124 HEREDITY IN RELATION TO EUGENICS 



mucous membrane of the throat — ^in some respects the most 

 vulnerable portion of the human body. Hence it is subject 

 to the weaknesses of that membrane. On account of its 

 very complexity it is especially liable to exhibit deformations 

 or deficiencies,^ In view of the great variety of changes any 

 one of which may result in deafness it is clear that deafness 

 can hardly be a unit defect. Consequently it will not be 

 inherited as a simple character. 



The facts justify the a priori conclusions. Deafness of 

 certain sorts is clearly hereditary but it is not possible to 

 predict certainly the outcome of a particular mating. Never- 

 theless something can be done; and it will be worth while 

 to learn what is known of the actual incidence of deafness in 

 the offspring of deaf parents. 



Inheritable deafness is of three general types, (a) That 

 due to defects or changes before birth or shortly after, giving 

 rise to deaf mutism; (b) otosclerosis^ or hardness of hearing, 

 with usually progressive symptoms; (c) catarrhal weakness 

 of the mucous membranes, rendering them hable to infection 

 with inflamma,tion and suppuration. 



a. Deaf Mutism. — This kind of deafness is characterized 

 by its early appearance in life, before speech has been ac- 

 quired. It is the less likely, consequently, to be due to dis- 

 ease and, as a matter of fact, it is that form which shows 

 clearest evidence of pure inheritance. So clear is the evi- 

 dence of inheritance of congenital deafness that some coun- 



1 Politzer (1807) gives among others the following anatomical causes of con- 

 genital deafness : impaired development or absence of middle ear, defects and 

 rachitic deformities of the labyrinthine windows; narrowing of the recess of 

 the round window to a cleft with connective tissue; atresia of the same; atrophy 

 of the cochlear nerve and spinal ganglion in the first turn of the cochlea; ab- 

 normalities of the membrance of the otoliths, organ of Corti and ductus coch- 

 learis; faulty development of the sensory epithelium; defects of the crista and 

 sulcus spiralis; lack of development of the labyrinth and of the auditory nerve; 

 malformations of the central nervous system. In addition there are numerous 

 changes in structure due to inflanunations. 



