THE CONTRACTION OF MUSCLE 545 



in a fresh state, the triceps femoris of the rabbit gives a quick response ; 

 when fatigued, it yields a long-drawn-out curve. 



The Period of Relaxation is from the maximum shortening of the 

 muscle to the position of rest. The relaxation has been thought to 

 be due to an active process, but there is little evidence of this beyond 

 the fact that there aro special changes going on during it (see p. 554). 



The Conditions which affect Muscular Contraction. Beside the 

 constitution of the muscle and the effect of drugs, such as veratrine, 

 the muscle response is affected by other conditions, such as the strength 

 of stimulus, the temperature, the amount of load, and previous activity. 



The Strength of Stimulus. Striated muscle gives a graduated 

 response according to the strength of stimulus. With a weak (minimal) 

 stimulus, it just contracts. The contraction then increases in amount 

 with the strength of stimulus until a maximal contraction is obtained, 

 after which no increased contraction is obtained, however much the 

 current be strengthened. The minimal contraction is due to the 



Fi3. 282. CONTINUATION OF THE EXPERIMENT IN Fo. 231. SINGLE CONTRACTIONS 

 OF THE GASTROCNEMIUS WITH DIFFERENT LOADS. (A. P. Beddard.) 



The figures on the curves represent the weights in grammes hung at the axis of the 

 lever; actual load on muscle was in each case one-fifth. Magnification, o. 

 Temperature, 12 C. 



weak stimulus only spreading to a few fibres. Probably any stimulus 

 which is effectual makes the fibres effected contract maximally. 



Temperature. Cold (0 to 3 C.) lengthens the whole curve, especi- 

 ally the latent period and the period of contraction. The rate pf con- 

 duction is also lessened by cold (Fig. 280). Frequently the relaxation 

 is incomplete, showing a " contraction remainder." At first the height 

 of the contraction is increased, then diminished. Gentle warmth 

 <25 to 35 C.) increases the rate of all stages of the curve, and greatly 

 increases its height, partly due to an inertia effect of the lever (Fig. 27H). 

 Heat (42 C. in the frog) coagulates the muscle proteins, and brings 

 .about a condition of " heat rigor." 



Load. An increase in load is found generally to decrease the 

 amount of contraction, and lengthen the latent period (Figs. 281, 282). 

 When, however, a muscle is fresh and in good condition, the first 

 fc -\\ increments of load may give an increased height to the contraction. 

 When the work done by the muscle is calculated 



Work done -= load lifted x height 



