664 
ME.  J.  LISTEE  ON  THE  EAELT  STAOES  OF  IXFLA3niATI0N. 
contraction,  yet  the  capillaries  retained  their  full  average  dimensions.  After  a while 
the  artery  became  so  much  more  contracted  as  only  to  admit  single  corpuscles  even 
through  the  main  trunk ; yet  still  the  capillaries  fed  by  it  did  not  appear  affected  in 
calibre.  This  is  but  one  example  of  what  I have  observed  times  without  number. 
The  capillaries,  though  not  contractile,  are  highly  elastic,  and  by  virtue  of  this  pro- 
perty are  capable  of  considerable  variation  in  capacity,  according  to  the  distending  force 
of  the  current  of  blood.  Figs.  3 and  4 of  Plate  XL VII.,  traced  with  the  camera  lucida, 
show,  besides  the  pigment  in  two  chromatophorous  cells  of  the  frog’s  foot,  part  of  a 
capillary  in  nearly  extreme  conditions  in  point  of  calibre.  In  fig.  3 the  vessel  is  about 
equal  in  diameter  to  the  length  of  a red  corpuscle,  while  in  fig.  4 it  is  so  narrow  that 
the  corpuscles  in  it  are  pinched  transversely  and  elongated.  VTien  the  capillaries  are 
most  distended,  their  parietes  are  much  thinner  than  when  shrunk  to  theii'  smallest 
dimensions;  an  estimate  may  be  formed  of  the  difference  by  comparing  the  close 
proximity  of  the  corpuscles  to  the  outer  bounding  line  of  the  vessel  in  fig.  3 -with  the 
considerable  interval  in  fig.  4,  that  interval  representing  the  apparent  thickness  of  the 
wall  of  the  vessel.  It  is  to  be  observed  that  the  frog  had  been  killed  in  a manner  in- 
volving considerable  hemorrhage  before  fig.  4 was  traced,  so  that  the  capillaries  were  then 
little,  if  at  all,  distended  with  blood.  The  thinness  of  the  Avails  of  the  capillaries,  as  com- 
pared with  the  small  arteries,  is,  doubtless,  calculated  to  favour  the  mutual  interchanges 
which  must  take  place  between  the  blood  in  them  and  the  tissues  in  their  Aicinity. 
It  is  believed  by  some  eminent  authorities  that  mutual  attractions  and  repulsions  sub- 
sisting between  the  nutrient  fiuid  and  the  tissues  among  which  it  fiows,  become  a soince 
of  movement  in  the  blood  and  assist  its  flow  through  the  capillaries ; while  others  regard 
the  heart  as  the  sole  cause  of  the  cnculation:  and  the  difference  of  opmion  on  this 
fundamental  point  in  physiology  involves  discordance  in  pathological  theory,  for  some 
who  hold  the  former  view  consider  the  changes  which  occur  in  the  ch’culation  at  the 
commencement  of  inflammation,  to  be  principally  owing  to  modifications  of  the  ‘ Altai  ’ 
moAing  force*.  The  view  that  such  a cause  of  movement  exists,  has  been  supported 
pai’tly  by  argument  draAvn  from  the  phenomena  of  mflammation : but  these,  as  we  shall 
■*  See  ‘ Outlines  of  Pathology  and  Practice  of  Medicine,’  by  W.  P.  Alisox,  M.D.,  F.E.S.E.,  &c.  1844, 
pp.  115  et  seq. 
