RESPONSE 
IN THE 
LIVING AND NON-LIVING 
CHAPTER I 
THE MECHANICAL RESPONSE OF LIVING SUBSTANCES 
Mechanical response —Different kinds of stimuli—Myograph—Character- 
istics of response-curve: period, amplitude, form — Modification of 
response-curves. 
OnE of the most striking effects of external disturbance 
on certain types of living substance is a visible change 
of form. Thus, a piece of muscle when pinched con- 
tracts. The external disturbance which produced this 
change is called the stimulus. The body which is thus 
capable of responding is said to be irritable or excit- 
able. A stimulus thus produces a state of excitability 
which may sometimes be expressed by change of form. 
Mechanical response to different kinds of stimuli.— 
This reaction under stimulus is seen even in the lowest 
organisms; in some of the ameeboid rhizopods, for 
instance. These lumpy protoplasmic bodies, usually 
elongated while creeping, if mechanically jarred, con- 
tract into a spherical form. If, instead of mechanical 
B 
