3g2 MOTION IN ANIMALS AND PLANTS. 



ical energy consequent on excitation of muscle, though by no means 

 an insohible, is still an unsolved, physical problem. We know how 

 much chemical energy is liberated, we know how much work is done, 

 and how much heat is wasted, but we can not explain how it happens, 

 it being difficult to suppose that the temperature required for such 

 transformation can exist in living muscle. 



The absence of a sufficient physical theory of the origin of muscular 

 force does not, however, deprive the mechanical manifestations of the 

 process of their value as simple, measurable, and controllable indica- 

 tions of functional activity. Whether we take the case in which a 

 muscle strives against a resistance which it can not overcome, or 

 shortens without resistance, or does both siuudtaneously, the change of 

 tension in the one case, of form in the other, or of both, are measurable 

 processes of which the time relations can be ascertained with great 

 accuracy. We are on safe ground, therefore, in using either change of 

 tension or change of form as a means of estimating the vital activity 

 of muscle, and in fact both are required. 



For every investigation in which muscular function is in question 

 three points come prominently forward: (1) The moment at which 

 mechanical energy comes into pla}" (2) the maximum energy dis- 

 played; and (3) the time at which that display culminates. As regards 

 the first point, the time occupied before the mechanical response begins 

 was for many years believed to be one one-hundredth second. This 

 estimate was accepted as though on the authority of Helmholtz, but 

 was really based on a misunderstanding of his experimental data. 

 But we now know that the change of form resulting from the action of 

 a single instantaneous stimulus begins in the muscle element not later 

 than four one-thousandths second after the moment of excitation, and 

 I may be permitted to show you how this result can be arrived at with 

 absolute certainty by the photographic method. (Photograph shown.) 



As regards the second and third points, we find it better to measure 

 contractile activity by change of tension rather than by change of form, 

 firstly, because the method of measuring tension is less liable to error, 

 and, secondly, because the process of development of tension is more 

 rapid than the development of change of form. For, although with 

 the exquisite methods we now possess of getting rid of inertia in our 

 recording apparatus, it is possible to measure the shortening with great 

 exactitude, yet it is easier to guard against errors of observation when 

 the other (isometric) method is used. 



Having seen how functional activity can be measured, we may 

 advert to the question that principally concerns us this afternoon— 

 the question, namely, whether the electrical phenomena may also be 

 regarded as expressions of functional activity. Assuming for the 

 moment the question to be answered in the affirmative, with what 

 part of our tension curve should we expect the electrical concomitant 



