Another importarit classification of the deaf from an educationalpoint of view is based upon the degree of deafness. There are all degrees of deafness, extending from a slight impairment of the hearing to a total inability to perceive through the ear the loudest sounds. Those who are only slightly deaf, who can hear well enough to be taught in ordinary schools, do not come within the scope of this article; but among those who are so deaf that they must he taught by special methods there are great differences in the degree of deafness. They are thus divided into two classes: the totally deaf, who have no hearing or scarcely any, and the semi-deaf, who can hear more or less. In a few cases of the semi-deaf the hearing power, even when before instruction it was so slight as to have remained unnoticed, may be so developed, or rather educated, by suitable acoustic training, with or without the aid of appliances, that those who entered school as deaf-mutes may be graduated as hard-of-hearing persons. Even where this result is unattainable, any hearing that exists (especially if it is sufficient for the pupil to hear his own voice) is a great aid in acquiring natural and agreeable speech — an acquisition that is almost impossible when the deafness i$ total. The semi-deaf, like the semi-mute, ate sometimes exhibited to the public as deaf-mutts, and the speech of the semi-deaf and the lan guage of the semi-mute are shown as the re sult of instruction given in the school. Mitich credit is often due to the school in both cases, but far more credit for the excellence of the speech is due to the partial hearing that the child possesses, and for the excellence of language to the fact that it was acquired before hearing was lost. In justice to the real deaf mutes, who are less favored by nature, an d in the interest of truth, the existence of pairtial hearing and of speech naturally acquired ought always to be explained to visitors. The semi deaf pupil may also be semi-mute; in that case he has a twofold advantage over his congeni tally or quasi-congenitally and totally deaf classmate.
Causes of The division of the deaf into the congenitally and adventitiously deaf is an important one with respect to the causes of deafness. Congenital deafness is not al ways hereditary but heredity is probably its most frequent cause. For this cause to be effective it is not necessary that one or both of the parents should be deaf ; though both parents may hear, the existence of deafness in other relatives, as grandparents and more remote ancestors, uncles and aunts, brothers and sisters and cousins, is an indication that the deafness is hereditary. Consanguineous marriages often result in deaf children and a large proportion of these children are congenitally deaf. In these cases the deaf ness is probably hereditary. It is not necessary to believe that consanguineous marriage in itself is a true cause of deafness, but a tendency to deafness exists in many families; when two members of such a marry each other, that tendency is transmitted to their offspring with increased intensity and deafness is the result. Another frequent cause of congenital deafness, though one rarely found in the statis tics of °assigned') causes, is constitutional syphi lis. While this cause is prenatal, and the re sponsibility for its results rests directly upon one or both of the parents, the deafness is not to be classed as hereditary. Other non-heredi
tary causes of congenital deafness are probably alcoholism, tuberculosis, scrofula and other dis eases in one or both of the parents, but it has not yet been determined how far these causes are effective. The causes of adventitious deaf ness are many and various. Even the pre-natal influences above mentioned that cause congenital deafness sometimes produce, or combine with other causes to produce, adventitious deafness. Accidents, such as concussion, falls, blows on the ear, etc., are responsible for some cases; but a large majority of the adventitiously deaf have lost their hearing from some disease. The dis eases of childhood that most frequently cause deafness are scarlet fever, cerebrospinal menin gitis, brain fever, catarrh, measles and typhoid fever, in t 'order named.
Histo y and The enumerators of the irteenth census reported 44,519 °deaf and mbp and 189 were subsequently added for/good reasons in the office, making a total of 44;708 in the United States in 1910; but, as Mated in the preliminary bulletin of the Bureau published in 1915, °it was impossible to say just what this total represented; op the one hand it fell short in all probability of including all deaf mutes, and on the other it included many who were not deaf-mutes at Probably the ac tual number of the deaf in the United States ex ceeds the returns of the thirteenth census re ports. The number of the deaf bears a cer tain relation to the population, being far greater in countries wherein squalor, privation, medical ignorance or unhygienic conditions prevail. The ratio of deaf-mutes to population is as follows, for the different countries named: Sardinia., 1 to 760; Nova Scotia, 1 to 880; Ireland and Norway. 1 to 970; France, 1 to 1,200• Prussia, 1 to 1,675; England, 1 to 1,970; Holland, 1 to 2,000; United States, 1 to 2,400. Whether deaf ness in the United States is increasing or di minishing it is impossible to say positively, for unhappily the decennial censuses of the deaf seed sown by Cardano was slow in springing up and bearing fruit. In the great intellec tual progress bf the 16th and 17th centuries the deaf had but a scanty share. A few fortunate individuals among them were taught by Ponce, Bonet and Carrion in Spain ; Wallis, Holder and Baker in England; Amman in Holland; Pasche, Kerger, Raphael and Lasius in Germany ; Fay, Pereire and Vanin in France. Some of these teachers, especially Bonet, Wallis and Amman, and one who was not a teacher, Dalgarno in Scot land, published expositions of their methods. But it was not until the middle of the 18th cen tury that the human conscience was aroused to the duty of educating the deaf generally. The first great impulse in this direction was given by the benevolent Abbe de ]'Epee,, whose school for the deaf, established at Pars about 1760, speedily became famous throughout Europe. At nearly the same time, independently of De l'Epee and of each other, schools were begun by Samuel Heincke in Dresden and Thomas Braid wood in Edinburgh. Similar schools soon fol lowed in other countries and they have contin ued to increase to the present time. There are now about 670 schools for the deaf in the world.