaining a true 

 cylinder ; it is therefore very important that a drum should never be 

 dropped or in any way dented. 



Minimal and Maximal Stimuli. If the strength of the stimulus 

 applied to a muscle be varied within certain limits, it is found 

 that the muscular response also varies, so that the greater the excita- 

 tion the greater is the shortening of the muscle. 



In order to demonstrate this, connect up a Daniell cell to an induc- 

 tion coil so as to give single induction shocks, placing a mercury key in 

 the primary circuit and a Du Bois key in the secondary circuit ; cover 

 and smoke a drum. Dissect out a gastrocnemius preparation and 

 attach it to the myograph lever, arrange the electrodes to stimulate the 

 muscle directly; one needle-electrode is used which passes through 



1 A rapidly drying varnish is made by dissolving 250 c.c. of the best white hard 

 varnish in a litre of methylated spirits and then adding 10 c.c. of castor oil. 



