What did Hecker say about laughter? Funny you should ask

The Darwin-Hecker hypothesis, viz. that laughter induced by tickling and humor share common underlying mechanisms, is so-called in part because of a quotation attributed to Ewald Hecker. However, a German counterpart of the quotation does not appear in the location cited. Some textual sleuthing is undertaken to find out what Hecker actually wrote and where he wrote it.