I.
The transmission of painful impressions from the periphery to the cortical centres is far from being determined in all the stages of its course.
The nerve terminations, from their outpost position, receive the first shock; but what part do they play? It is known that the nerves of the deep organs and the filaments of the great sympathetic have no specially constructed terminations. The nerves of special sense, on the contrary,—vision, hearing, smell, and taste,—possess special peripheral apparatus (retina, organ of Corti, etc.) of very complex anatomy; it is known that their rôle is specially sensorial; they are above all instruments of knowledge, seldom directly of pain or pleasure. So that the question of nerve terminations in relation to pain may chiefly be confined to the nerves of the tactile apparatus, taking the word in its largest sense. The extreme difficulty of isolating the purely peripheral impression from that which reaches the nerve itself renders almost insoluble the question as to the part played by these peripheral apparatus. Beaunis,[[12]] relying on the cases of localised anæsthesia in which the patient no longer feels pain but still perceives contact, thinks that analgesia would reach the nerves before acting on their terminations, shut up in more or less resistant capsules.
Are there special nerves for the transmission of pain? Goldscheider, well known for his researches on cold and heat points on the skin, at first maintained that there are.[[13]] According to him, the pain-bearing nerve filaments are interlaced with the sensorial nerves, more numerously with the nerves of general sensation (touch, heat, cold), less numerously with those of special sense. If the existence of these special pain nerves were well established, it would have as great an importance for our subject as the discoveries of Sachs and others, on the nervous filaments peculiar to the muscles, have had for the study of the kinæsthetic sense. But this physiologist has since repudiated his first assertion, and maintained that it was misunderstood; he admits pain points (points sensible to pain), but not a specific organ for pain nor special nerves to transmit it.[[14]] Frey, on the other hand, professes to have proved experimentally both pain nerves and appropriate terminal organs. His observations have been rejected as inaccurate. At the present time there is nothing to establish the existence of pain nerves, and most authors have given strong reasons against the probability of such a discovery. Rejecting this hypothesis, we may admit that an impression of pain, like any other impression, is transmitted by the nerves of general or special sensibility. When it has entered the spinal cord at the posterior roots, the road it follows to reach the higher centres has given rise to much investigation and discussion. According to Schiff, transmission takes place through the grey substance, tactile impressions passing by the posterior fibres; there would thus be two distinct paths, one for the feeling, the other for the sensation properly so-called. Brown-Séquard also admits distinct paths, but through the grey substance alone; the anterior region is devoted to touch, the median to temperature, the posterior to pain. According to Wundt, impressions of touch and temperature have a primary path through the white substance when stimulation is moderate, a secondary path through the grey substance acting as a surplus channel when stimulation is violent. The hypothesis of separate paths, whatever they may be, has the advantage of harmonising with the well-known fact, to which we shall return, that the transmission of pain is slower than sensorial transmission. Lehmann, who takes up a rigidly intellectualist position, cannot admit that the element of feeling has a certain independence in relation to the element of sensation, existing by itself. He believes that the delay is explained by the fact that “pain requires a stronger excitation in the sensorial region than sensation without pain, and that consequently pain is only produced after sensation, as the excitation increases in intensity.”[[15]] This explanation may be accepted, but it assumes that pain always depends on intensity of stimulus, which is not proved.
From the spinal cord we reach the medulla, to which some authors assign the chief part. The latest, Sergi, in his book Dolore e Piacere (Milan, 1894), makes it the seat of the affective phenomena in general (pains, pleasures, emotions). What in his opinion testifies to the importance of the medulla in the affective life is the number and nature of the nervous centres situated between the protuberance and the floor of the small ventricle, centres which act on the heart, the vessels, the lungs, the secretions, the intestinal movements. “The vital knot of Flourens is the vital centre and must also be the centre of pleasure and pain, which are merely alterations in the functions of organic life.”[[16]] In his opinion (which is mine also) the part played by the brain in the genesis of states of feeling has been exaggerated; it only acts in two ways—by rendering the disturbances of organic life, the physical basis of the feelings, apparent to consciousness, and as a cause of stimulation by means of ideas.
However disposed we may be to restrict the part played by the brain—that is to say the cortical layer—it remains a predominant factor and the final terminus in the process of transmission. Here we plunge into darkness. Researches into cerebral localisation teach us on this subject nothing which is generally admitted. During the first period of such studies, which may be called that of circumscribed localisation à outrance, Ferrier placed the seat of the emotions in the occipital lobes, because, in his opinion, that region of the cortex receives the visceral sensations, because the sexual instinct is dependent upon them, and finally, because these lobes are more developed in women than in men. It is needless to bring forward the numerous criticisms of this thesis. During the second and present period of localisation, which may be called that of disseminated localisation, functional rather than anatomical, authors are little inclined to admit a particular centre for the affective life in general, and still less for pain. All the sensory centres, and even all the motor centres (perhaps there are fundamentally only sensori-motor centres with preponderance of one or the other element), may under certain conditions of activity produce in consciousness a feeling of pleasure or pain.
The hypothesis of a cortical centre is not, therefore, probable; I shall return to this point in discussing the emotions.