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HEREDITY IN THE CASE OF DEMENTIA PRÆCOX AND RELATED DISEASES 119 
or several — basal factors, which may have been present in each of 
the parents without manifesting itself in them or their antecedents. 
It is obvious, that the investigation of such families requires a con- 
siderable amount of preliminary work. Information regarding the more 
remote portion of the family which comes to the knowledge of the 
mental specialist, is usually and must necessarily be incomplete and 
uncertain, where it is a question of something so indefinite as weak- 
mindedness and lesser mental deficiencies; but the conception of the 
genetic connection between these and Dementia præcox, which is dealt 
with here, is greatly strengthened by the relation of this disease to 
other related diseases, and especially to the serious eye disease known 
by the name of Retinilis pigmentosa. Here we have very characteristic, 
somatic features in the picture of the disease, namely, a peculiar pig- 
ment degeneration in the retina, which leads to a progressive con- 
striction of the field of vision with hemeralopia,, which usually enables 
the disease to be easily diagnosed. But even here lighter forms may 
occur without very obvious changes in the retina, though as a rule 
these are only the incipient stages and thus have little interest. The 
distinct cases occur just like many other inherited diseases sometimes 
apparently quite isolated or in several members of a family, sometimes 
in more or fewer, successive generations. Exceptionally, but very 
rarely, the disease may skip one or several generations, but this is not 
in conflict with the fact, that it is inherited as a dominant charac- 
teristic, since like Dementia præcox it may be regarded as dihybrid or 
polyhybrid. Just as the dominant basal factors, on which it presumably 
depends, may be separated after being together through some gene- 
rations, they may also collect together again and become manifest 
without the disease therewith losing its dominant character. 
Since Retinitis pigmentosa occurs comparatively frequently in the 
same families as Dementia præcox and inherited imbecility, it is rea- 
sonable to conclude, that one of the dominant factors, which conditions 
it, is the same as that which connects Dementia preecox with imbecility, 
whilst the second is an accessory special factor, which conditions the 
degeneration in the peripheral organ. Again, the disease likewise 
appears comparatively frequently in families where there is also in- 
herited deafness and not rarely in the same persons. The somatic 
changes, which form the basis of this inherited deafness, are not clearly 
known, but it is probable that they consist in a degeneration of the 
nervus acusticus, corresponding to the degeneration of the nervus 
opticus in Retinitis pigmentosa, that is to say, it is only a matter 
