ii CHANGE OF FORM IN MUSCLE DURING ACTIVITY 69 



energetic contractions, while the longitudinal fibrils in this 

 case also produce permanent closure. In Anodonta, where a 

 similar differentiation of the adductor muscle is shown in two 

 sections visible to the naked eye, neither Engelmann nor Bieder- 

 inann was able to detect any perceptible difference in rate 

 of contraction between the differently coloured parts of the 

 muscle (19). 



II. DEPENDENCE OF MUSCULAR CONTRACTION UPON STRENGTH 



OF EXCITATION 



A systematic inquiry into this point is best effected with the 

 aid of electrical excitation, in the form of single induction shocks, 

 the strength of which can be graduated in the finest proportions. 

 We are thus in a position to determine the law of dependence of 

 contraction upon strength of excitation. It is easily proved that 

 below a certain minimal limit of intensity (the threshold of 

 stimulation} the excitation produces no visible effect ; the dis- 

 charge of a contraction begins first with a given strength of 

 excitation, and its magnitude (height) increases for some time, 

 according to Fick, in proportion with increased strength of 

 stimulus. Beyond a certain point, however, the increase in con- 

 traction ceases, and the existing maximum is maintained for each 

 increment of stimulation. This maximal limit is usually but 

 little above that at which the first just perceptible contraction 

 was yielded. The entire process of this greatest contraction and 

 extension is known as a maximal contraction. It may be 

 described in Tick's words (20) by saying, "Each impact of 

 excitation discharges either a maximal contraction or no contrac- 

 tion at all ; it is only in a limited interval of the scale of 

 excitation (often hard to find on account of its narrow propor- 

 tions) that sub-maximal, so to say, imperfect, contractions are 

 given." We shall presently see that there are muscles (heart) 

 which yield only maximal contractions. 



The law of approximately proportional increase in height of 

 contraction within the given narrow interval, deduced by Eick 

 from experiments on indirect excitation of skeletal muscle, was 

 subsequently disputed by Tigerstedt (21), who found (with direct 

 excitation of curarised muscle also) " that with uniform increase of 

 strength in the electrical stimulus, the muscular contractions 



