ISOTONIC MUSCULAR CONTRACTION. 



to the rod of a pendulum (Pick's pendulum-myograph) , or a surface set in motion 

 by gravity (Jendrassik's gravity-myograph) or by means of a spring (Du Bois- 

 Reymond) or a rotating convex surface (Rosenthal's rotating myograph) , may be 

 employed. Under the myogram a time-curve is traced by means of a vibrating 

 tuning-fork. The apparatus is, in addition, provided with an arrangement for 

 indicating in the tracing the moment of stimulation. 



The curve may be traced advantageously on the vibrating plate of a tuning- 

 fork (Fig. 194, I). The time-units are thus registered in all parts of the curve, 

 each complete vibration being equal to 0.01613 second. The moment of stimula- 

 tion coincides with the beginning of the vibration of the fork, which is at first 

 moved to one side for a time, without vibrating. This is accomplished by re- 

 leasing a clamp, which at the same time opens a galvanic circuit, and sends an 

 induction (opening) shock of the secondary coil through the muscle. The tuning- 

 fork can also be set in vibration by a blow on one of its prongs. If under such 

 circumstances the nerve is so placed upon the fork as to be struck by the blow, 

 the latter acts at the same time as a mechanical nerve-stimulus. 



The balance, together with the imposed weights, is jerked upward at the 

 commencement of the contraction. As a result the curve is distorted, because the 

 muscle is no longer weighted after the moment of occurrence of the jerk. For 

 this reason the muscle has been 

 made to draw up an elastic spring. 

 In this way, however, a stronger 

 pull must be made on the muscle 

 as the spring is raised higher and 

 higher. To avoid this Grutzner 

 constructed a spring that exerts 

 a steadily diminishing tension on 

 the apparatus as the muscle pro- 

 gressively contracts. 



If it be desired to record only 

 the extent (height) of the contrac- 

 tion, the tracing is made on a 

 stationary surface, which is dis- 

 placed slightly after each move- 

 ment (Pfliiger's myograph) . 



Muscular contractions may 

 also be recorded in the case of 

 man. It is best to transfer the 

 increase in thickness attending 

 contraction either to a lever or 

 to a drum covered with rubber, 

 for example that of Brondgeest's 

 pansphygmograph (p. 101). 



FIG. 192. Diagrammatic Representation of v. Helmholtz's 

 Myograph: M, the muscle, fastened at K; F, the writing- 

 style, suspended from the arm of the lever that is to be 

 raised; P, a counter-weight for maintaining equilibrium; 

 W, scale-pan for weighting the muscle as desired; S S, posts 

 supporting the balancing lever. 



If a single stimulus of 

 momentary duration be ap- 

 plied to a freely movable mus- 

 cle, the latter executes a simple contraction, that is it shortens rapidly 

 and also returns quickly to the relaxed condition. Under such circum- 

 stances the internal tension of the muscle remains the same during the 

 course of the entire contraction, and for this reason the resulting curve 

 is designated an isotonic myogram. 



The following details can be noted in an isotonic contraction-curve 

 described by a muscle that has to lift only the light writing lever, and is 

 not overweighted by any other attached weights: i. The stage of 

 latent stimulation (Fig. 193, a b), which arises from the fact that the con- 

 traction of the muscle does not begin at the moment of stimulation, but 

 always somewhat later. If the momentary stimulus, for example an 

 induction-shock, be applied directly to the entire muscle, the latent period 

 is about o.oi second. 



In man the stage of latent stimulation varies from 0.004 to o.oi second. 

 36 



