ME.  J.  LISTEE  ON  THE  EAELY  STAGES  OE  INFLAMMATION. 
SECTION  II. 
On  the  Structure  and  Functions  of  the  Bloodvessels. 
An  acquaintance  with  the  anatomy  and  physiology  of  the  vascular  system  is  indis- 
pensable to  a successful  study  of  the  deviations  from  health  exhibited  in  the  circu- 
lation of  the  blood  through  the  vessels  of  an  inflamed  part ; it  is  not,  however, 
intended  to  give  here  a full  account  of  the  subject,  but  merely  to  dwell  upon  some 
important  points  on  which  differences  of  opinion  prevail. 
It  has  long  been  a debated  question  whether  or  not  the  capillaries  possess  contracti- 
lity, and  there  is  still  some  difference  of  opinion  among  authorities  upon  the  subject. 
With  a \iew  to  throwing  light  upon  this  important  point,  I investigated  carefully  the 
structure  of  the  minute  vessels  of  the  frog’s  foot ; dissecting  them  out  from  between  the 
layers  of  skin  composing  the  web,  so  as  to  render  their  constituent  material  capable  of 
clear  definition  with  the  microscope.  The  chief  results  have  been  communicated  to  the 
Koyal  Society  of  Edinbiu’gh,  in  a paper  that  vrill  shortly  appear  in  their  Transactions, 
‘•On  the  Structure  of  Involuntary  Muscular  Fibre*.”  I need  therefore  merely  repeat 
here,  that  while  the  capillaries  were  proved  to  consist,  as  has  been  long  known,  merely 
of  a delicate  homogeneous  membrane  beset  with  occasional  nuclei,  the  minute  arteries, 
some  of  them  even  less  in  calibre  than  average  capillaries,  were  found  to  possess  three 
distinct  coats,  namely,  an  external  layer  of  cellular  tissue,  in  variable  quantity,  longi- 
tudinally arranged,  an  internal  extremely  delicate  lining  membrane,  and  an  intermediate 
ch’cular  coat,  which  constituted  the  principal  bulk  of  the  vascular  parietes,  and  which, 
when  highly  magnified,  was  found  to  consist  of  a single  layer  of  muscular  fibre-cells, 
each  wound  spu’ally  round  the  internal  membrane  so  as  to  encircle  it  from  one  and  a 
half  to  two  and  a half  times. 
Now  when  we  consider  the  properties  of  muscular  fibre-cells,  which,  as  is  shown  in  the 
paper  referred  to,  are  capable  of  contracting  in  the  pig’s  intestine  as  much  as  to  one- 
tenth  of  their  length,  it  is  impos.sible  to  conceive  a more  efficient  mechanism  for  the 
constriction  of  a tube  than  is  provided  in  these  minute  arteries.  On  the  other  hand,  the 
capillaries  are  totally  destitute  of  any  structure  known  to  be  contractile.  The  changes 
of  calibre  which  occur  in  the  vessels  of  the  living  web  are  in  perfect  harmony  with  this 
anatomical  description ; for  while  the  arteries,  even  to  their  smallest  branches,  are  some- 
times constricted  to  absolute  closure,  and  at  other  times  widely  dilated,  the  capillaries 
are  never  found  to  be  entirely  closed,  nor  to  present  any  variations  in  diameter,  which 
are  not  explicable  by  elasticity  of  their  parietesf. 
The  sketch  at  the  top  of  the  next  page  represents  the  calibre  of  an  artery  dividing 
into  minute  branches,  with  the  capillaries  into  which  they  poured  their  blood.  At  the 
time  when  it  was  drawn,  the  artery  and  its  branches  were  in  a state  of  spontaneous 
* Transactions  of  the  Eoyal  Society  of  Edinburgh,  vol.  xxi.  pt.  4. 
t In  this  respect  I merely  confirm  the  observations  long  since  made  by  Messrs.  Paoet  and  Wharton 
Jones. 
