658 
ME.  J.  LISTEE  ON  THE  EAELT  STAGES  OE  ENTLA^OIATIOX. 
spicuous  do  the  veins  become  in  consequence  of  this  change  in  the  quality  of  their  con- 
tents, that  it  may  be  extremely  dithcult  to  distinguish  them  from  other  tissues ; the 
appearance  of  the  web  on  superficial  observation  being  as  if  it  possessed  no  blood-vessels 
at  all.  This  remarkable  condition,  which,  so  far  as  I know,  has  not  been  before 
described,  may  last  for  several  minutes  in  consequence  of  irritation  of  the  cord,  and  in 
one  case  I observed  it  occur  spontaneously,  and  continue  for  five  minutes  together.  It 
appears  to  be  comparable  to  the  dead  whiteness  of  the  human  fingers  when  benumbed 
with  cold,  or  the  perfect  pallor  of  the  cheek  in  faintness ; while  blushing  is  no  doubt 
caused  by  full  dilatation  of  the  arteries. 
Such,  according  to  my  experience,  are  the  efiects  produced  upon  the  cuculation  by 
changes  of  calibre  in  the  vessels  of  a perfectly  healthy  web.  The  arteries  regulate  by 
their  contractility  the  amount  of  blood  transmitted  in  a given  time  through  the  capil- 
laries, but  neither  full  dilatation,  extreme  constriction,  nor  any  intermediate  state  of  the 
former  is  capable  ^er  se  of  producing  accumulation  of  corpuscles  in  the  latter. 
SECTION  III. 
On  the  Effects  of  Irritants  upon  the  Circulation. 
It  is  well  known  that  the  application  of  an  irritant  substance  to  the  web  of  the  fi-og's 
foot  is  followed  by  changes  of  calibre  in  the  blood-vessels,  and  also  by  an  abnormal 
accumulation  within  them  of  the  corpuscular  elements  of  the  blood.  The  first  experi- 
ments which  I performed  upon  the  frog  Avere  directed  to  the  solution  of  the  much- 
debated  question,  whether  or  no  the  latter  were  a mere  consequence  of  the  former ; and 
although  it  has,  I think,  been  sufficiently  shown  at  the  conclusion  of  the  last  section  that 
such  cannot  be  the  case,  yet  it  will  be  well  to  allude  shortly  to  these  experiments  on 
account  of  their  further  bearing  upon  the  subject  of  this  inquiry. 
It  occurred  to  me,  that  if,  instead  of  the  powerful  irritants  commonly  used  in  these 
investigations,  some  exceedingly  mild  stimulant  Avere  employed,  the  changes  in  the 
calibre  of  the  vessels  might  perhaps  be  produced  Avithout  concomitant  alterations  of  the 
blood.  The  material  which  appeared  most  suitable  for  this  pui'pose  Avas  warm  water, 
which  is  known  to  cause,  in  the  human  subject,  increased  redness  AAithout  infiammatiou 
of  the  part  to  which  it  is  applied. 
Accordingly,  in  September  1855  I endeavoured  to  ascertain  its  effects  upon  the  frog. 
In  most  of  the  experiments,  the  foot  of  the  animal  being  stretched  under  the  micro- 
scope upon  a glass  plate  somewhat  inclined,  so  that  any  fluid  upon  it  might  run  off 
quickly,  an  assistant  threw  a stream  of  water  of  knoAvn  temperature  upon  it  by  means 
of  a syringe,  the  eye  of  the  observer  being  kept  over  the  microscope,  AA'hich  Avas  proAided 
Avith  a micrometer  in  its  eyepiece.  In  this  way  the  effects  produced  by  the  AAUter  could 
be  seen  almost  immediately  after  it  had  ceased  to  play  upon  the  Aveb,  and  the  changes 
of  calibre  in  any  artery  selected  for  observation  Avere  noted  AAith  precision.  It  Avas 
found  that  the  result  of  the  warm  application  Avas  constriction  ot  the  arteries  to  absolute 
