Origin of Cold, Warm, Pain and Touch Spots.

The hair-clad skin of primitive man provided ample raw material for the eventual differentia­tion of both end-organs and sensorial areas which is found to-day. Not only did he possess what is called Common Sensation in his skin but in the individual hairs lay a delicate tactile structure, which, though probably inferior in delicacy, serves a similar purpose to that of the vibrissæ on the muzzle of Felidæ. Each hair, being deeply inserted into the skin and supplied with fine nerve fibrils, when it is bent, acts as a lever communicating an impulse to an afferent nerve trunk. In an animal covered with thick hair the sensory impulse conveyed might be exceedingly delicate, but, from the nature of the case, of much more limited range than in one like man in whom the hair is so greatly diminished in length and thickness.

It would be fruitless to speculate as to which of these four forms of stimuli was the earliest to become effective in developing man.