82 EXPERIMENTAL PHYSIOLOGY 



under the myograph lever. Before either writing-point is allowed to touch 

 the smoked surface, the drum should be set in motion to see that the front of it 

 rotates from right to left. The secondary coil is now brought into such a 

 position thai maximal contractions are obtained when the key k is opened. 

 The drum is now rotated until the rod a is a little in front of the arm of the 

 break key k'. Adjust the writing-point of the lever L- to touch the drum 

 surface whilst the tuning-fork f is not in contact. The key k 1 is closed and 

 then k 2 is opened. The drum is next very slowly rotated by hand until 

 the arm a breaks the key k 1 and the shock thus produced in the secondary 

 coil causes a twitch of the muscle, which is recorded as a vertical line on the 

 smoked surface. The key K 3 is again closed. We now know that, no matter 

 at what rate the drum be rotating, at the instant at which the arm A breaks 

 the primary circuit the writing-point must be exactly opposite the vertical 

 line just recorded. In other words, this vertical line represents the instant at 

 which the stimulus will be sent into the nerve, i.e. it is the point of stimula- 

 tion. Now rotate the drum through about a half-revolution, set the tuning- 

 fork vibrating, and bring its writing-point in contact with the surface. Close 

 the key K 1 and open K-. Set the drum revolving by switching the friction 

 wheel below the coned pulley p into contact with the wheel h, the arm a 

 breaks the contact of K 1 , a stimulus is sent to the nerve, and the muscle con- 

 tracts. As soon as the writing lever has returned to rest, the drum is stopped. 

 This takes as a rule about half a revolution. The key k- is closed and the 

 writing-point of the tuning-fork removed from the surface. The writing- 

 point of the lever is once more brought accurately on to the abscissa line, and 

 the drum rotated so that a horizontal line is recorded on the drum. This is 

 the zero- abscissa line. 



The drum is again rotated till the writing-point is brought to the line 

 marking the point of stimulation, when the lever is depressed until it cuts 

 the time tracing. In a similar way vertical arcs are drawn opposite the 

 following three points : (1) the point at which the tracing leaves the zero- 

 abscissa line ; (2) the highest point of the curve ; and (3) the point when it 

 regains the abscissa line. One or two of such curves should be taken, and 

 the curves given by different muscles should also be recorded. A tracing by 

 a hyoglossus preparation is especially useful. This may be stimulated 

 directly, for which purpose one electrode wire is wound round the pin fixing 

 the hyoid cartilage to the cork plate, and the other may be attached to the 

 bent pin passing through the tip of the muscle, or it may be passed directly 

 through the tongue from side to side. This wire should be very fine. The 

 paper may now be removed from the drum, a note of the nature of the experi- 

 ment may be written upon it by a finely pointed pen, and it may then be 

 fixed by drawing it through a dish of varnish, 1 afterwards allowing it to dry. 



In this way the curve of fig. 38 was obtained. The point of stimu- 

 lation is marked at a, and b, c, d are the other three points mentioned 

 above. The rate of vibration of the tuning-fork is 200 per second. 

 The muscle from which it was obtained was a hyoglossus, and the 

 writing-point magnified the movement of the muscle three times. The 

 curved lines a a', b b', c c', and clcV, written while the smoked surface 

 was stationary, are taken for the purpose of making the time measure- 

 ments more accurately. The curve is seen to fall naturally into three 

 parts : — 



1 A very convenient varnish consists of 250 c.c. of best white-hard varnish to 

 which 1 litre of methylated spirit and 10 c.c. of castor oil are added. This dries 

 quickly and gives a dull surface to the tracing. 



