27 
TUBERCULOSIS 
As  REGARDS  HEREDITY  IN  CAUSATION  AND 
Elimination  from  Infected  Herds. 
PART  i. 
The  Importance  of  Heredity  in  the  Causation  of 
Tuberculosis. 
Before  proceeding  to  discuss  the  question  whether  the 
hereditary  transmission  of  tuberculosis  plays  a part  of  any 
importance  in  the  dissemination  of  the  disease,  it  is  necessary 
to  draw  a clear  distinction  between  the  inheritance  of  the 
disease  itself  and  the  inheritance  of  a special  predisposition  to 
it.  When  tuberculosis  is  inherited  in  the  former  sense  the 
young  animal  comes  into  the  world  with  tubercle  bacilli 
already  in  its  system,  these  bacilli  having  been  furnished  by 
one  or  other  of  the  parents.  When  predisposition  only  is 
inherited  the  young  animal  is  born  free  from  the  bacilli,  but 
it  possesses  by  descent  the  exceptionally  low  power  of  resisting 
infection  which  characterised  its  parents.  It  is  to  be  observed 
that  in  this  latter  case  it  is  not  necessary  that  the  parents 
themselves  should  be  actually  tuberculous,  for,  although 
specially  predisposed,  by  accident  or  design  they  may  never 
have  been  exposed  to  any  chance  of  infection. 
The  belief  that  tuberculosis  is  frequently  inherited  dates  from 
a time  when  it  was  not  known,  and  for  the  most  part  not  even 
suspected,  that  the  disease  was  caused  by  a germ  or  bacterium, 
and  hence  in  the  first  instance  it  was  supposed  to  be  the 
predisposition  or  susceptibility  that  was  transmitted  from  the 
tuberculous  parent  to  the  offspring.  At  the  time  here  referred 
to  very  vague  notions  were  prevalent  regarding  the  actual 
nature  of  tuberculous  disease,  but  the  greatest  importance  was 
assigned  to  a peculiar  weakness  of  the  cells  or  tissues,  in 
consequence  of  which  inflammations  arising  from  various 
causes  assumed  a specially  pernicious  course,  with  little  or  no 
tendency  to  recovery,  such  as  they  exhibited  in  ordinary 
individuals.  A fact  which  everywhere  forced  itself  upon 
attention  was  that  the  disease  among  human  beings  often  ran 
in  families  ; and  quite  naturally,  since  there  was  no  suspicion 
that  the  disease  was  contagious,  this  fact  was  ascribed  to  the 
hereditary  transmission  of  the  peculiar  weakness  which  was 
regarded  as  the  main  factor  in  the  causation  of  tuberculosis. 
The  question,  of  course,  assumed  an  entirely  new  aspect 
when  Koch  discovered  the  tubercle  bacillus,  and  proved  that 
the  essence  of  the  disease  was  the  introduction  of  this 
