The peculiar sensations felt in the teeth from a noise which grates upon the ears, is sympathetic of the irritation of the auditory nerve. Practitioners are well aware how many morbid sensations in parts remote from the intestinal canal may be cured by the removal of scybala or other accumulations from it. Painful affections of the nerves of the face, and of other parts, are often due to a cause of this kind. The irritation of a stone in the bladder gives rise to pains in the thighs, or to itching at the end of the penis ; and uterine irritation, whether from disease or from the enlargement of that organ in con nection with the early stage of pregnancy, causes similar pains in the nerves of the thighs.
Headache and defective vision are fre quently produced by disordered stomach. A draught of very cold water, or ice, taken quickly into the stomach, may occasion acute pain in the course of either frontal nerve. This same nerve on one side is frequently the seat of pain after the imprudent use of acid wines or other fermented liquors.
Movements, excited by the operation of a stimulus applied at a distance, form a large proportion of the instances of sympathetic phenomena. All the ordinary physical ner vous actions in which motions are excited by stimulating a sentient surface, may be regarded as examples of sympathetic actions.* The contraction of the iris upon the application of the stimulus of light to the retina, or of the pharyngeal muscles by stimulating the mucous membrane of the fauces, are instances in point where the stimulus acts indirectly upon the contracting fibre. Nothing is more sure than that in these instances the change wrought by the stimulus in certain sentient nerves, travels by a circuitous route through a nervous centre to the muscles which are called into action. Akin to these actions are the forcible respira tory movements which may be excited by ir ritation of the tracheal membrane, as coughing ; or sneezing, by stimulating the nasal mem brane ; or vomiting, by irritating the fauces. Spasmodic affections are often instances of morbid actions in sympathy with intestinal irritation, or the irritation of teething in chil dren. Partial or general convulsions are very frequently due to either or both these causes. We have known the most violent opisthotonos co-existing for a considerable time with the pre sence of lumbricoid ascarides in the intestine ; but ceasing immediately on the removal of the worms. Vomiting is commonly sympathetic of diseased kidney, or of the passage of a calculus along the ureter ; or of the passage of a gall-stone along the gall duct ; or it may be induced by the introduction of a catheter into the urethra.
The consentaneous action of symmetrical parts is no doubt due to a similar cause to that by which most of the sympathetic actions are excited, and more especially in those parts where symmetry of action is constant, al though liable to be interrupted by the influence of the will.
A distinct class of sympathetic actions con sists of those in which certain parts enlarge or become developed simultaneously with, and to a certain extent in effect of, the increase of others. The penis, the beard, the vocal
organs, experience a marked increase of de velopment at the adult period of life simul taneously with the enlargement of the testes; and, it may be added, in effect of their increase, because the early removal of these organs pre vents the growth of the others. And so like wise as the ovaria are developed, the uterus, the vulva, the mamma;, increase in size ; the ovarian and uterine irritation which accom panies the menstrual flux causes enlargement of the breasts, which subsides as soon as that period has gone by.
The various examples enumerated in the preceding paragraphs may be classed under three heads : first, sympathies between dif ferent individuals ; secondly, those which affect the mind, and, through it, the body ; and, thirdly, those which are strictly organic, and therefore physical.
Of the first class of sympathies we can offer no physical explanation. Whether the ner vous system of one individual can directly affect that of another, or whether the effect is produced on the imagination, and after wards on the nervous system, are questions still sub judice. The serpent fascinates his prey, apparently by the power of his eyes, and it is well known that one man can exert a marked control over another by a mere look ; and in the same way man can control other animals, even the fiercest carnivora, by a firm and decided glance of the eyes. It is no explanation of sympathetic phenomena of this kind to ascribe them to the effect of a tendency to imitation. Imitation is voluntary ; these actions are involuntary, or take place even in despite of the In the second class of sympathetic pheno mena, an affection of the mind is a necessary link. But why that affection of the mind should produce its peculiar effect is a question difficult to solve. Why should the perception .of certain odours produce in one case in creased action of the salivary glands, and in the other case cause syncope ? The only reply which can be made to this question is, that in these instances the impression on the senso riuzn causes a change there analogous to that which an original affection of the mind of similar kind would and therefore gives rise to effects of the same nature as those resulting from that mental change. Thus .the smell of savoury food excites in the mind the idea of food, which in a hungry man would, if it occurred spontaneously, occasion a flow of saliva. And the odour which oc casions syncope, creates in the mind an emo tion of disgust, which., if it arose independently of the physical impression, would affect the heart through the centre of emotion. It is plain, however, that that portion of the ner vous centre which is affected in such cases, must have a direct influence -upon the parts in which the sympathetic phenomena appear ; and this through commissural fibres, or the continuity of its gray matter with that of the centre from which its nerves immediately spring; thus, in the instances referred to, the centre of sensation, which is first affected, is, through the medulla oblongata, connected with the salivary glands by the fifth nerve, and with the heart by the vagus.