322  MR.  R.  OWEN'S  DESCRIPTION  OF  A  MICROSCOPIC  ENTOZOON 
part :  it  appears  to  be  a  layer  only  of  the  external  cyst,  which,  as  is  often  seen  in  cysts 
of  corresponding  structure  not  formed  by  Hydatids^,  is  more  or  less  detached  from  the 
outer  layer. 
In  almost  every  instance  in  which  I  have  succeeded  in  opening  the  cyst  without 
injury  to  the  worm  within,  it  has  been  expelled  entire,  together  with  the  fluid  matter 
surrounding  it,  by  pressure  upon  the  cyst.  Occasionally,  however,  a  part  of  the  worm 
remains  adherent ;  but  this  has  been  accompanied  with  a  glairy  adhesive  state  of  the  fluid 
secretion  of  the  cyst,  and  has  been,  I  believe,  dependent  upon  it ;  for  when  the  broken 
pieces  have  been  extracted  and  examined  with  a  high  power,  both  extremities  have 
presented  the  same  entire  surface  and  uniform  rounded  appearance  as  in  the  worms 
which  are  extracted  whole. 
The  structure  and  relations  of  the  cyst,  therefore,  and  the  absence  of  all  organical 
connexion  between  it  and  the  contained  worm,  lead  to  the  conclusion  that  the  cyst  is 
adventitious,  foreign  to  the  Entozoon,  and  composed  of  the  cellular  substance  of  the 
body  infested,  morbidly  altered  by  the  irritation  of  the  worm. 
From  the  tenacity  of  irritability  manifested  by  the  Trichina  under  circumstances  so 
opposite  to  those  under  which  it  was  developed,  from  its  small  size  compared  with  the 
cavity  of  the  cyst,  and  from  the  quantity  of  fluid  in  which  it  is  immersed,  it  is  highly 
probable  that  in  its  natural  condition  it  enjoys  active  powers  of  motion.  If  in  such 
movements  the  extremities  of  the  worm  were  repeatedly  pressed  against  the  surrounding 
capsule,  this  would  yield  and  become  elongated  in  the  directions  where  there  was  least 
resistance  ;  viz.  where  the  muscular  fasciculi  would  most  readily  separate,  and  obser- 
vation shows  that  it  is  in  the  direction  of  the  fasciculi  that  the  cysts  are  elongated. 
If  the  germ  of  a  Trichina,  or  a  portion  of  the  worm  separated  by  spontaneous  fission, 
be  deposited  at  the  end  of  one  of  the  elongated  axes  of  the  cyst,  it  might  in  the  process 
of  development  excite  the  adhesive  inflammation,  which  would  then  cut  off"  the  commu- 
nication between  the  smaller  cyst  and  that  of  the  parent  worm,  while  the  former  would 
be  stimulated  to  secrete  from  its  inner  surface  a  serous  fluid,  and  so  go  on  enlarging  in 
size,  through  the  influence  of  the  same  causes  as  occasioned  the  formation  of  the  cyst  of 
the  parent.  Smaller  cysts  of  different  sizes  are  occasionally  seen  thus  attached  to  one 
end  of  larger  cysts,  and  I  am  inclined  to  account  in  this  manner  for  their  formation. 
Cysts  filled  with  opake  matter  are  also  occasionally  seen.  In  these  the  worm  may 
have  perished,  or  its  germ,  after  exciting  the  cyst  to  secrete,  may  not  have  been  deve- 
loped, and  the  enlargement  of  the  cyst  may  be  occasioned  by  the  accumulation  of  its 
own  secretion.  But  these  appearances  are  not  sufficient  to  establish  the  independent 
vitality  or  existence  of  the  cyst,  in  opposition  to  those  analogies  which  so  plainly  point 
out  its  true  nature  and  origin. 
'  'Ilils  separation  of  cysts  alternating  with  a  secretion  of  fluid  is  the  cause  of  the  Pill-box  Hydatid  of 
Mr.  Hunter,  which  is  not  a  distinct  animal  or  true  Entozoon. 
