STIMULATION OF MUSCLES AND OF NERVES 415 



A loaded lever lifted in this way suffers a certain acceleration in its move- 

 ment upward, which is often so great that from a certain moment onward the 

 lever moves of its own inertia, and not at the instance of the muscle. It is 

 evident that a muscle curve recorded under such circumstances can be trusted 

 only for information as to the very beginning of the contraction. In order to 

 prevent this " throw " due to inertia a very light lever is employed, and the load 

 is applied as near to the axis as possible (Fig. 160), while the muscle is attached 

 at a greater distance therefrom. Since it is assumed that the tension of the 

 muscle remains the same throughout, this sort of a contraction is called isotonic. 

 An actual condition of isotony, however, is scarcely ever to be had (cf. page 435). 



To be able to analyze the temporal course of the muscular contraction ex- 

 actly, one must record it on a writing surface moving with sufficient speed (300- 

 600 mm. per second). 



2. The simple contraction. The muscular contractions caused by the vari- 

 ous stimuli are either simple or summated. By simple contraction, or merely 

 contraction, we mean that act of the muscle which is discharged by a single 



FIG. 154. Simple contraction curve of the frog's gastrocnemius. The vertical line at the left 

 marks the movement of stimulation. The interval between this line and the point at which 

 the contraction curve leaves the base line is the latent period. 



stimulus. By summated contractions we understand the contractions dis- 

 charged by a series of stimuli following each other in rapid succession (cf. 

 page 51). 



When a muscle receives a stimulus, a measurable time always elapses be- 

 tween the instant of stimulation and the appearance of a visible effect, and 

 this time is designated as the latent period (Helmholtz, 1850). 



The general procedure in making exact determinations of this as well as of 

 other physiological periods may be explained by the following method: On the 

 lower edge of the drum of a kymograph a metallic peg is securely fastened, so 

 that when the drum is revolving this peg can break an electric contact. The 

 contact forms a part of the primary current to an induction coil. If now the 

 secondary current be conveyed to the muscle, and the drum be set going so that 

 the muscle lever makes a tracing, it is clear that the instant the peg opens (or 

 closes) the primary current the muscle will receive a shock. But it is equally 

 clear that, owing to the latent period, the instant the resulting contraction begins 

 will not be the instant of stimulation. To find on the tracing the instant when 

 the muscle receives the shock, let the drum be moved very slowly until the peg 

 once more breaks the primary current. Since the drum is now as good as stand- 

 ing still the resulting contraction will trace a vertical line (Fig. 154) which 

 marks the instant of stimulation. The interval between this vertical line and 

 the rise of the muscle curve above the base line is the latent period. The period 

 can of course be measured in fractions of a second if the vibrations of a tuning 



