a dog or man, and that they would show less response to trauma.
In two carefully conducted experiments on skunks and two on armadillos
(an insufficient number) the energy discharged in response to severe
and protracted trauma of the abdominal viscera was very much less than
in similar experiments on dogs, opossums, pigs, sheep, and rabbits.
It was indeed relatively difficult to exhaust the skunks and armadillos
by trauma. These experiments are too few to be conclusive,
but they are of some value and furnish an excellent lead.
It seems more than a coincidence that proneness to fear,
distribution of nociceptors, and susceptibility to shock go
hand-in-hand in these comparative observations (Figs. 6, 7, and 8).

The discharge of energy caused by an adequate mechanical stimulation
of the nociceptors is best explained in accordance with the law
of phylogenetic association. That is, injuries awaken those reflex
actions which by natural selection have been developed for the purpose
of self-protection. Adequate stimulation of the nociceptors for pain
is not the only means by which a discharge of nervous energy is caused.
Nervous energy may be discharged also by adequate stimulation
of the various ticklish regions of the body; the entire skin
surface of the body contains delicate ticklish receptors.
These receptors are closely related to the nociceptors for pain,
and their adequate stimulation by an insect-like touch
causes a discharge of energy,--a nerve-muscular reaction,--
resembling that developed for the purpose of brushing off insects.
This reflex is similar to the scratch reflex in the dog.
The discharge of energy is almost wholly independent of the will
and is a self-protective action in the same sense as is the response
to pain stimuli. The ear in man and in animals is acutely ticklish,
the adequate stimulus being any foreign body, especially a buzzing,