154

Influence of the Nervi Vagi on Digestion.

was then compelled to swallow food, which was found to be converted into chyme, and to furnish afterwards an abundant quantity of chyle".

When, however, the nervi vagi are divided in the neck, the production of chyme appears very imperfect in those cases wherein it is not entirely prevented; but it is probable that these results ensue indirectly and are to be attributed to the derangement of other functions. The researches of Dr. W. Philip, confirmed by those of MM. Breschet, Edwards, and Vavassour, tend to make it appear that the galvanic influence directed upon the stomach after the division of the nervi vagi in the neck restores its digestive properties; and that the removal of a portion of each nervus vagus interferes with digestion considerably more than the simple division of these nerves. But whatever light has been thrown upon this subject generally by the researches of the physiologists I have mentioned, and by the experiments of Mr. Broughton and of Mr. Cutler, we must admit that it remains involved in great uncertainty.

It appears from experiments by M. Majendie, that when the cerebrum and great part of the cerebellum have been removed in ducks, the instinct of seeking food is lost in every instance, and the instinct of deglutition in many; nevertheless, food that has been introduced into the stomach is found to be digested.

The division of the sympathetic nerves in the neck of rabbits produces no apparent disturbance of any function. The influence of the splanchnic nerves and of the semilunar ganglia has not, that I know of, been put to the test of experiment.

Majendie, Elémens de Physiologie, tome ii. p. 103.

A history of the functions of the stomach would be incomplete without an account of the means by which it relieves itself of offending matter.

In 1686 it appears that M. Chirac determined by experiment that the stomach is passive during vomiting. Corrosive sublimate was given to a dog upon bread, which was almost immediately thrown up, but nausea and violent retching continued. Upon exposing the cavity of the abdomen, the stomach exhibited a peristaltic motion so feeble as to persuade the operator that the expulsion of its contents could not result from this cause. The wound in the abdomen was then closed; and while the animal continued its efforts to vomit, the finger was introduced and applied to the stomach, which was found to remain free from contraction, and only to be flattened and compressed by the abdominal muscles and diaphragm, at each effort to expel its contents.

Subsequently, however, the opinion of several physiologists seemed to incline to the supposition that the fibres of the stomach are the principal agents in vomiting. Lieutaud and Haller were of this party. But others from the time of Chirac to Hunter continued to hold the first opinion.

"In vomiting," Mr. Hunter observes, "the muscles of the cavity of the abdomen act, in which is to be included the diaphragm; so that the capacity of the abdomen is lessened, and the action of the diaphragm rather raises the ribs; and there is also an attempt to raise them by their proper muscles, to make a kind of vacuum in the thorax, that the oesophagus may be rather opened than shut, while the glottis is shut so as to let no air into the lungs. The muscles of the throat and fauces act to

156

Experiments to show the Means

dilate the fauces, which is easily felt by the hand, making there a vacuum, or what is commonly called a suction; so that when all these actions take place together, the stomach is immediately emptied "."

The following experiments by M. Majendie confirm the opinion of Chirac and Hunter; they include the notice of an additional fact, which Dr. Haighton had observed, that the division of the par vagum does not prevent vomiting, and present other curious matter for reflection.

If two grains of tartarized antimony dissolved in an ounce and a half of water be thrown into the crural vein of a dog, nausea is produced almost instantaneously; if the stomach be then drawn through a wound in the abdomen, the spasm of retching takes place in the diaphragin and abdominal muscles, but the stomach remains without movement, and no vomiting ensues. If the stomach be, then replaced in the cavity of the abdomen, it may be felt by the finger applied to it to remain relaxed, at the time that its contents are expelled through the renewed efforts of retching.

If the nervi vagi be divided, and the emetic substance be introduced as above, nausea and vomiting follow.

If the abdominal muscles be removed leaving the linea alba entire, upon the injection of the emetic substance nausea and vomiting take place, the stomach being compressed between the diaphragm and linea alba.

If the phrenic nerves be divided, and the emetic substance injected, nausea occurs, and vomiting, but more feebly than in the preceding experiment. The diaphragm receives a few twigs from the eleventh and twelfth dorsal

• Hunter on the Animal Economy.

by which Vomiting takes place.

157

nerves, which enable it to act partially in opposition to the abdominal muscles.

Finally, if the stomach be removed, and a pig's bladder substituted in its place communicating artificially with the oesophagus, the injection of tartarized antimony into a vein is followed by nausea, by retching, and the expulsion of the contents of the bladder by the fauces.

Animals are observed instinctively to swallow a large quantity of air previously to vomiting, which acts like the draughts of liquid prescribed after an emetic by distending the stomach; so that it resists the spasm of the diaphragm and abdominal muscles, and prevents the necessity for their extreme and painful contraction P.

The stomach is remarkable for its sympathies. A blow upon the head produces nausea and vomiting; indigestion produces irritation in the lungs, palpitation of the heart, clouded intellect and depression of spirits; a violent blow on the stomach is instantly fatal.

SECTION 4.

Of the Formation of Chyle.

As fast as the chyme accumulates in the pyloric extremity of the stomach, it is carried into the duodenum ; so that two or three ounces of chyme form the largest quantity ever found in the part of the stomach adjoining the pylorus. Upon watching the passage of the chyme into the small intestine, the peristaltic action is observed to commence upon the duodenum and to be carried backwards towards the stomach: then a vermicular action commences upon the stomach, and is continued over the

'Mémoire sur le Vomissement.

158

Of the Valve of the Pylorus.

pylorus to the duodenum, carrying chyme before it. These phenomena are repeated at intervals, and are not observed to be suspended by the section of the par vagum.

What is termed the valve of the pylorus consists of muscular fibres triple the thickness of the muscular coat of the stomach, which form a strong circular band projecting into the alimentary canal. The mucous passage is so much the more straitened by the projecting fibres of the valve, that the alimentary canal is externally much narrower at this part than elsewhere.

When air is blown into the duodenum, it readily finds its way into the stomach; but if blown from the oesophagus into the stomach, the latter yields to a great degree of distension before the pylorus allows of the passage of

air into the duodenum.

Upon entering the duodenum the chyme becomes mixed with the bile, the pancreatic secretion, and the mucus of the intestine. The latter is viscid, of a salt and acid savour, and quickly renewed, when wiped from the surface of the bowel.

The bile in living animals is seen to exude from the ductus choledochus at intervals, a drop appearing at the orifice and diffusing itself over the neighbouring surface, about twice in a minute. The pancreatic secretion is yet slower in its separation from the gland when thus examined. The bile quickly imparts its sensible qualities to the chyme,—its colour and bitterness. In a short time a spontaneous change is observed to take place in the compound. It separates into a whitish tenacious liquid termed chyle, and a yellow pulp. The former is the recrementitious part of the aliment; the latter, the excrementitious portion, which after undergoing a further change is to be eliminated. Both together are