By MAYO COLLIER, M.S. Lond., F.R.C.S. Eng.; Senior
Assistant Surgeon, North-West London Hospital; Assistant Demonstrator
of Anatomy, London Hospital Medical College.
We may take it for granted that all gases generated in the jejunum,
ileum, and large intestines pass onward toward the anus, and there
sooner or later escape. Fetid gases--except those generated in the
stomach and duodenum--never pass upward, not even during vomiting due
to hernia, obstruction, and other causes. Physiologists, it would
appear, have never busied themselves to find an explanation for this
apparent breach of the laws of gravity. The intestinal canal is a tube
with various dilatations and constrictions, but at no spot except the
pylorus does the constriction completely obliterate the lumen of the
tube, and here only periodically. It is perfectly evident, then, that,
unless some system of trap exists in the canal, gases are free to
travel from below upward in obedience to the laws of gravity, and
would, as a matter of fact, sooner or later do so. From the straight,
course and vertical position of the oesophagus, a very slight
pressure of gas in the stomach easily overcomes the closure of its
cardiac sphincter and allows of escape. When the stomach has digested
its contents and the pylorus is relaxed, gases generated in the
duodenum can and do ascend into the stomach and so escape. Normally,
no fetid gases are generated in the stomach or duodenum. If we follow
the course of the intestines down, we find that the duodenum presents
a remarkable curve.
Now, there are some points of great interest in connection with this
remarkable, almost circular, curve of the duodenum. In the first
place, this curve is a constant feature in all mammalians. Mr. Treves
says it is one of the most constant features in the anatomy of the
intestines in man, and, speaking of mammalians in general, that the
curve of the duodenum varies in shape, but is never absent, becoming
more complex in some of the higher primates, but seldom less distinct
than in man. In birds the duodenum always forms a long loop embracing
the pancreas.
A second point of great interest is the absolute constancy and
fixation of its terminal portion at the point of junction with the
jejunum, more correctly termed second ascending or fourth portion. Mr.
Treves says that this fourth portion is never less than an inch, and
is practically 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 kept full not only with fluid,
but, as I would suggest, antiseptic fluid. There is no other trap in
the intestinal canal, but the peculiar position of the colon would no
doubt have more or less effect in preventing gases ascending through
the ileo-cæcal valve.--_Lancet._
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