When  the  mother  has  contracted  an  acute  infection,  as  measlea  or  enteric
fever,  the  child  may  be  born  during  the  period  of  incubation  or  with  the
symptoms  of  the  disease  already  manifest.  A  number  of  nervous  diseases
are  clearly  hereditary.  As  examples  may  be  mentioned  progressive  mus-
cular atrophy,  hereditary  chorea,  Friedreich's  ataxia,  and  migraine.  The
definite  symptoms  may  not  show  themselves  for  some  years  after  birth,
in  some  cases  not  until  adult  life.  Much  more  commonly  it  is  the  pre-
disposition that  is  transmitted.  This  is  especially  the  case  in  tuberculosis.
The  peculiar  exposure  of  the  young  infant  to  infection  from  a  tuberculous
mother  and  the  length  of  time  that  the  tuberculous  lesions  in  many  in-
stances remain  locaHzed  render  it  in  the  highest  degree  probable  that  the
predisposition  to  tuberculosis  rather  than  the  disease  itself  is  hereditary.
This  view  is  confirmed  by  the  results  of  pathological  and  bacteriological
investigations.  The  direct  transmission  of  tuberculosis  from  the  mother
to  the  foetus  in  the  human  being  is  of  uncommon  occurrence.  The  doc-
trine of  the  direct  hereditary  transmission  of  tuberculosis,  so  long  enter-
tained but  now  fortunately  abandoned,  was  a  stumbling  block  in  the  way
of  the  recognition  of  the  infectious  character  of  this  disease.  That  the
predisposition  rather  than  the  disease  is  hereditary  is  also  true  of  cancer.
