HEREDITY IN THE CASE OF DEMENTIA 
PRAECOX AND RELATED DISEASES 
BY SOREN HANSEN 
COPENHAGEN ” 
eB studying the inheritance of mental disease it is necessary to remem- 
ber, that what we call mental disease is only the manifestation of 
some faulty condition in the brain, about whose nature and existence 
we usually know practically nothing. This has been so often said, that 
it may seem superfluous repetition, yet it is necessary for the reason that 
just in this field the inherited conditions are so difficult to unravel, that 
the starting point must be perfectly clear if the work is to lead to results 
in any way satisfactory. The material we work with is of such a 
nature that the greatest care is needed. The apparent symptoms of 
disease are in great part not easily distinguished from quite simple men- 
tal disturbances and are often blurred by complications of very dif- 
ferent nature. Even in distinct cases of inherited mental diseases it is . 
extremely difficult to determine how great a part heredity plays and 
what importance should be ascribed to the various outer and inner 
participating causes, which must of necessity be considered. The aim 
in the study of the inheritance of mental disease is not however to 
deepen or further our knowledge of heredity itself. Such a material is 
not suited for that purpose, but by reasonable treatment it may throw 
light on the causes of the inherited diseases and their mutual connec- 
tion; and to lighten this task it is both permissible and necessary to 
keep to the manifestations and exclude their still unknown, deeper 
causes. 
The mental disease dealt with here, owing to its great frequency 
and serious character, is the one which has the greatest interest of all. 
Like most of the other diseases it is not a sharply defined entity; it 
must rather be regarded as a group of nearly related diseases, without 
our knowing however, what it is that determines the differences in the 
appearance of the disease. It may be that different outer or inner 
causes modify the same basal weakness, or there may be a different 
localisation of the same diseased condition in the central nervous sy- 
stem or a different diseased condition in the same part of this, but 
