The reports of various investigators, as to the relative number of deaf-mutes with total deafness, differ considerably, for, while some have found that only about one-fourth of the deaf-mutes ex amined were totally deaf, others have found a much larger proportion, the principal cause of this discrepancy being probably the fact that there is generally a distinct relationship between the deaf ness and its cause. This relationship is most distinctly seen by comparing the power of hearing of congenital deaf mutes with that of deaf-mutes with ac quired deafness. All investigators, with a few exceptions, have, namely, found a much greater number of cases of total deafness among deaf-mutes with ac quired deafness than among deaf-mutes with congenital deafness.
The reason why so many more cases of total deafness are met with among deaf mutes with acquired deafness than among those with congenital deafness is owed to the fact that post-natal proc esses in the ear causing deafness are much more destructive than the same processes occurring during fcetal life: a circumstance which has been previously pointed out. Most authors have also found that congenital deaf-mutes are more frequently in possession of a con siderable degree of hearing (hearing of vowels or even of words) than deaf-mutes with acquired deafness.
It may be mentioned, finally, that Bezold examined the hearing power of deaf-mutes by means of a graduated series of tuning-forks and found that frequently "islands" of perception of notes alternated with total defects of hearing. These defects appeared most frequently in the lower end of the scale —a fact which has been corroborated by L`chermann.
Munsm.-111utism was in early times believed to be the primary and essential symptom of deaf-mutism, but it is known now to be a secondary phenomena which is the consequence of the deafness. That this is the case is also evident, from the fact that the degree of mutism is, as a rule, in exact relation to the degree of deafness, and also to the period at which the deafness makes its appearance. Thus congenital deafness, or deafness acquired in early infancy, is always accompanied by complete mutism (excepting in cases in which the mutism is removed by special methods of education), while in eases of acquired deafness, in which the deafness is either not total or arises after the child has learned to speak., a certain degree of speech is respectively acquired or retained. The explanation is simple, speech being, under normal circum stances, acquired through the ear, the child imitating the words which it hears spoken by those about it. It may, how
ever, be mentioned that even children totally devoid of hearing produce sounds which sometimes resemble words, such as "ma-ma," "ba-ba," etc., and sometimes also imitate animals, often thus causing their friends to suppose that they are capable of hearing. This may be because the above-mentioned sounds and the voices of certain animals are produced by very simple movements of the vocal organs which can be imitated by spontaneous observation. Finally, it is possible that the vibrations caused by such loud sounds as the barking of a dog, bellowing of a cow, etc., may be perceived by the aid of touch, which sense is often highly developed in deaf children, and consequently guides them in imitating the sounds.
The question as to the degree of deaf ness which must exist, or, in acquired cases, the age at which the deafness must appear in order to cause mutism result ing in deaf-mutism, cannot be answered decidedly. To begin with, the applica tion of the term "deaf-mutism" is en tirely arbitrary in cases in which there is some power of hearing or of speech, and the distinction between a deaf-mute child and a child with deficient power of hear ing must in some cases depend entirely upon practical considerations, of which the method of instruction which is requi site for the child's education is, as a rule, decisive. Thus, for instance, a child of well-to-do parents, who is able to hear tunes and to a certain extent reproduce them, will scarcely be considered deaf and dumb and sent to an asylum, while a child with the same degree of hearing, but of poor parents, will be treated as a deaf-mute, because the parents are un able to give it the special education which it requires. The non-development or deficient development of the power of speech in cases of congenital partial deaf ness, and its complete or partial loss in cases of acquired deafness, are also often dependent upon the assiduity with which a child's friends attend to its develop ment or preservation. Some children, too, seem to have a greater aptitude for developing or retaining the power of speech than others, and this seems to be not only dependent upon their intellect ual faculties, but also upon other un known conditions. Thus, a child with comparatively very slight power of hear ing, or with deafness acquired soon after birth, may exhibit a comparatively con siderable power of speech, while another child with greater powers of hearing and later acquired deafness may be entirely without it.