﻿1875.] 
  Influence 
  of 
  Electricity 
  on 
  the 
  Hearts 
  of 
  Mollusks. 
  321 
  

  

  duration. 
  Since 
  the 
  eye 
  has 
  been 
  our 
  only 
  means 
  of 
  judging 
  of 
  the 
  cha- 
  

   racters 
  of 
  the 
  contraction, 
  it 
  being 
  impossible 
  to 
  register 
  the 
  movements 
  

   by 
  the 
  graphic 
  method, 
  we 
  speak 
  with 
  great 
  uncertainty 
  on 
  this 
  point. 
  

  

  The 
  stronger 
  the 
  shock 
  the 
  more 
  marked, 
  within 
  certain 
  limits, 
  is 
  the 
  

   contraction. 
  The 
  effect, 
  of 
  course, 
  in 
  any 
  particular 
  case 
  will 
  depend 
  

   not 
  only 
  upon 
  the 
  absolute 
  strength 
  of 
  the 
  current, 
  but 
  on 
  its 
  strength 
  

   in 
  relation 
  to 
  the 
  irritability 
  of 
  the 
  heart. 
  Thus 
  a 
  heart 
  with 
  frequent 
  

   and 
  strong 
  beats 
  is 
  affected 
  by 
  a 
  weak 
  current 
  to 
  the 
  same 
  extent 
  as 
  is 
  a 
  

   heart 
  beating 
  slowly 
  and 
  feebly 
  by 
  a 
  strong 
  current. 
  

  

  The 
  contraction 
  or 
  beat 
  thus 
  produced 
  by 
  a 
  single 
  induction-shock 
  is 
  

   generally 
  followed 
  by 
  a 
  pause, 
  much 
  more 
  evident 
  in 
  some 
  cases 
  than 
  in 
  

   others. 
  We 
  have 
  not 
  been 
  able 
  to 
  discover 
  the 
  exact 
  conditions 
  which 
  

   determine 
  the 
  duration 
  of 
  the 
  pause 
  ; 
  but 
  it 
  seems 
  to 
  depend 
  much 
  more 
  

   on 
  the 
  state 
  of 
  the 
  heart 
  than 
  on 
  the 
  strength 
  of 
  the 
  current. 
  There 
  

   appears 
  to 
  be 
  no 
  necessary 
  relation 
  between 
  the 
  amount 
  of 
  the 
  contrac- 
  

   tion 
  and 
  the 
  length 
  of 
  the 
  consequent 
  pause. 
  Thus 
  a 
  contraction 
  twice 
  

   the 
  strength 
  of 
  the 
  normal 
  beat 
  is 
  not 
  invariably 
  followed 
  by 
  a 
  pause 
  of 
  

   twice 
  the 
  length 
  of 
  the 
  normal 
  diastole 
  ; 
  it 
  may 
  be 
  more 
  than 
  twice 
  as 
  

   great, 
  it 
  may 
  be 
  hardly 
  greater. 
  On 
  this 
  point, 
  again, 
  we 
  speak 
  with 
  

   hesitation, 
  from 
  inability 
  to 
  measure 
  accurately 
  the 
  force 
  or 
  extent 
  of 
  

   the 
  beats. 
  

  

  We 
  have 
  thrown 
  in 
  the 
  single 
  induction-shock 
  at 
  all 
  phases 
  of 
  the 
  car- 
  

   diac 
  cycle, 
  at 
  the 
  height 
  of 
  systole, 
  immediately 
  before 
  and 
  immediately 
  

   after 
  systole, 
  in 
  the 
  middle 
  of 
  diastole, 
  &c, 
  but 
  have 
  been 
  unable 
  to 
  

   detect 
  any 
  marked 
  differences 
  in 
  the 
  result. 
  It 
  can 
  hardly 
  be 
  doubted 
  

   that, 
  whatever 
  be 
  the 
  way 
  in 
  which 
  the 
  shock 
  causes 
  a 
  contraction, 
  the 
  

   heart 
  must 
  be 
  differently 
  disposed 
  towards 
  a 
  stimulus, 
  according 
  as 
  it 
  is 
  

   about 
  to 
  make 
  or 
  has 
  just 
  made 
  a 
  beat 
  ; 
  nevertheless 
  we 
  have 
  not 
  suc- 
  

   ceeded 
  in 
  observing 
  any 
  marked 
  differences. 
  

  

  Not 
  only 
  has 
  the 
  contraction 
  produced 
  by 
  a 
  single 
  induction-shock 
  the 
  

   general 
  characters 
  of 
  a 
  normal 
  "beat" 
  rather 
  than 
  those 
  of 
  a 
  simple 
  

   muscular 
  contraction, 
  but, 
  as 
  far 
  as 
  can 
  be 
  judged 
  by 
  the 
  eye, 
  the 
  effect 
  

   of 
  the 
  shock 
  on 
  the 
  ventricle 
  differs 
  from 
  the 
  effects 
  produced 
  in 
  any 
  

   ordinary 
  muscular 
  fibre 
  in 
  the 
  following 
  respects. 
  In 
  passing 
  gradually 
  

   from 
  extremely 
  feeble 
  to 
  stronger 
  currents, 
  there 
  is 
  not 
  witnessed 
  a 
  

   gradual 
  increase 
  of 
  effect 
  ; 
  on 
  the 
  contrary, 
  the 
  change 
  is 
  a 
  sudden 
  one, 
  

   from 
  apparently 
  no 
  effect 
  at 
  all 
  to 
  the 
  production 
  of 
  a 
  well-marked 
  beat. 
  

   This 
  leads 
  us 
  to 
  infer 
  that 
  the 
  mechanism 
  by 
  which 
  feeble 
  currents 
  pro- 
  

   duce 
  contractions 
  in 
  the 
  heart 
  differs 
  somewhat 
  from 
  that 
  whereby 
  con- 
  

   tractions 
  are 
  caused 
  by 
  stimulating 
  an 
  ordinary 
  muscular 
  fibre 
  *. 
  The 
  

   stronger 
  contractions, 
  on 
  the 
  other 
  hand, 
  produced 
  by 
  stronger 
  shocks, 
  

   seem 
  to 
  share 
  more 
  distinctly 
  in 
  the 
  characters 
  of 
  an 
  ordinary 
  muscular 
  

   contraction. 
  

  

  * 
  Compare 
  Bowditch, 
  Arbeiten 
  phys. 
  Anstalt, 
  Leipzig, 
  vi. 
  Jahrgang 
  (1871), 
  p. 
  139. 
  

  

  