So much for the clonic tics of mastication: we pass on to review the tonic forms, the most curious of which has received the name of _mental trismus_. MENTAL TRISMUS The characteristic feature of this tonic tic is an all but permanent contraction of the masseters, which may, however, be completely relaxed by making the subject put out his tongue, show his throat, etc. It may be maintained during the act of speaking. Its intensity and its persistence alike stand in rigorous relation to the nature and degree of the mental affection that provides its occasion. In the insane it may become so absolute an obstacle to nutrition that recourse must be made to nasal feeding. Mental trismus resembles mental torticollis in that any proceeding to which the patient attributes a special inhibitory virtue is adequate to correct it, as, for instance, the insertion of a cork between the teeth,[75] or the placing of a finger on the incisors.[76] It must of course be clearly understood that the diagnosis of mental trismus can be arrived at only after previous elimination of every possible source of confusion, such as tetanus, more rarely tetany, meningitis, and acute bulbar paralysis, in addition to other mesencephalic and perhaps also certain cortical lesions. One is inclined to be less dogmatic where tonic or clonic convulsions of the jaws succeed violent fright, as in a case of trismus of nine months' duration recorded by Billot and Francotte. For that matter, trismus is met with in hysteria, and may be regarded as a manifestation of that disease, although this cannot always be invoked as its cause. We are not attracted by Kocher's idea of assigning it to an "idiopathic spastic neurosis," preferring to ally it to tics of the tonic variety. Among the crowd of circumstances that reflexly give rise to trismus may be enumerated abscess, caries, alveolo-dental periostitis, eruption of the wisdom teeth, disease of the maxilla and the neighbouring soft parts, and less commonly myositis or injury to the masseters. But so long as any one of these causes is in operation, and especially if the affection be attended with pain, we are dealing with a trismus spasm, not a trismus tic. S., whose psychical imperfections have already formed the subject of remark, supplies an example of the combination of mental trismus and torticollis, the former being the outcome of an inopportunely reiterated voluntary act, and therefore comparable to the tics.