[*] The vagus nerve, when artificially stimulated, has a primary, brief inhibitory effect on the stomach and small intestine; its main function, however, as already stated, is to produce increased tone and contraction in these organs. This double action of the vagus is marked thus, ∓, in [Fig. 1].
Sherrington has demonstrated that the setting of skeletal muscles in opposed groups about a joint or system of joints—as in flexors and extensors—is associated with an internal organization of the central nervous system that provides for relaxation of one group of the opposed muscles when the other group is made to contract. This “reciprocal innervation of antagonistic muscles,” as Sherrington has called it,[6] is thus a device for orderly action in the body. As the above description has shown, there are peripheral oppositions in the viscera corresponding to the oppositions between flexor and extensor muscles. In all probability these opposed innervations of the viscera have counterparts in the organization of neurones in the central nervous system. Sherrington has noticed, and I can confirm the observation, that even though the sympathetic supply to the eye is severed and is therefore incapable of causing dilation of the pupil, nevertheless the pupil dilates in a paroxysm of anger—due, no doubt (because the response is too rapid to be mediated by the blood stream), to central inhibition of the cranial nerve supply to the constrictor muscles—i. e., an inhibition of the muscles which naturally oppose the dilator action of the sympathetic. Pain, the major emotions—fear and rage—and also intense excitement, are manifested in the activities of the sympathetic division. When in these states impulses rush out over the neurones of this division they produce all the changes typical of sympathetic excitation, such as dilating the pupils, inhibiting digestion, causing pallor, accelerating the heart, and various other well-known effects. The impulses of the sympathetic neurones, as indicated by their dominance over the digestive process, are capable of readily overwhelming the conditions established by neurones of the cranial division of the autonomic system.
Neurones of the Sympathetic Division and Adrenal Secretion Have the Same Action
Lying anterior to each kidney is a small body—the adrenal gland. It is composed of an external portion or cortex, and a central portion or medulla. From the medulla can be extracted a substance, called variously suprarenin, adrenin, epinephrin or “adrenalin,”[*] which, in extraordinarily minute amounts, affects the structures innervated by the sympathetic division of the autonomic system precisely as if they were receiving nervous impulses. For example, when adrenin is injected into the blood, it will cause pupils to dilate, hairs to stand erect, blood vessels to be constricted, the activities of the alimentary canal to be inhibited, and sugar to be liberated from the liver. These effects are not produced by action of the substance on the central nervous system, but by direct action on the organ itself.[7] And the effects occur even after the structures have been removed from the body and kept alive artificially.
[*] The name “adrenalin” is proprietary. “Epinephrin” and “adrenin” have been suggested as terms free from commercial suggestions. As adrenin is shorter and more clearly related to the common adjectival form, adrenal, I have followed Schäfer in using adrenin to designate the substance produced physiologically by the adrenal glands.
The adrenals are glands of internal secretion, i. e., like the thyroid, parathyroid, and pituitary glands, for example; they have no connection with the surface of the body, and they give out into the blood the material which they elaborate. The blood is carried away from each of them by the lumbo-adrenal vein which empties either into the renal vein or directly into the inferior vena cava just anterior to the openings of the renal veins. The adrenal glands are supplied by preganglionic fibres of the autonomic group,[8] shown in solid line in [Fig. 1]. This seems an exception to the general rule that gland cells have an outlying neurone between them and the neurones of the central nervous system. The medulla of the adrenal gland, however, is composed of modified nerve cells, and may therefore be regarded as offering exceptional conditions.
The foregoing brief sketch of the organization of the autonomic system brings out a number of points that should be of importance as bearing on the nature of the emotions which manifest themselves in the operations of this system. Thus it is highly probable that the sympathetic division, because arranged for diffuse discharge, is likely to be brought into activity as a whole, whereas the sacral and cranial divisions, arranged for particular action on separate organs, may operate in parts. Also, because antagonisms exist between the middle and either end division of the autonomic, affective states may be classified according to their expression in the middle or an end division and these states would be, like the nerves, antagonistic in character. And finally, since the adrenal glands are innervated by autonomic fibres of the mid-division, and since adrenal secretion stimulates the same activities that are stimulated nervously by this division, it is possible that disturbances in the realm of the sympathetic, although initiated by nervous discharge, are automatically augmented and prolonged through chemical effects of the adrenal secretion.
REFERENCES
[1] Cannon: The American Journal of Psychology, 1914, xxv, p. 257.
[2] For a summary of his studies of the organization of the autonomic system, see Langley: Ergebnisse der Physiologie, Wiesbaden, 1903, ii2, p. 818.