T
he intermarriage of blood-relations is doubtless one cause. In one school for the deaf and dumb 25 per cent., in another 20 per cent., and in others 15 per cent. of the pupils are said to be the off-spring of marriages between blood-relations.

Davy mentions the following case observed by MeniÈre:—A married couple, being cousins, who enjoyed excellent health, had eight children, of whom four were born deaf mutes, another was idiotic, another died when five years of age, and two others suffered from absolute deafness, which only made its appearance later on.

In the Institution at Derby there are three sisters, and also a younger sister at home, all born deaf and dumb, the parents being own cousins. On the other hand, there are at the same Institution several children having deaf and dumb brothers or sisters, where no relationship exists between the parents, notably one family, in which both parents are healthy, and in which there are four deaf and dumb children. In the same county there are other cases of three, four, and even six in a family, yet there has been no relationship between the parents. In addition to the born deaf, measles, bronchitis, typhus, scarletina, and other diseases are the causes of deafness, and consequently dumbness.

Cases are known to have resulted from lightning, fright, boxing on the ears, and where young children have been allowed to fall on the head, etc.

Damp houses are said to be a cause of deafness. In Paris among eight children born in a family, five were born deaf, and these had all been born in a damp house. The family who had previously lived in the same house had three children, among whom were two born deaf and dumb. It is sometimes said that certain unhealthy trades in which the parent or parents are engaged are responsible for some of these cases, but the only complete statistics are those of Nassau, the results of which are as follows:—

Meckel says that in the families of tradespeople, who are constantly exposed to a damp unhealthy atmosphere or other injurious influences, deaf-mutism occurs most frequently. Moreover, Meckel has found that deaf-mutism is more frequently met with in flour-mills than elsewhere. Among 990 millers in Nassau, there were found eight deaf mutes, or one deaf mute to 123.7.