30 
Tuberculosis. 
and  179  of  these,  or  12  per  cent.,  were  diseased.  At  Schwerin, 
from  1893  to  1903,  48,449  calves  were  slaughtered,  of  which 
103,  or  '21  per  cent.,  were  tuberculous. 
One  might  have  supposed  that  in  face  of  the  evidence 
furnished  by  these  slaughter-house  statistics  the  view  that  any 
considerable  proportion  of  the  progeny  of  tuberculous  cows  are 
infected  prior  to  birth  would  have  been  generally  abandoned. 
As  a matter  of  fact,  however,  imagination  has  suggested  a 
method  of  accounting  for  the  rarity  of  tuberculosis  in  new-born 
calves  while  still  maintaining  that  the  direct  inheritance  of  the 
disease  is  common  and  practically  important.  It  is  contended 
that  the  absence  of  visible  lesions  in  new-born  calves  ought 
never  to  be  accepted  as  evidence  that  the  young  animal  is  not 
infected,  inasmuch  as  the  tubercle  bacilli  may  be  present  in 
what  is  called  a latent  condition.  Furthermore,  it  is  held  that 
the  frequent  development  of  actual  tuberculous  disease  in  the 
progeny  of  tuberculous  parents  before  the  period  of  adult  life 
is  reached  harmonises  very  well  with  the  view  that  the  bacilli 
are  often  present  in  the  organs  or  tissues  of  new-born  animals 
in  which  no  visible  lesions  can  be  detected  by  the  most  searching 
post-mortem  examination.  It  would  be  quite  permissible  to 
reject  this  view  as  a pure  hypothesis  in  support  of  which  no 
evidence  has  ever  been  produced.  Here,  however,  it  may  be 
well  to  state  other  reasons  for  declining  to  accept  it. 
The  possibility  of  latency  in  tuberculosis  must  be  admitted 
in  the  sense  that  when  a tubercle  bacillus  gains  access  to  any 
part  of  the  body  and  starts  to  multiply,  a certain  time  elapses 
before  the  resulting  tubercle  acquires  what  may  be  called 
naked-eye  size.  What  this  period  usually  is  is  well  known 
from  observations  made  on  animals  experimentally  infected. 
Broadly  speaking,  it  is  some  three  or  four  weeks  at  the  outside. 
Knowledge  of  this  fact  compels  one  to  admit  that  probably 
some  of  the  apparently  sound  calves  of  tuberculous  cows  are 
born  infected,  the  infection  having  been  of  too  recent  a date 
to  permit  the  development  of  visible  tubercles.  This,  however, 
is  the  only  sense  in  which  latency  can  be  admitted,  and  even 
when  the  fullest  weight  is  allowed  to  it,  it  cannot  be  held  that 
the  fact  seriously  invalidates  the  conclusion  based  on  the 
proved  rarity  of  visible  lesions  in  new-born  calves. 
But  this  is  not  at  all  the  sense  in  which  it  is  maintained  that 
tuberculosis  is  latent  in  young  calves.  The  view  put  forward 
is  that  owing  to  some  peculiarity  of  the  tissues  in  such  young 
animals  any  tubercle  bacilli  present  are  restrained  from 
multiplying,  and  thus  lie  dormant  for  months  or  years,  like 
the  seeds  of  plants  kept  in  circumstances  unfavourable  for 
germination.  The  answer  to  this  contention  has  been  fur- 
nished by  numerous  experiments  which  show  that  when 
