764 KEPORT— 1889. 



The eye is not the only organ of sense wliicli exhibits a tendency to the pro- 

 duction of hereditary congenital defects. The ear is similarly affected, and inti- 

 mately associated with congenital deafness is an inability to speak articulately, 

 •which occasions the condition termed Deaf-mutism. Statisticians have given some 

 attention to this subject, both as regards its relative fi-equency and its hereditary 

 character. The writer of the article ' Vital Statistics,' in the Report of the Irish 

 Census Commissioners during the decades ending 1851, 1861, 1871, has discussed at 

 some length the subject of congenital deaf-mutism, and has produced a mass of 

 evidence which proves that it is often hereditarily transmitted. In the Census 

 Report for 1871,' 3,297 persons were returned as belonging to this class, and in 393 

 cases the previous or collateral branches of the family were also mute. In 211 of 

 these the condition was transmitted through the father ; in 182 through the mother. 

 In 2,579 cases there was one deaf-mute in a family ; in 379 instances, two ; in 191 

 families, three ; in 53, four ; in 21, five ; in 5, six ; and in each of two families no 

 fewer than seven deaf-mutes were born of the same parents. In one of these two 

 families neither hereditary predisposition nor any other probable physiological or 

 pathological reason was assigned to account for the peculiarity, but in the other 

 family the parents were first cousins. Mr. David Buxton, who has paid great 

 attention to this subject,- states that the probability of congenital deafness in the 

 ■offspi'ing is nearly seven times greater when both parents are deaf than when only 

 one is so ; in the latter case the chance of a child being born deaf is less than 

 three-quarters percent.; in the former, the chances are that 5 per cent, of the 

 children will be deaf-mutes. Mr. Buxton refers to several families where the 

 deaf-mutism has been transmitted through three successive generations, though 

 in some instances the affection passes over one generation to re-appear in the next. 

 He also relates a case of a familj' of sixteen persons, eight of whom were born deaf 

 and dumb, and one at least of the members of which transmitted the affection to 

 his descendants as far as the third generation. There can be little doubt that con- 

 genital deaf-mutism, in the great majority of instances, is associated veith a 

 defective development, and therefore a structural variation of the organ of hearing, 

 though in some cases, perhaps, the defect may be in the development of the brain 

 itself. 



Although a sufficient number of cases has now been put on record to prove that 

 in some families one or other kind of congenital deformity may be hereditarily 

 transmitted, j^et I do not wish it to be supposed that congenital malformations may 

 not arise in individuals in whom no hereditary tendency can be traced. It is 

 undoubtedly true that family histories are in many cases very defective, and fre- 

 quently cannot be followed back for more than one, or, at the most, two genera- 

 tions ; so that it is not unlikely that an hereditary predisposition may exist in many 

 instances where it cannot be proved. Still, allowing even for a considerable 

 proportion of such cases, a sufficient number will remain to warrant the statement 

 that malformations or va.riations in structure which have not been displayed by 

 their ancestors may arise in individuals belonging to a particular generation. 



The variations which I have spoken of as congenital malformations arise, as a 

 rule, before the time of birth, during the early development of the individual; but 

 there is an important class of cases, in which the evidence for hereditary trans- 

 mission is more or less strong, which may not exhibit their peculiarities until 

 months, or even years, after the birth of the individual. This class is spoken of as 

 Hereditary Diseases, and the structural and functional changes which they produce 

 exercise most momentous influences. Sometimes these diseases may occasion 

 changes in the tissues and organs of the body of considerable magnitude, but at 

 other times the alteration is much more subtle, is molecular in its character, 

 requires the microscope for its determination, or is even incapable of being recog- 

 nised by that instrument. 



Had one been discussing the subject of hereditary disease twenty years ago, the 

 first example probably that would have been adduced would have been tubercu- 



' Vol. Ixxii., Part II., ' Report on the Status of Disease,' p. 1, 1873. 

 2 Idverjwol Medico- Chimrg. Journ., July 1857; Jan. 1859. 



