The Origin and Nature of Emotions | Page 7

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. Then various portions of the hemispheres were slowly but completely destroyed by rubbing them with pieces of gauze. In some instances a hemisphere was destroyed by burning. In no case was there more than a slight response of the centers governing circulation and respiration, and no morphologic change was noted in an histologic study of the brain-cells of the uninjured hemisphere. The experiment was as completely negative as were the experiments on the "spinal dog." Clinically I have confirmed these experimental findings when I have explored the brains of conscious patients with a probe to determine the presence of brain tumors. Such explorations elicited neither pain nor any evidence of altered physiologic functions. The brain, therefore, contains no mechanism-- no nociceptors--the direct stimulation of which can cause a discharge of nervous energy in a self-defensive action. That is to say, direct injury of the brain can cause no purposeful nerve-muscular action, while direct injury of the finger does cause purposeful nerve-muscular action. In like manner, the deeper portions of the spinal region have been sheltered from trauma and they, too, show but little power of causing a discharge of nervous energy on receiving trauma. The various tissues and organs of the body are differently endowed with injury receptors--the nociceptors of Sherrington. The abdomen and chest when traumatized stand first in their facility for causing the discharge of nervous energy, _i. e_., THEY STAND FIRST IN SHOCK PRODUCTION. Then follow the extremities, the neck, and the back. It is an interesting fact also that different types of trauma elicit different responses as far as the consequent discharge of energy is concerned.
Because it is such a commonplace observation, one scarcely realizes the importance of the fact that clean-cut wounds inflicted by a razor-like knife cause the least reaction, while a tearing, crushing trauma causes the greatest response. It is a suggestive fact that the greatest shock is produced by any technic which imitates the methods of attack and of slaughter used by the carnivora. _*In the course of evolution, injuries thus produced may well have been the predominating type of traumata to which our progenitors were subjected_. In one particular respect there is an analogy between the response to trauma of some parts of the body of the individuals of a species susceptible to shock and the response to trauma of the individuals in certain other great divisions of the animal kingdom. Natural selection has protected the crustaceans against their enemies by protective armor, _e. g_., the turtle and the armadillo; to the birds, it has given sharp eyes and wings, as, for instance, the wild goose to another species--the skunk--it has given a noisome odor for its protection. The turtle, protected by its armor against trauma, is in a very similar position to that of the sheltered brain of man and, like the brain, the turtle does not respond to trauma by an especially active self-protective nerve-muscular response, but merely withdraws its head and legs within the armored protection. It is proverbially difficult to exhaust or to kill this animal by trauma. The brain and other phylogenetically sheltered parts likewise give no exhausting self-protective nerve-muscular response to trauma. The skunk is quite effectively protected from violence by its peculiar odor. This is indicated not only by the protective value of the odor itself, but also by the fact that the skunk has no efficient nerve-muscular