MOTION IN ANIMALS AND PLANTS. 333 



to coincide!' Assuredly with the first and second hundredths of a 

 second after excitation, i. e. , with the period of greatest activity of the 

 unknown process by which chemical is replaced by mechanical energy, 

 and, for a reason which I will at once explain, with the ver}^ begin- 

 ning of that process. For, as I have already indicated, the transition 

 does not occur at the same moment everywhere, and inasmuch as the 

 method which we use for the investigation of electrical change takes 

 cognizance only of what happens within an area of a couple of milli- 

 meters, we should expect it to occur not at a moment corresponding 

 to the maximum development of tension in the whole muscle, but at 

 the moment at which the transition process is going on with the 

 greatest rapidity in the elements immediately concerned within that 

 limited area. Assuming for the moment that this rapidity is expres- 

 sible as a measurable electromotive force, we should expect the appear- 

 ance and disappearance of that electromotive force to be represented, 

 not by a. curve resembling the tension curve, but M^ a curve of the 

 form indicated in the diagram. (Curve P' in diagram 4.) 



Before losing sight of the mechanical changes which have for the 

 last few minutes been occupying our attention, there are two other 

 points which must be shortly adverted to on account of their bearing 

 on what follows. The one is the terminableness and the cyclical char- 

 acter of the mechanical process. The muscle returns to its status quo 

 at a certain time after it has been disturbed, a time strictly dependent 

 on temperature and other well-ascertained physiological conditions. 

 We do not know as yet how it relaxes, whether it is merely a physical 

 reaction or whether it is by the intervention of a new chemical process. 

 This is a qu^estio vexata which for the present must remain open. 



The second point is that although the mechanical process is limited 

 in time, it is not limited in space. If it were possible to imagine a 

 continuum of contractile protoplasm, an excitation once started would 

 go on forever, i. e., it would be propagated from element to element- 

 in every direction if it were of the nature of cardiac muscle, in two 

 directions only if it were of the nature of skeletal muscle. For this 

 process to take place we suppose that each element excites its neigh])or. 

 In each transmission the time lost is almost infinitesimal, yet by sum- 

 mation it acquires a definite value, so that the relation between distance 

 traveled and time occupied can, when the temperature and other con- 

 ditions are known, be foretold. In so far as each element transmits 

 its state of change to its neigh))or without loss it resembles the propa- 

 gation of light and sound, but the velocity of propagation is of so 

 ditierent an order that the comparison uuist not be carried too far. 



METHOD OF OBSERVATION. 



We are now in a position to enter on the inquiry which more innue- 

 diately concerns us. Having the order of the mechanical changes 



