MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 721 



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 experimental condi- 

 tions at least, weak stimuli cause a smaller contraction than strong, 

 not only because each stimulated fibre contracts less, but because a 

 smaller number of fibres are excited (p. 155). The objects used for the 

 study of muscular contraction contain many fibres, and it is not in 



Fig. 245. Curve of a Single Muscular Contraction or Twitch taken on Smoked Glass 

 with Spring Myograph and photographed. Vertical Jine A marks the point at 

 which the muscle was stimulated; time tracing shows ^ r of a second (reduced). 



general possible to distribute the stimulus equally to all. This is true 

 for smooth muscle as well as for striped. Finally, increase of the load 

 per unit of cross-section of the muscle diminishes above a certain limit 

 the ' height of the lift.' 



Influences which affect the Time-Relations of the Muscular Contrac- 

 tion. Many circumstances affect the form of the muscle curve and its 

 time-relations. 



(a) Influence of the Load Isotonic and Isometric Contraction. The 

 first effect of contraction is to suddenly stretch the muscle, and the 

 more the muscle is loaded the greater will this 

 elongation be. So that at the beginning of the 

 actual shortening part of the energy of contraction 

 is already expended without visible effect, and has 

 to be recovered from the elastic reaction during 

 the ascent of the lever. 



The contraction of a muscle loaded by a weight 

 which is not increased or diminished during the 

 contraction is said to be isotonic, for here the 

 tension of the muscle is the same throughout, and 

 its length alters. When the muscle is attached very 

 near the fulcrum of the lever, so that it acts upon 

 a short arm, while the long arm carrying the 

 writing-point is prevented from moving much by 

 a spring, the muscle can only shorten itself very 

 slightly; but the changes of tension in it will be 

 related to those in the spring, and therefore to the 

 curve traced by the writing-point. Such a curve 

 is called isometric, since the length of the muscle 

 remains almost unaltered. In the body muscles 

 usually contract under conditions more nearly 

 allied to those of the isometric than to those of 

 the isotonic contraction. 



The work done by a muscle in raising a weight is equal to the product 

 of the weight by the height to which it is raised. Beginning with no 

 load at all, it is found that the weight can be increased up to a certain 

 limit without diminishing the height of the contraction; perhaps the 

 height may even increase. Up to this limit, then, the work evidently 

 increases with the load. If the weight is made still greater, the con- 



46 



Fig. 246. Contrac- 

 tions of Smooth Mus 

 cle: Cat's Bladder 

 (C. C. Stewart). 

 Stimulated with pro- 

 gressively stronger 

 induction shocks. 

 The lowest line is the 

 time trace (lo-second 

 intervals). Immedi- 

 ately below the mus. 

 cular contractions are 

 marked the points at 

 which the stimuli 

 were thrown in. 



