constant. It extends along the side of the left crus of
the diaphragm opposite the second lumbar vertebra, and is there firmly
fixed to the front of the aorta and crus of the diaphragm by a strong
fibro-muscular band, slinging it up and absolutely retaining it in
position. This band has been termed the "musculus suspensorius
duodeni," but is chiefly composed of white fibrous tissue, and is more
of the native of a ligament than a muscle. This ligament is always
present, and its position is never altered. The curve of the duodenum
may descend as far as the iliac fossa, but the terminal portion is
always maintained by this band in its normal position.

Another point of great constancy is the position of the pancreas and
its relation to the curve of the duodenum. The duodenum always curves
round the head of the pancreas and is, as it were, moulded on it and
retained in position by it. In birds the duodenum always forms a long
loop embracing the pancreas. Further, the ducts of the liver and
pancreas always open into the center Of the duodenum, either
separately or by a common opening.

[Illustration]

Now, the absolute constancy of the curve of the duodenum, the complete
fixation of its fourth portion, the position of the pancreas, and the
place of entry of the ducts of the pancreas and liver, are all
component parts of a siphon trap, whereby gases generated below the
duodenum are prevented from passing upward. A reference to the
accompanying diagrams will make this quite clear. A is a diagram of a
siphon trap copied from Parkes' hygiene. B is a very diagrammatic
outline of the stomach and duodenum, _a_ is intended to mark the
position of the fibrous band, or musculus suspensorius duodeni; and
_b_ the position of entry of the ducts of the liver and pancreas. The
duodenum, then, is a siphon trap, and a most efficient one. Now, the
efficiency of a siphon trap depends not only on its shape, but what is
absolutely essential is that the curve must be kept constantly full of
fluid, without which it ceases to be a trap, and would allow gases to
ascend freely. The position of the place of entry of the ducts of the
pancreas and liver assures that this _sine qua non_ shall be present.
The discharge of the secretions of the pancreas and liver, although
more active during and after feeding, is practically constant, and so
insures in an admirable manner that the curve on which the efficiency
of the trap depends shall be constantly ke