568 



ISOMETRIC MUSCULAR CONTRACTION. 



Isometric Muscular Activity. While the experiments discussed in 

 the foregoing are concerned with the determination of the changes in the 

 length of a muscle on stimulation and the movement of a weight sup- 

 ported by it, Pick has investigated the changes that take place in the 

 tension of a muscle under the influence of stimuli, when its length is kept 

 constant. Pick designates this process the isometric muscular act. 



The following apparatus will serve to demonstrate the isometric muscular 

 act (Fig. 197) : The angular frame R is provided at its base with a long writing- 

 lever S (abbreviated in the illustration), which is movable at the hinge- joint p. 

 The muscle M, suspended from above, is connected with the writing-lever 

 near its point of attachment. A strong spiral spring F is connected with the 

 other arm of the writing-lever, and during the activity of the muscle, permits 

 only the slightest degree of shortening to take place. This, however, is sufficiently 

 magnified by the great length of the lever. A momentary electric stimulus is 



applied to the muscle 

 by means of two elec- 

 trodes (r r), and the 

 writing-lever records 

 the isometric curve. 



The isometric 



. , x - - |>s contraction-curve is, 



X on the whole, similar 



to the isotonic curve, 

 as a comparison of the 

 curves in Fig. 197 will 

 show. Nevertheless, 

 the following differ- 

 ences exist: (i) The 

 contracting muscle 

 attains its maximum 

 tension in the isome- 

 tric muscular act 

 more rapidly than it 

 attains its maximum 

 shortening in the iso- 

 tonic act. (2) The 

 FIG. 197. Isometric Muscular Act. contracting muscle in 



the isometric act 

 maintains the degree 



of highest tension somewhat longer, while in the isotonic act it recedes more 

 rapidly from the highest degree of shortening. 



In the isometric muscular act in man, voluntary excitation gives rise to a 

 higher degree of tension than electric stimulation. In the frog, the tension of 

 the muscle in a state of tetanus is about twice as great as it is in a maximal con- 

 traction; in human muscle it may be even ten times as great. During extension 

 of the tetanized muscle, as during its contraction, equal degrees of tension cor- 

 respond to smaller lengths. 



In the case of unstriated muscles, the entire curve is much shorter in the 

 isometric act than during the isotonic act, and its form is almost symmetrical. 



RAPIDITY OF PROPAGATION OF MUSCULAR CONTRACTION. 



If a muscle of considerable length is stimulated at one extremity a 

 contraction occurs at that point, and rapidly traverses in a wave-like 

 manner the entire length of the muscle to its other extremity. The 

 excitation is therefore communicated to each successive part of the 

 muscle by virtue of a special conductive capacity on the part of the 

 muscle to enter into a state of contraction. In the frog the wave of 

 contraction has an average velocity of from 3 or 4 to 6 meters in a second, 

 in the rabbit of from 4 to 5 meters, in the lobster of only i meter, in 



