In the case of phosphorus, I think it is now pretty generally believed that its poison has very little effect in the mouth unless there exist a precedent caries of a tooth or its socket. These facts almost suffice to take these agents out of the group of constitutionally acting into that of peripherally irritant causes. In this class of agents, as in the preceding one, the first indication in treatment is the complete removal of the sufferer from their baneful influence.
We have now briefly reviewed the main agencies which act constitutionally in the production of pain. It is apparent, to recur to our simple illustration, that they must have their main efficiency in the action they have on the central cell, and not on any modification of the impulses sent to that cell. It cannot be denied that in rare instances these various agents are productive of pain referred to a particular nerve, when we cannot find anything in the nerve itself or in the tissues supplied by it to account for the morbid manifestation. We are, therefore, constrained to believe, at least for the present, that morbid manifestations, sensations of pain, may originate in the cell itself and thence be referred outward. But I would remind you that the whole tendency of modern medical thought is to more accurately localize the starting point of disease, and to circumscribe the area of cases in which such outward cause of disease is unknown. So long as men were satisfied to cover up their ignorance in such vague phrases as “humors of the blood,” “rheumatic diathesis,” etc., etc., few were tempted to carefully examine the local conditions for an explanation. But the last fifty years have seen an enormous change in our attitude of mind to these problems. It is a change which is one of the greatest in the history of the human mind. And while I do not for a moment wish to underrate the great importance of a due regard to the constitutional causes of pain, especially of the malnutrition of the nerve cells, I believe that in the main they must be classed as predisposing causes and not as efficient ones. When we come to the question of why pain is located or referred to a particular nerve, I believe the answer in the overwhelming majority of cases will be because there is some peripheral abnormality in that nerve or in some other nerve with which it is intimately associated; for we have to recognize in the philosophy of pain the same fact that we do in the philosophy of the human mind, namely, that our ideas are so closely associated that one thought will almost necessarily suggest another. Just as, if we have always been accustomed to see Smith and Jones together, we can hardly think of Smith without Jones also putting his nose in; so in feeling sensations, certain ones get so closely intertwined that one will almost inevitably causes the other. This, then, leads us naturally to the second great division of our subject, and that is the influence of peripheral irritation in causing pain.
From what I have just said, this may be of two kinds—a reflex or associated pain expressed in some other nerve than the one affected, or else it may be due to direct irritation in the nerve itself.
A very common example of the former is seen in the headaches from which many women suffer, from the menstrual congestion (irritation of the nerves) of the ovaries and uterus. It is, however, quite outside the scope of this paper to enlarge on this curious and obscure part of our subject. I prefer to take up the more understood and more common form of direct peripheral irritation, and especially the irritation arising from diseases of the teeth and jaws.
In that delightful book, “Rest and Pain,” by Mr. John Hilton, the eminent London surgeon, he narrates a case, which is so instructive in illustrating the mode in which peripheral irritation may cause not only pain, but local disease, that I cannot forbear from quoting it:
“A gentleman, aged 63, came to consult me about an ulcer situated upon the left side of his tongue. On examination, I found an elongated, very ugly-looking ulcer, nearly as large as a bitter almond, and of much the same shape. The surrounding parts were swollen, hard, red, and much inflamed, and a lymphatic gland was enlarged below the horizontal ramus of the lower jaw on the same side. I saw in the mouth a rugged tooth, with several projecting points upon it, opposite the ulcer. This gentleman observed to me: “Having suffered a good deal from earache on the left side for a long time, without experiencing any relief from medical treatment, it was thought that I must be gouty, and I went to a surgeon who treats gouty affections of the ear. This surgeon paid great attention to my ear, but certainly did not do it the slightest degree of good. I accidentally mentioned to him that I had for some time past something the matter with my tongue. On seeing it, he immediately began to apply caustic vigorously; moreover, not satisfied with applying it himself, he gave it to my wife that she might apply it at home. I have gone on in this way from day to day, until the pain in my ear is very considerably increased, and the ulcer on my tongue is enlarging; so I have come to you for your opinion regarding my state; for, to tell you the truth, I am afraid of a cancer in my tongue.” I thought I saw the explanation of this patient’s symptoms. The pain in the ear was expressed by the fifth nerve, and there was a rugged tooth with little projections on it, some of which touched a small filament of the lingual-gustatory branch of the fifth nerve in the surface of the ulcer. I detected this little filament by placing upon it the end of a blunt probe. It was situated near the centre of the ulcer, and was by far its most exquisitely painful part. This exposed nerve caused the pain in the auditory canal which led him to go to the aurist, and the aurist, instead of confining himself to his own department, seized the tongue, put nitrate of silver upon the whole of the ulcer, and increased the mischief. I simply desired that the ulcer should be left at rest; that the patient, to avoid touching the tooth, should neither talk nor move his tongue more than necessary; that he should wash his mouth with some poppy fermentation, and take a little soda and sarsaparilla twice a day. In three days about one-third of the ulcer was healed up, actually cicatrized, the enlarged gland nearly gone, and the earache much diminished.
“This rapid improvement might appear something like exaggeration, but all surgeons know that the tongue has those elements within it which contribute to the most rapid repair of injury. I do not know any tissue that repairs itself more rapidly. It is abundantly supplied with capillaries filled with arterial blood, and has an enormous distribution of nerves, and these are two elements that contribute to rapid reparation. It was quite clear that the treatment was in the right direction, viz., that of giving rest to the tongue and ulcer. After a few more days I requested him to consult a dental surgeon with respect to the propriety of taking off the points of the tooth. This was afterwards done, and the patient soon lost his anxiety about cancer, his earache, and all his other severe symptoms.”
I cannot doubt that the starting point of a large number of similar painful ulcers and of true cancer of the mouth is to be looked for in disease of the teeth.
When we come to the teeth themselves, the pain lies in the irritated nerves of the pulp. Of course it cannot be denied that the pulp itself may be the original seat of the pain, but, if so, the number of such cases must be few. When we reflect on the mode of nutrition of the tooth, it seems almost self-evident that any depressing agency which could lead to disease of the pulp must, by an augmented action, cause greater disease in the structures which depend on the pulp for their nutrition to start with. At most, disease and pain in the pulp alone must be nothing less than a pathological curiosity. Such, however, is not the case in the vast multitude of cases dependent on caries, in which the pulp has lost in part or in whole its protection from external morbid influences. The origin of caries is one of the most interesting subjects in the whole domain of surgery. I have been astonished to find that among dentists it is not more definitely settled. So able a writer as Wm. Henry Potter (of Harvard) says: “In the first place, it may be said that caries of the teeth does not resemble caries of the bone. The term caries, as applied to the teeth, is a misnomer, given at a time when the true nature of the process was not understood.... The pathological change which occurs in caries is a decalcification and disintegration of the several tissues of the teeth.”
I confess that strikes me as a very excellent description of what surgeons usually term caries, namely, a molecular death of bone tissue. Nor can I see any difference in the essential nature of the two processes, if you make due allowance for the morphological modifications of tooth structure from bone structure, and the different environment under which the process takes place. If I were asked to define dental caries, I should say it was a molecular death of the tooth structures, especially the dentine, due to the action of micro-organisms; that in the course of the process lactic or other acid is developed, which decalcifies the teeth, is doubtless true, but the very presence of acid fermentation in a normally alkaline cavity necessitates the assumption of the action of micro-organisms. I would remind you that the conditions favorable to the activity of such organisms are all apt to be present. They are: