EEGIJLATING  THE  CONTRACTIONS  OE  THE  ARTERIES. 
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duced  upon  the  arteries ; one  selected  for  micrometrical  observation,  the  largest  of  the 
web,  measuring,  as  it  had  done  before,  nearly  4°,  which  was  a state  of  full  dilatation. 
I then  treated  in  a similar  manner  the  posterior  dark-coloured  portion  of  the  brain, 
including  the  optic  lobes,  cerebellum  and  medulla  oblongata,  which  were  not  distin- 
guished from  one  another  in  the  experiment.  As  I continued  this  treatment  for  a few 
seconds,  keeping  my  eye  over  the  microscope,  the  artery  became  contracted  to  1°, 
which  was  the  length  of  a red  corpuscle.  The  leg  then  became  spasmodically  extended, 
and  the  artery  was  carried  out  of  the  field ; but  when  I next  looked  at  the  web  after 
removal  of  the  needle,  the  vessels  had  dilated  again  to  pretty  full  size.  Having  selected 
a main  artery  of  another  web  more  conveniently  placed,  I repeated  the  experiment  of 
thrusting  the  needle  into  the  posterior  portion  of  the  brain.  This  vessel,  which  just 
before,  though  by  no  means  at  its  largest  size,  measured  2^°,  became  contracted  to 
almost  absolute  closure,  and  remained  so  till  the  needle  was  removed,  after  which  it 
gradually  dilated,  and  in  three  minutes  measured  2°;  forty  seconds  later  2|°;  and 
about  a minute  afterwards  3°.  The  experiment  was  repeated  several  times  with  similar 
results,  “ the  invariable  rule  ” (to  quote  from  my  notes)  “ being  contraction  of  the  artery 
up  to  a certain  point,  and  maintenance  in  the  contracted  state  during  the  whole  time, 
often  several  minutes,  that  the  needle  was  sthred  about  in  the  brain ; and  then  expan- 
sion, beginning  almost  immediately  after  withdrawal  of  the  needle,  and  advancing  to  a 
certain  point  at  which  it  remained  till  the  needle  was  again  introduced.”  As  the  brain 
became  more  and  more  broken  up,  the  contractions  grew  less  and  less  energetic,  and  the 
dilatations  were  increased,  till  the  needle  failed  to  produce  greater  contraction  than  from 
4°  to  3°.  I then  thrust  the  needle  into  the  spinal  canal  and  withdrew  it  immediately. 
The  hind  legs  started,  and,  after  a few  seconds,  when  I first  caught  sight  of  one  of  the 
webs,  it  was  almost  bloodless,  and  the  arteries  were  invisible  through  extreme  constric- 
tion. Four  minutes  later  the  artery  before  observed  had  begun  to  dilate  and  mea- 
sured 1°,  and  after  five  minutes  more  it  was  3°.  A repetition  of  this  experiment 
produced  similar  effects*. 
* The  constriction  of  the  arteries  of  the  webs  on  irritation  of  the  cord  may  be  readily  demonstrated  in 
the  following  simple  manner.  The  head  of  the  frog  being  depressed  so  as  to  stretch  the  ligament  between 
the  occiput  and  first  vertebra,  a sharp  knife  is  carried  across  the  spinal  canal  immediately  behind  the  head, 
so  as  to  divide  the  cord  from  the  brain.  The  toes  may  now  be  tied  out  and  any  observation  made  upon  the 
web  without  the  inconvenience  generally  produced  by  voluntary  struggles  on  the  part  of  the  animal,  w’hile 
at  the  same  time  the  use  of  chloroform  is  avoided ; which  is  very  desirable,  on  account  of  the  irritating  effect 
of  its  vapour  on  the  web  and  the  constant  care  required  for  its  administration.  If  the  webs  be  examined 
immediately  after  the  operation,  they  will  be  found  exsanguine  from  extreme  constriction  of  the  arteries  ; 
but  in  a few  minutes  this  state  will  give  place  to  dilatation  with  free  flow  of  blood.  If  now  a fine  needle, 
curved  at  the  end,  be  introduced  through  the  wound  into  the  spinal  canal,  so  that  its  point  may  penetrate 
a short  distance  into  the  cord,  while  the  eye  of  the  observer  is  kept  over  the  microscope,  the  arteries  will  be 
seen  to  become  constricted  to  absolute  closure,  and  dilate  again  after  withdrawal  of  the  needle.  The  expe- 
riment may  be  repeated  as  often  as  may  be  desired  till  the  cord  becomes  disorganized. 
I have  lately  found  the  above-mentioned  mode  of  preparing  the  frog  the  best  adapted  also  for  experiments 
