MUSCLE 



643 



contraction of the muscle the moment it began. In this way he 

 obtained the value of y^ second for the latent period of frog's muscle. 

 The tendency of later observations has been to make the latent period 

 shorter. Burdon Sanderson found that the change of form begins in 

 muscle with direct stimulation in T?T 4 oo second after, and the electrical 

 change (p. 721) simultaneously with, the excitation. It is known 

 that the apparent latent period depends upon the resistance which the 

 muscle has to overcome in beginning its contraction. A heavily- 

 weighted muscle, for instance, cannot begin to shorten until as much 

 energy has been developed as is necessary to raise the weight ; and its 

 latent period will be distinctly longer than that of unweighted or very 

 slightly weighted muscles, such as those with which Sanderson worked. 



FIG. 228. SPRING MYOGRAPH. 



A, B, iron uprights, between which are stretched the guide-wires on which the 

 travelling plate a runs ; k, pieces of cork on the guides to gradually check the plate 

 at the end of its excursion, and prevent jarring ; b, spring, the release of which 

 shoots the plate along ; h, trigger -key, which is opened by the pin d on the frame 

 of the plate. 



The maximum shortening, or ' height of the lift,' depends upon 

 the length of the muscle, the direction of the fibres, the strength of 

 the stimulus, the excitability of the tissue, and the- load it-has to raise. 



In a long muscle, other things being equal, the absolute shortening 

 and therefore the maximum height of the curve, will be greater than 

 in a short muscle ; in a muscle with fibres parallel to its length 

 the sartorius, for instance it will be greater than in a';muscle like 

 the gastrocnemius, with the fibres directed at various angles to the 

 long axis. For stimuli less than maximal, the absolute contraction 

 increases with the strength of stimulation, and a given stimulus 

 will cause a greater contraction in a muscle with a given excitability 

 than in a muscle which is less excitable. Under ordinary experi- 

 mental conditions at least, weak stimuli cause a smaller contraction 

 than strong, not only because each stimulated fibre contracts less, but 



412 



