Future investigations will in all proba bility decide how far total acquired deaf ness results in total mutism. Hartmann states that deafness acquired before the age of seven causes secondary mutism, and this opinion is, no doubt, correct. On the other hand, there are reports from various places to the effect that deaf mutism may appear at the age of 14 or 15 or even later. In these cases, however, it is probable that the term deaf-mutism is incorrect, though, of course, such acci dental circumstances as feeble-minded ness, blindness, etc., may necessitate the registration of persons who have lost the power of hearing so late in life as deaf mutes, because they are unable to read from the lips, or unable to pronounce so distinctly fIS to be understood.
As mentioned above, mutism in deaf mutes may be either total—i.e., the power of speech may be entirely wanting—or it may be partial, in which latter case the power of speech is developed, or, in ac quired deaf-mutism, it is retained to a certain extent. This power of speech is frequently considerable; so that such persons cannot, properly speaking, be termed mutes. There are, however, cer tain peculiarities which always attach themselves to the speech even of persons who are only partially deaf from their birth, or who have become deaf during childhood. These peculiarities, which are still more pronounced in true deaf mutes, consist in the absence of accentua tion of syllables and of words, the result being that speech becomes monotonous. Besides this, the speech of such persons is generally dull-sounding and feeble, and the control of respiration is also de ficient. The stock of words is also some times limited, though this peculiarity is, under ordinary circumstances, not very noticeable, excepting in cases where the power of hearing is very slight, or where the deafness appears comparatively early.
These physical deficiencies in the speech of deaf-mutes are easily accounted for, because the power of hearing is not only important in the development of speech by enabling a child to imitate the speech of others, but it also enables it to regulate the modulation, sound, and force of its voice by the aid of the vibrations which reach the labyrinth through the bones of the cranium.
The power of hearing plays so great a part in the above-mentioned physical qualities of speech that its loss cannot be completely compensated for by any other sense. It is, however, possible, by aid of sight and touch, to teach a great number of deaf-mutes to speak well enough to be able to use speech as a means of communication. Persons who have been totally deaf from birth can also be taught, by a special method of instruction, to speak so that they can be understood, though with the peculiarities above mentioned. Owing to these pecul iarities, such speech has received the name of "articulation." It is not always an easy matter for the deaf-mute to re tain the power of speech which he has gained with so much difficulty, when he enters the world and comes in contact with persons who cannot, or can only partially, understand him. In such cases the deaf-mute generally abandons the use of speech as a means of communica tion, especially as lip-reading requires great attention and well-developed sight.