class="an ">&ensp;</span>pig <span  title="" class="an ">&ensp;</span>ileum <span  title="" class="an ">&ensp;</span>in <span  title="" class="an ">&ensp;</span>comparison <span  title="" class="an ">&ensp;</span>with <span  title="" class="an ">&ensp;</span>an <span  title="" class="an ">&ensp;</span>opiate <span  title="" class="an ">&ensp;</span>methionine <span  title="" class="an ">&ensp;</span>enkephalin <span  title="" class="an ">&ensp;</span>These <span  title="" class="an ">&ensp;</span>contractio