212 



MEMOIRS OF THE NATIONAL ACADEMY OF SOIBNOES. 



The ordinates of the ciuves represent, respectively, the number of congenital and non-con- 

 genitiil deaf-mutes who became deaf in the decades indicated by the abscissje. In the case of the 

 congenital deaf-mutes the ordinates also represent the number who were born in the decades given, 

 but this is not true of the uon-cougeuitals. It will be observed that the number of deaf-mutes re- 

 turned who became deaf in the last decade, 1871-'80, is less than the number who became deaf in 

 the preceding decade. This does not necessarily mean that the number actually was less, but more 

 probably indicates that the returns for the last decade are imperfect. Mr. Wines says that "In 

 proi)ortion to the degree of their youth the younger deaf-mutes are not enumerated. Fewer deaf- 

 mutes who are babes in arms are enumerated than at the age of three years, and fewer at three 

 years than at seven. The apparent maximum at seven is not the actual maximum ; the actual 

 maximum is at some younger age not yet ascertained." 



In the above diagram those portions of the curves that are believed to be unreliable from this 

 cause are indicated by dotted lines. 



It will be observed that among the older deaf mutes the congenitals are more numerous than 

 the noTi-congenitals ; whereas among the younger the reverse appears to be the case. There is no 

 apparent diminution in the numbers of the cougenitally deaf born of late years; and the reversal 

 of the relation between the two classes must be attributed to an abnormal increase iu the number 

 of those who became deaf from disease or accident. It looks as if a wave of deafness-producing 

 disease had swept over the continent about the time of the late civil war. 



Fli:. 9.— The ilark lines indicate tliose pupils who were liorn ileuf, and the light lines those who bceanie deaf from disease or accident. 



