handbooks of psychology to illustrate the process of sensory stimulus of a
nerve-centre and the succeeding motor reaction. The diagram is usually
drawn after this fashion:

Sensory stimulus Nerve-centre Motor Reaction
________________________________O______________________________
--------------------> -------------------->

The process is thus described by William James:
[Footnote: _Psychology, Briefer Course_, American Science Series, p. 91.
Henry Holt.]

"The afferent nerves, when excited by some physical irritant, be this as
gross in its mode of operation as a chopping axe or as subtle as the
waves of light, convey the excitement to the nervous centres. The
commotion set up in the centres does not stop there, but discharges
through the efferent nerves, exciting movements which vary with the
animal and with the irritant applied."

The familiar laboratory experiment irritates with a drop of acid the hind
leg of a frog. Even if the frog's brain has been removed, leaving the
spinal cord alone to represent the nervous system, the stimulus of the
acid results in an instant movement of the leg. Sensory stimulus,
consequent excitement of the nerve centre and then motor reaction is the
law. Thus an alarmed cuttlefish secretes an inky fluid which colors the
sea-water and serves as his protection. Such illustrations may be
multiplied indefinitely.
[Footnote: See the extremely interesting statement by Sara Teasdale,
quoted in Miss Wilkinson's _New Voices_, p. 199. Macmillan, 1919.]
It may seem fanciful to insist upon the analogy between a frightened