genious hypothesis has been formulated by Vanlair, which may be mentioned. According to Pierret there is a relation between the length of a nerve fibre and the size of its cell. The sheath nerves will vary in length according to the distance from the centre at which they terminate. If the cells discharge in the order of their size, beginning with the smallest, the sensation will seem to dart centrifugally. For the apparently centripetal direction, a still more complex hypothesis has been suggested. Most nerves contain recurrent fibres which proceed from adjacent nerves [Magendie, Arloing, and Tripier]. These recurrent fibres ascend the branches and trunk, but all cease before the foramina of exit are reached. If these end in the nerve sheath, the same hypothesis applied to the recurrent fibres will serve to explain the centripetal darting; the longest fibres, which end nearest the centre, will have the largest cells, and discharge last. Unfortunately, we do not know that any of the recurrent fibres end in the nerve sheath, and if they do, it is probable that their central connection is the same as that of the direct fibres. It seems more probable that the arrangement of the cells in the centre depends on the distribution of the fibres in the sheath, and that this arrangement, and not the size of the cells, determines the order of the discharge, which may traverse the centre in opposite directions in different cases, just as in one epileptic an aura may pass down, and in another up, the arm.)

The irradiation of the pain, in severe attacks, to neighboring nerve areas is clearly, as already mentioned, a central phenomenon. An intense discharge always tends to spread to other connected cells, in proportion to its intensity, as the phenomena of epilepsy abundantly illustrate.

The origin of the tender points is obscure, and has been the subject of much speculation. (The hypothesis [of Cartaz and others] which connects these tender points with the distribution of the recurrent nerves, rests on too many unprovable assumptions to deserve detailed description.) Their localization to the places at which nerves emerge from deeper structures, or divide, suggests their dependence on mechanical causes. Accidental pressure, and traction in movement, will have most influence on the nerves at such places, and cause there a greater degree of stimulation of the nervi nervorum (Vanlair). Many phenomena of neuralgia suggest, moreover,

that a neuralgia which is at first purely central may not remain so. We have seen that the pain often causes secondary vascular disturbance in the territory of the nerve. It is most unlikely that such disturbance, conspicuous in the skin, is confined to the surface. It probably involves, also, the deeper structures, and especially the nerve sheaths, in which the pain is especially localized. Such secondary vascular disturbance, and the tissue changes to which it ultimately leads, must constitute a source of irritation of the nervi nervorum, and, in a truly "vicious circle," must intensify the malady, which, at first central, may be thus, at last, peripheral also. It is probable that this mechanism takes an important share in the production of the tender points, and is also the cause of the intractability of some neuralgias.

Not only is it probable that peripheral disturbance takes part in the pathogenesis of central neuralgia, but it is certain that central disturbance is concerned in some neuralgias of peripheral origin. A traumatic cause, an injury to a nerve branch, may induce pain far wider in area than the distribution of the injured branch, or even of the nerve from which it comes. In some cases, again, the pain is felt not in the area of the nerve injured, but in that of some other nerve (reflex neuralgia). An interval usually elapses after the injury before the pain is felt. Lastly, in some traumatic cases, division of the nerve does not cure the neuralgia. These facts can only be explained by assuming that the chief cause of the pain is a morbid state of the central cells, excited by, but to some extent independent of, the peripheral lesion. Doubtless, in all neuralgias of "symptomatic" character, the symptoms depend, in varying degree, on an induced central disturbance.

Various Forms.-Cases of neuralgia differ much according to their situation, character, and cause, and hence it is necessary to describe in some detail the varieties of the disease. According to situation, we have to consider separately those which occupy the head, neck, arm, trunk, and leg. According to character, we have "epileptiform neuralgia," and "reflex" or "sympathetic neuralgia," while of those which depend on special causes the most important are the traumatic, herpetic, anæmic, malarial, syphilitic, and diabetic forms.

It is important to remember that neuralgia is often not confined to a single nerve. Those who are liable to the affec

tion in a high degree sometimes suffer from neuralgia in many situations, simultaneously or in succession.

VARIETIES DEPENDING ON SITUATION.

Neuralgia of the Fifth Nerve. Trifacial or Trigeminal Neuralgia; Tic Douloureux; Prosopalgia.- Under these various designations the most common form of neuralgia has been described. Neuralgia of the fifth is probably more frequent than all the other varieties together, and it presents, in most typical forms, the characteristic symptoms of the disease. Nor is this surprising when we consider that the fifth is incomparably the most important nerve of common sensibility in the body.

The causes of this form are all those that have been described in the section on etiology; indeed, the general history of neuralgia is, to a large extent, based on the symptoms of this variety. It is equally common on the two sides. The seat of the pain may be in any of the three divisions of the nerve; and it more commonly occupies one or two of the divisions than all three. The tender points are often well marked, and in them the pain has its chief intensity.

Neuralgia of the first division is felt chiefly in the supraorbital branch, and hence is often called supra-orbital neuralgia. The frequency with which it was formerly due to malaria has left for it the popular name of "brow ague," although this cause is now rarely operative in this country. The pain radiates from the supra-orbital notch over the anterior half of the head, and is often felt in the eyelids and even in the eye, and in the side of the nose. The most important tender points are the supra-orbital, just above the notch or foramen of that name; a palpebral, in the outer part of the upper eyelid; a nasal, at the emergence of the nasal branch at the lower edge of this bone, and sometimes an ocular, within the eyeball. Pain felt just above the eyebrows is sometimes due to a morbid state of the frontal sinuses, but pain from this cause is generally double, and is often secondary to coryza. The lining membrane of the sinus is supplied by the fifth nerve, and it has been conjectured that the pain occurs when the small opening of the sinus into the nasal cavity becomes closed. Seeligmüller thinks that this is the cause of the pain even in

malarial cases. But the nerves of the sinus (or their centres) seem to be particularly obnoxious to certain influences, as is shown by the peculiar pain, evidently referred to these sinuses, which many persons experience after eating ices. We cannot, therefore, conclude that because the pain occupies this situation, it is necessarily due to a local cause. The supra-orbital region is a not uncommon seat of pain that is apparently neuralgic. This does not follow the course of the nerves, but it may be felt sometimes over one eye, sometimes over both.

Ocular Neuralgia.--The eyeball is an occasional seat of neuralgic pain, often of considerable severity. This is sometimes associated with some error of refraction, especially hypermetropia, but occurs also independently of any abnormality of the eye itself. Either one or both eyes may be affected. The pain may occur spontaneously, or may be brought on by use of the eyes; it is not often accompanied by photophobia. When severe, there is occasionally dimness of sight, apparently of inhibitory origin, and the amblyopia may be accompanied by a peripheral restriction of the field of vision. Ocular neuralgia may exist alone, or be associated with pain in adjacent parts, and sometimes with pain that extends far beyond the limits of the fifth nerve. Bilateral pain sometimes passes from the eyes over or through the head to the occipital region, and even down the neck. Anæmic girls often complain of a peculiar dragging pain at the back of the eyes, increased by their use. Ocular pain is often associated with rheumatism; the subjects of rheumatic iritis are often liable to pain in the eyeballs, which seems to be neuralgic in character.

In neuralgia of the second division, infra-orbital neuralgia, the pain occupies the area between the orbit and the mouth and extends over a great part of the cheek, and to the ala nasi. The chief foci of pain and tender points are an infraorbital, at the emergence of the nerve beneath the orbit; a nasal, at the side of the nose; a malar over the most prominent part of that bone, and a gingival line below that bone, along the line of the gums of the upper jaw; very rarely there is a point in the palate, or in the upper lip. The most acute nerve, as, for in.

pain is often felt only in one portion of the stance, the side of the nose; but it usually radiates, in a slighter degree, over a wider extent.

When the third division is affected, the pain often extends

over a large area, occupying the parietal eminence and the temple, the ear, the lower jaw, and the tongue. The chief tender points are the inferior dental, at the foramen of that name; the temporal, in the posterior part of the temple on the auriculo-temporal branch; it may be a little lower down, just above the zygoma in front of the ear, and is a very common focus. The parietal, over the parietal eminence, is common to this and to occipital neuralgias. Sometimes there is a focus of pain in the tongue. Separate portions of this branch are sometimes affected alone, especially the inferior dental and the auriculo-temporal. A boring pain limited to the temporal point is especially common. Most intense neuralgia is sometimes confined to the lingual branch. Occasionally, a tender point exists in the cervical spines, at the first two or at the fifth (Armaingaud), its exact cause is obscure.

The pain in trigeminal neuralgia is often peculiarly intense and presents every variety. It may radiate from one part of the fifth nerve to the next, and even to other nerve regions. Thus, in one case of violent neuralgia of the second division of the fifth, the pain often radiated to the occipital region and sometimes to the shoulder on that side. The effects of cold and contact in exciting the pain are well marked, and it is often increased by movement of the face or jaw, so that, in severe cases mastication may be impossible and it may be difficult to give sufficient food. If the ear is the seat of pain, either alone or with other parts, the attack may be accompanied or followed by auditory hyperesthesia. When the pain is very acute and sudden, reflex muscular spasm may occur in the face (the “tic convulsif" of the French). Paralytic phenomena are rare, but transient paralysis of the third nerve has been observed to follow each paroxysm of pain in the supra-orbital branch. Sometimes paroxysms of severe pain in the fifth nerve are accompanied by subjective flashes of light, especially when the eyeballs are the seat of pain. The vaso-motor disturbance already described is frequently seen, flushing, sweating, permanent dilatation of the vessels (often conspicuous in the eye), salivation, increased secretion of mucus in the nose, lachrymation. Trophic disturbances occasionally occur; acute, as erythema; or chronic, as thickening of the periosteum, loss of hair, or local grayness. Even unilateral atrophy of the face has been met with (Boisson). It is probable that in many of