250 LECTURES TO SCIENCE TEACHERS. 



circuit along which the instantaneous induction current, by which the sciatic 

 nerve is stimulated, passes each time that the primary current is closed. Upon 

 the cylinder are inscribed the records of ten experiments. The first five were 

 written by the muscle in its normal state. In the last five it was becoming 

 gradually cooled by placing near it a small block of ice. The last contraction 

 has just been completed. The vertical lines by which the tracings are crossed 

 were drawn before the experiments were made, by first bringing the cylinder 

 into the exact position at which the arm h comes into contact with the trigger, 

 and then working the rack and pinion. It indicates the point in each tracing 

 which corresponds to the moment of excitation. 



I need scarcely tell you that the curve is produced by 

 the contraction of the muscle. It reveals to us that the 

 apparently instantaneous effect which is produced, when a 

 nerve is instantaneously excited, is in reality a process of 

 three stages, each of which has a definite duration, viz. a 

 period of latency which intervenes between the moment of 

 stimulation and the commencement of change in the 

 muscle, and corresponds to so much of the tracing to the 

 right of the vertical, as is horizontal, and therefore to a 

 period of about one hundredth of a second or thereabouts : 

 Secondly, a period about three times as long, during which 

 the muscle is contracting, and lastly a still longer period, 

 during which it is returning to its previous condition. 



As I before hinted, the knowledge of the mechanism 

 and condition of muscular action is of fundamental im- 

 portance in physiology. It is so because it is by muscular 

 action that the most essential functions of our bodies 

 are discharged, by it our circulation and respiration 

 are maintained, and by it we maintain our relation with 

 other persons and with the world around us. Muscle 

 constitutes fully two-fifths of the weight of the body. 

 By muscle, in a certain very true sense, we live and move 

 and have our being; by muscle every action is per- 

 formed, every thought expressed. It is therefore of great 

 moment to the physiologist to be able to apply to the 

 phenomena of muscular contraction more exact methods 

 of research than to those of any other function ; and this 

 is so, not merely as regards the mechanism by which a 

 muscle, when stimulated, shortens itself and thereby per- 

 forms mechanical work, but quite as much as regards the 

 molecular changes which in the living substance of muscle 

 are associated with the change of form. Of all this the 

 experiment I have shown you is a mere sample. It may 

 enable you to understand something of the nature of the 

 work in which physiologists are now engaged. Many vital 



