insect-like contact. The discharge of nervous energy in horses
and in cattle on adequate stimulation of the ticklish receptors
of the ear is so extraordinary that in the course of evolution it
must have been of great importance to the safety of the animal.
A similar ticklish zone guards the nasal chambers, the discharge of energy
here taking a form which effectively dislodges the foreign body.
The larynx is exquisitely ticklish, and, in response to any adequate
stimulus, energy is discharged in the production of a vigorous cough.
The mouth and pharynx have active receptors which cause the rejection
of noxious substances. The conjunctival reflex, though not
classed as ticklish, is a most efficient self-protective reflex.
I assume that there is no doubt as to the relation between
the adequate stimuli and the nerve-muscular response of the various
ticklish receptors of the surface of the skin, of the ear,
the nose, the eye, and the larynx. These mechanisms were developed
by natural selection as protective measures against the intrusion
of insects and foreign bodies into regions of great importance.
The discharge of energy in these instances is in accordance
with the laws of inheritance and association. The other ticklish
points which are capable of discharging vast amounts of energy
are the lateral chest-wall, the abdomen, the loins, the neck,
and the soles of the feet. The type of adequate stimuli of the soles
of the feet, the distribution of the ticklish points upon them,
and the associated response, leave no doubt that these ticklish points
were long ago established as a means of protection from injury.
Under present conditions they are of little value to man.

The adequate stimulus for the ticklish points of the ribs,
the loins, the abdomen, and the neck is deep isolated pressure,
probably the most adequate being pressure by a tooth-shaped body.