2 RESPONSE IN THE LIVING AND NON-LIVING 



disturbance, we apply salt solution, they again contract, 

 in the same way as before. Similar effects are produced 

 by sudden illumination, or by rise of temperature, or by 

 electric shock. A living substance may thus be put 

 into an excitatory state by either mechanical, chemical, 

 thermal, electrical, or light stimulus. Not only does 

 the point stimulated show the effect of stimulus, but 

 that effect may sometimes be conducted even to a con- 

 siderable distance. This power of conducting stimulus, 

 though common to all living substances, is present in 

 very different degrees. While in some forms of animal 

 tissue irritation spreads, at a very slow rate, only to 

 points in close neighbourhood, in other forms, as for 

 example in nerves, conduction is very rapid and reaches 

 far. 



The visible mode of response by change of form may 

 perhaps be best studied in a piece of muscle. When 

 this is pinched, or an electrical shock is sent through it, 

 it becomes shorter and broader. A responsive twitch 

 is thus produced. The excitatory state then dis- 

 appears, and the muscle is seen to relax into its 

 normal form. 



Mechanical lever recorder. In the case of contrac- 

 tion of muscle, the effect is very quick, the twitch 

 takes place in too short a time for detailed observation 

 by ordinary means. A myographic apparatus is there- 

 fore used, by means of which the changes in the muscle 

 are self-recorded. Thus we obtain a history of its 

 change and recovery from the change. The muscle is 

 connected to one end of a writing lever. When the 

 muscle contracts, the tracing point is pulled up in one 



