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 heht 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 im one 
