THE RECORDING OF MUSCULAR CONTRACTIONS 3! 



working order, and at what distance of the secondary from the primary 

 the break shock is effective in causing a full contraction when the 

 drum is made to revolve. Do not allow the muscle to become fatigued 

 by many excitations before recording its contraction. 



Now bring the lever point so as lightly to touch the blackened 

 paper, using the stop of the myograph stand to prevent the point 

 pressing too hard against the paper. When the stop is used in this 

 way the lever point can be removed at any time from the paper 

 and brought back again so as to press with exactly the same force as 

 before ; it is absolutely essential to make use of the stop in all recording 

 experiments in which comparisons of different curves upon the same 

 surface have to be made failure to do so will invalidate all results. 



Start the drum revolving, but keep the short-circuit key (&') closed 

 so that no stimulus reaches the nerve ; the lever point will describe a 

 horizontal line (abscissa). Whilst the drum is still revolving open the 

 short-circuit key, but close it again the instant the muscle has con- 

 tracted ; immediately afterwards remove the lever point from the 

 drum, before this has had time to perform another revolution. A 

 simple muscle curve will thus be described. 



To mark the point of stimulation, move the drum slowly round by 

 hand until the projecting pin just touches the needle where contact is 

 made (as in the diagram, Fig. 34) ; bring the lever point against the 

 smoked surface as far as the stop will allow, and raise the lever about 

 half an inch by the finger. The distance between this mark, which 

 indicates the moment when the stimulus was put into the nerve, and 

 the rise of the curve, which indicates the commencement of the con- 

 traction of the muscle, gives the period of latent stimulation. To 

 measure this period, as well as the duration of the contraction and 

 relaxation of the muscle, remove the lever point from the smoked 

 surface, set the drum revolving at the same rate as before, and allow 

 a tuning-fork of known rate, e.g., one hundred vibrations per second, 

 to record a time tracing, either directly or by an electro-magnet; putting 

 the writing point attached to the tuning-fork or electro-magnet against 

 the drum during a single revolution only. Cut through the paper 

 without scoring the surface of the drum. Lay it on the table, and 

 write upon it date and description. Then pass it through the varnish- 

 ing trough, 1 and hang it up to dry. When dry, cut out the part of the 

 tracing which is required. 



Effect of heating and cooling the muscle upon its contraction. 

 The same nerve-muscle preparation may be used, the apparatus being 

 arranged exactly as in the last experiment. Mark on a new abscissa 



1 A good varnishing fluid is furnished by dissolving 100 grammes rosin in a 

 litre of rectified spirit. 



