38 CONTRACTILITY 



1. The Change in Form 



When a muscle such as the frog's gastrocnemius is con- 

 nected with recording apparatus and is stimulated by means 

 of a single induction shock apphed to its nerve, the 

 mechanical result consists of three parts^ — the latent period, 

 the period of contraction, and the period of relaxation. 

 The latent period is due partly to the inertia of the apparatus, 

 partly to the time occupied in the transmission of the 

 impulse along the nerve and across the nerve ending. But 

 when these have been discounted there remains an interval 

 of time, estimated at -0025 sec, during which changes 

 preparatory to contraction are taking place in the muscle 

 itself. This is known as the true latent period. 



The period of contraction occupies about y^ sec, and 

 the period of relaxation shghtly longer — about xfo sec. 

 It is important to realise that the curve of contraction 

 obtained in this way is but a caricature of the actual change 

 in form, so great is the distortion caused by the inertia of 

 the recording apparatus. 



Factors Modifying the Change in Form 



1. Temperature. — On raising the temperature all three 

 constituents of the curve — ^latent period, upstroke and down- 

 stroke — are shortened. As commonly recorded there is, 

 in addition, an increase in the height of the curve. This, 

 however, can be shown by means of an arrested lever to 

 be instrumental in origin. Alterations in temperature, 

 therefore, do not influence the height of contraction. 



2. Load. — Beginning with a very hght weight, increase 

 in the load is at first a stimulus to increased contraction. 

 Beyond a certain weight any further addition leads to a 

 diminution in the height to which it is hfted. There is 

 thus for every muscle a certain load which stimulates it to 

 the maximum work — work being the product of the 

 weight and the height to which the weight is raised. 



3. Strength of Stimulus. — In the ordinary gastrocnemius 



