5 6 2 A MANUAL OF PHYSIOLOGY 



known that the apparent latent period depends upon the re- 

 sistance which the muscle has to overcome in beginning its 

 contraction. A heavily-weighted muscle, for instance, can- 

 not 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 Sander- 

 son worked. 



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 



FIG. 169. CURVE OF A SINGLE MUSCULAR CONTRACTION OR TWITCH TAKEN 

 ON SMOKED GLASS WITH SPRING MYOGRAPH AND PHOTOGRAPHED. 



Vertical line A marks the point at which the muscle vras stimulated ; time-tracing 

 shows TOO of a second (reduced). 



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. 

 Finally, increase of the load per unit of cross-section of the 

 muscle diminishes above a certain limit the ' height of the 

 lift/ although below that limit it may increase it. 



Influences which affect the Time-relations of the Muscular 

 Contraction. Many circumstances affect the form of the 

 muscle-curve and its. time-relations. 



(a) Influence of the Load. The first effect of contraction 

 is to suddenly stretch the muscle, and the more the muscle 



