It has long been recognized that the most characteristic feature of reflexes is their “purposive” nature, or their utility either in preserving the welfare of the organism or in safeguarding it against injury. The reflexes of sucking, swallowing, vomiting and coughing, for instance, need only to be mentioned to indicate the variety of ways in which reflexes favor the continuance of existence. When, therefore, these automatic responses accompanying pain and fear and rage—the increased discharge of adrenin and sugar—are under consideration, it is reasonable to inquire first as to their utility.
Numerous ingenious suggestions have been offered to account for the more obvious changes accompanying emotional states—as, for example, the terrifying aspect produced by the bristling of the hair and the uncovering of the teeth in an access of rage.[1] The most widely applicable explanation proposed for these spontaneous reactions is that during the long course of racial experience they have been developed for quick service in the struggle for existence. Earlier writers on organic evolution pointed out the anticipatory character of these responses. According to Spencer,[2] “Fear, when strong, expresses itself in cries, in efforts to hide or escape, in palpitations and tremblings; and these are just the manifestations that would accompany an actual experience of the evil feared. The destructive passions are shown in a general tension of the muscular system, in gnashing of the teeth and protrusion of the claws, in dilated eyes and nostrils, in growls; and these are weaker forms of the actions that accompany the killing of prey.” McDougall[3] has developed this idea systematically and has suggested that an association has become established between peculiar emotions and peculiar instinctive reactions; thus the emotion of fear is associated with the instinct for flight, and the emotion of anger or rage with the instinct for fighting or attack. Crile[4] likewise in giving recent expression to the same view has emphasized the importance of adaptation and natural selection, operative through myriads of years of racial experience, in enabling us to account for the already channeled responses which we find established in our nervous organization. And on a principle of “phylogenetic association” he assumes that fear, born of innumerable injuries in the course of evolution, has developed into portentous foreshadowing of possible injury and has become, therefore, capable of arousing in the body all the offensive and defensive activities that favor the survival of the organism.
Because the increase of adrenin and the increase of sugar in the blood, following painful or strong emotional experiences, are reflex in character, and because reflexes as a rule are useful responses, we are justified in the assumption that under these circumstances these reactions are useful. What, then, is their possible value?
In order that these reactions may be useful they must be prompt. Such is the case. Some observations made by one of my students, Mr. H. Osgood, show that the latent period of adrenal secretion, when the splanchnic nerve is stimulated below the diaphragm, is not longer than 16 seconds; and Macleod[5] states that within a few minutes after splanchnic stimulation the sugar in the blood rises between 10 and 30 per cent. The two secretions are, therefore, almost instantly ready for service.
Conceivably the two secretions might act in conjunction, or each might have its own function alone. Thus adrenin might serve in coöperation with nervous excitement to produce increase of blood sugar, or it might have that function and other functions quite apart from that. Before these possibilities are considered, however, the value of the increased blood sugar itself will be discussed.
The Utility of the Increased Blood Sugar as a Source of Muscular Energy
When we were working on emotional glycosuria a clue to the significance of the increase of sugar in the blood was found in McDougall’s suggestion of a relation between “flight instinct” and “fear emotion,” and “pugnacity instinct” and “anger emotion.” And the point was made that, since the fear emotion and the anger emotion are, in wild life, likely to be followed by activities (running or fighting) which require contraction of great muscular masses in supreme and prolonged struggle, a mobilization of sugar in the blood might be of signal service to the laboring muscles. Pain—and fighting is almost certain to involve pain—would, if possible, call forth even greater muscular effort. “In the agony of pain almost every muscle of the body is brought into strong action,” Darwin[6] wrote, for “great pain urges all animals, and has urged them during endless generations, to make the most violent and diversified efforts to escape from the cause of suffering.”[*]
[*] It is recognized that both pain and the major emotions may have at times depressive rather than stimulating effects. For example, Martin and Lacey have shown (American Journal of Physiology, 1914, xxxiii, p. 212) that such stimuli as would induce pain may cause a fall of blood pressure, and they suggest that the rise of blood pressure commonly reported at times of painful experience is due to the psychic disturbance that is simultaneously aroused. Conceivably there is a relation between recognizing the possibility of escape (with the psychic consequences of that possibility) and the degree of stimulating effect. Thus pains originating from the interior of the body, or from injuries sure to be made more painful by action, would not likely lead to action. On the other hand, the whip and spur illustrate the well-known excitant effect of painful stimuli.
Similarly in the case of the strong emotions, the effect may be paralyzing until there is a definite deed to perform. Thus terror may be the most depressing of all emotions, but, as Darwin pointed out (Loc. cit., p. 81), “a man or animal driven through terror to desperation is endowed with wonderful strength, and is notoriously dangerous in the highest degree.”
That muscular work is performed by energy supplied in carbonaceous material is shown by the great increase of carbon-dioxide output in severe muscular work, which may exceed twenty times the output during rest. Furthermore, the storage of glycogen in muscle, and the disappearance of this glycogen deposit from excised muscle stimulated to activity,[7] or its reduction after excessive contractions produced by strychnine,[8] and the lessened ability of muscles to work if their glycogen store has been reduced,[9] and the simple chemical relation between sugar and the lactic acid which appears when muscles are repeatedly made to contract, are all indications that carbohydrate (sugar and glycogen) is the elective source of energy for contraction. This conclusion is supported in recent careful studies by Benedict and Cathcart,[10] who have shown that a small but distinct increase in the ratio between the carbon-dioxide breathed out and the oxygen breathed in during a given period (the respiratory quotient) occurs during muscular work, and that a decrease in the quotient follows, thus pointing to a larger proportion of carbohydrate burned during muscular work than before or after—i. e., a call on the carbohydrate deposits of the body.