554 
Diseases  of  Cattle  and  Sheep. 
and  so  on,  dotted  about  in  this  singular  manner.  Sometimes, 
after  having  carried  off  several  cattle  from  a farm,  it  would 
absent  itself  for  a few  weeks,  and  then  suddenly  return  without 
any  assignable  reason.  Such  temporary  absence  cannot  be  ac- 
counted for  in  many  instances,  otherwise  than  by  supposing  the 
germs  of  the  disease  to  have  during  that  time  been  brooding  ; and 
this  by  the  by  seems  to  favour  the  opinion  now  gaining  ground 
amongst  medical  practitioners,  that  malignant  epidemics  are  the 
result  of  the  operation  of  living  parasites — vegetable  or  animal — 
disturbing  in  sundry  ways  the  functions  and  structures  of  the 
body,  each  after  its  own  kind.  The  parasitic  theory  receives 
some  support  from  the  well-known  fact  that  the  pleuro-pneumonia 
was  increased  in  virulence  in  crowded,  dirty,  dairy  establishments 
of  the  metropolis  and  other  large  towns.  Nothing  tends  to  pro- 
mote the  spread  of  infectious  disease  more  than  the  crowding 
together  several  animals  who  are  suffering  under  it.  Each  one  is 
a separate  source  of  contagion,  and,  if  these  sources  are  multiplied 
in  a cow-house,  the  air  will  be  contaminated  in  proportion. 
Warmth , closeness,  and  filth  increase  the  virulence  of  contagion,  of 
whatever  hind  it  mag  be,  and  become  as  it  were  a nursery  of  pesti- 
lence. 
The  first  operation  of  malaria  and  aerial  poisons  is  evidently 
on  the  blood,  affecting  both  its  quality  and  distribution,  which  be- 
comes frequently  darker  in  colour  and  otherwise  altered,  and 
accumulates  to  an  extraordinary  amount  in  some  particular  organ 
or  structure.  Hence  the  lungs  and  its  membranes  are  the  seat  of 
pleuro-pneumonia,  and  here  we  have  congestion  with  dropsical 
effusion  of  lymph  and  serum  into  the  interlobular  structure  of  the 
lungs  and  the  cavity  of  the  chest.  In  the  “ variola  ovinafi  or 
“ sheep-pox ,”  the  poison  spreads  its  destructive  ravages  on  the  skin  : 
the  internal  affections  in  this  dreadful  disease  are  simply  inflam- 
matory, and  do  not  partake  of  the  specific  character  of  the  cuta- 
neous disease.  The  last  disease  may  be  promulgated  either  by 
inoculation  or  diffusion  through  the  air. 
4.  Inattention  to  contagious  diseases  has  occasioned 
much  mischief  and  loss  of  property  ; for  there  cannot  be  a ques- 
tion, if  but  proper  precautions  were  taken  on  the  first  appearance 
of  a visitation  of  this  kind  in  a neighbourhood,  that  its  ravages 
might  be  considerably  mitigated,  if  not  altogether  prevented  in 
many  instances. 
When  a disease  of  an  infectious  character  threatens  a farmer, 
lie  should  immediately  establish  a rigid  quarantine,  and  admit  no 
new  animals  into  stock  until  several  weeks  have  established  their 
freedom  from  any  unhealthy  symptoms.  The  wide  spread  of 
pleuro-pneumonia  was  occasioned  by  parties  sending  their  beasts 
