For this reason we think it preferable to exclude stammering, stuttering, and all defects of phonation or articulation whose existence is revealed only in the act of speaking. At the same time reference must be made to facts linking these functional anomalies to the tics, and to instances of the latter existing with or succeeding the former. Such is the case with stammering. According to Letulle,[118] stammering is a tic of speech whose beginning is a functional disturbance of nervous centres, as is that of tics in general. Holding as we do, however, that one of the features of tic is its appearance in season and out of season, we cannot class stammering as a tic, since its exhibition is restricted to the exercise of a certain function, viz. speech. It is therefore comparable to a "professional cramp," and we may briefly note the analogies it offers to the tics. Stammering,[119] which in more than fifty per cent. of cases is hereditary, and associated with a neuropathic diathesis, usually betrays itself in childhood and becomes aggravated at puberty. The old idea which credited stammerers with exceptional intellectual powers, in whom, however, rapidity of thought surpassed rapidity of action on the part of the muscles of articulation, is exploded, and to-day those thus afflicted are assigned their true place among the volitionally infirm. In a few rare cases stammering has been due to organic disease of the centres for articulate speech, or of bulbo-pontine nuclei; it has been supposed also to result from genuine spasm on a reflex arc, and this is a possible explanation; as a general rule, however, the pathogeny of stammering is identical with that of tic. Its dependence on such affections of nose, larynx, and pharynx as hypertrophic rhinitis and adenoids has been emphasised by Biaggi[120]; and Derevoge,[121] in directing attention to the association of volitional enfeeblement with respiratory troubles, remarks that stammerers sometimes have a phobia for certain words. Many observers have been convinced of the psychical nature of the affection from the fact of its disappearance during singing, as well as from the effect anger, elation, and other stimuli have in momentarily inhibiting it. The same is of course true of the tics.