and Plants to the Electrical Phenomena associated with it. 41 



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 by 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 character 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 tempera- 

 ture 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 

 qua'stio 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 con- 

 tinuum of contractile protoplasm, an excitation once started would go on 

 for ever, 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 neighbour. In each 

 transmission the time lost is almost infinitesimal, yet by summation it 

 acquires a definite value, so that the relation between distance travelled 

 and time occupied can, when the temperature and other conditions are 

 known, be foretold. In so far as each element transmits its state of 

 change to its neighbour without loss it resembles the propagation of 

 light and sound, but the velocity of propagation is of so different an 

 order that the comparison must not- be carried too far. 



Method of Observation. 



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

 immediately concerns us. Having the order of the mechanical changes 

 which constitute muscular action before us, it will be our purpose to 

 compare with this order that of its concomitant electrical phenomena. 

 Before I proceed with this comparison it is desirable to say that 

 it should be understood that no reference will be made to electrical 

 theory. We merely derive our modes of observation and of measure- 

 ment from the exact sciences, and aim at the utmost attainable 

 precision ; but the phenomena have their chief interest as outward 

 and visible signs of intimate vital processes, of which they afford us the 

 only knowledge that is within our reach. 



