LETTER VIII.

THE STOMACH.

Once in the _oesophagus_ (you remember this is the name of the tube
which leads to the stomach), the mouthful of food has nothing to do but
to proceed on its way. All along this tube there is a succession
of small elastic rings, [Footnote: Properly, _contractile circular
fibres_.] which contract behind the food to force it forward, and
widen before it to give it free passage. They thus propel it forward,
one after another, till it reaches the entrance to the stomach, into
which the last ring pushes it, closing upon it at the same time.

Have you ever observed a worm or a leech in motion? You see a successive
swelling up of the whole surface of its body, as the creature gradually
pushes forward, just as if there was something in its inside rolling
along from the tail to the head. Such is precisely the appearance which
the _oesophagus_ would present to you, as the food passes down it, if
you had the opportunity of seeing it in action; and this has been called
_the vermicular movement_, in consequence of its resemblance to the
movement of a worm.

Here I wish to draw your attention to the very important fact, that
this movement is in one respect of a quite different nature from that
of your thumb when you take hold of a bit of bread, or that of your
jaw when you bite with your teeth, or of your tongue, &c., when you
swallow. All these actions belong to yourself, to a certain extent;