The Origin and Nature of Emotions | Page 8

mechanism for escape or defense; it can neither run fast nor can it climb a tree. Moreover, in encounters it shows no fear and backs rather than runs. The armadillo rolls itself into a ball for defense. On these premises we should conclude that the turtle, the armadillo, and the skunk have fewer nociceptors than has 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, 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. The response to tickling in these regions is actively and obviously self-defensive. The horse discharges energy in the form of a kick; the dog wriggles and makes a counter-bite; the man makes efforts at defense and escape.
There is strong evidence that the deep ticklish points of the body were developed through vast periods of fighting with teeth and claws (Fig. 9). Even puppies at play bite each other in their ticklish points and thus give a recapitulation of their ancestral battles and of the real battles to come (Fig. 10). The mere fact that animals fight effectively in the dark and always according to the habit of their species supports the belief that the fighting of animals is not an intellectual but a reflex process. There are no rules which govern the conduct of a fight between animals.