416 THE FUNCTIONS OF CROSS-STRIATED MUSCLES 



fork be recorded while the drum is going at the same rate of speed. Sometimes 

 it is difficult to say just when the muscle curve rises from the base line. The 

 exact moment can be determined if the experiment be so devised that the mus- 

 cular contraction, the instant it begins, opens the current to an electric signal. 



The length of the latent period, which has generally been determined on 

 the frog's muscle, depends upon various circumstances. With a maximal 

 break induction shock and at ordinary room temperature (17-19 C.), the 

 mean length is about 0.004 second; at a higher temperature it is shorter, at 

 a lower temperature longer. The latent period increases also as the height 

 of the contraction decreases. On the other hand, under circumstances other- 

 wise the same, it is influenced very little by the load or by the tension of the 

 muscle (i. e., up to a certain limit). 



If a muscle prepared for stimulation be placed in a horizontal position 

 and a lever be attached to each of its two ends in such a way that any increase 

 in the thickness of the muscle at either end will be recorded, and if the muscle 

 be now stimulated at one end, it is found that the response spreads from the 

 point of stimulation throughout the muscle at a measurable rate of speed 

 (Abey). This rate of propagation, which according to Engelmann is inde- 

 pendent of the strength of the stimulus, amounts in the frog muscle to 3-4 m. 

 per second (Bernstein, Hermann), or 5-6 m. (Engelmann), and in human 



muscles to 10-13 m. per second (Hermann). 







If in an experiment like the one cited above for the determination of the 

 latent period, the two electrodes be placed on opposite ends of the muscle, the 

 excitation will start from the negative electrode (cf. page 59), and will spread 

 from there throughout the muscle. But before the lever can be raised, the exci- 

 tation must have reached the entire muscle; whence it is evident that the 

 mechanical latent period of the part first excited must be shorter than that indi- 

 cated for the whole muscle. 



After the latent period the muscle curve rises to its maximum height and 

 then falls. Accordingly in every muscular contraction we have to distinguish : 

 (1) latent period, (2) period of shortening, (3) the summit, and (4) the 

 period of relaxation. In the frog's gastrocnemius the period of shortening 

 lasts 0.05-0.07 second, the period of relaxation somewhat longer. 



The course of the simple contraction may be very different in different 

 muscles, and in point of time we meet with all possible gradations from the 

 extremely short twitch of certain insects' muscles, lasting only 0.0033 second, 

 to the contraction of smooth muscles continuing for several seconds. 



Ranvier first directed attention to the fact that the skeletal muscles of the 

 same animal which differ in color, differ also in their physiological properties. 

 Thus in such animals as the rabbit, we can easily distinguish re d and white 

 muscles. With the red the latent period is longer, the height of the contrac- 

 tion is less the descending limb of the curve especially being very much 

 drawn out ; but the force and endurance are greater than in the white muscles. 

 The former are therefore more capable of severe work. Griitzner showed 

 later that individual muscles generally are composed of red and white sec- 

 tions, and that the mixture of the two kinds of fibers is often very intimate. 



