Disregarding for the present the expenditure of energy for procuring
food and for procreation, let us consider the discharge of energy
for self-preservation. The mechanisms for self-defense which we
now possess were developed in the course of vast periods of time
through innumerable intermediary stages from those possessed by
the lowest forms of life. One would suppose, therefore, that we must
now be in possession of mechanisms which still discharge energy on
adequate stimulation, but which are not suited to our present needs.
We shall point out some examples of such unnecessary mechanisms.
As Sherrington has stated, our skin, in which are implanted many
receptors for receiving specific stimuli which are transmitted
to the brain, is interposed between ourselves and the environment
in which we are immersed. When these stimuli reach the brain,
there is a specific response, principally in the form of
muscular action. Now, each receptor can be adequately stimulated
only by the particular factor or factors in the environment
which created the necessity for the existence of that receptor.
Thus there have arisen receptors for touch, for temperature,
for pain, etc. The receptors for pain have been designated _nociceptors_
(nocuous or harmful) by Sherrington.

On the basis of natural selection, nociceptors could have developed
in only those regions of the body which have been exposed to injury
during long periods of time. On this ground the finger, because it
is exposed, should have many nociceptors, while the brain, though the
most important organ of the body, should have no nociceptors because,
during a vast period of time, it has been protected by a skull.
Realizing that this point is a crucial one, Dr. Sloan and I made a series
of careful experiments. The cerebral hemispheres of dogs were exposed
by removing the skull and dura under ether and local anesthesia.