Movement of the contents of the alimentary canal may be favoured by judicious pressure, or massage. From the description of the situation of its several parts given above, it will be understood that if the right hand be placed on the abdomen immediately beneath the ribs, with the fingers well round to the left side, the stomach will be covered. Pressure from left to right will tend to drive its contents towards the pyloric valve. The small intestine is so irregular in its course as to preclude the possibility of following it with the hand. Pressure first on one side and then on the other, with a general tendency to work from above downwards, tends to press forward its contents; but, owing to its circular form and strong muscular walls, it is not in much need of help. Very different is the position of the large intestine in this respect. Its calibre is much greater, its wall is sacculated, its contents comparatively firm. If the palm of the hand be placed above the right groin and pressure directed upwards, the cæcum coli and ascending colon are emptied. If pressure be directed from the extreme right side just below the ribs, across the middle line to the left side, the transverse colon is emptied. The descending colon needs pressure from above downwards on the left side; the sigmoid flexure, pressure above the left groin, downwards, and towards the middle line.
The inner wall of the œsophagus is smooth, save for the wrinkles into which it is thrown when not distended; but from the cardiac orifice of the stomach onwards the mucous membrane of the alimentary canal exhibits folds and other projections which serve many purposes. They serve to delay the food, keeping it longer in contact with the secreting surface. They increase the area pitted with tubular glands; they increase also the area through which absorption of the products of digestion occurs. On the inner surface of the stomach the folds produce a reticulated pattern. In the upper portion of the small intestine, especially the duodenum, there are prominent transverse shelves (valvulæ conniventes). No definite folds occur below the upper three-fourths of the small intestine, with the exception of the constrictions of the transverse colon already referred to, which affect the whole thickness of its wall. Throughout the whole of the small intestine the mucous membrane projects in finger-like processes, or villi, which give it a characteristic velvety appearance. The villi are longest in the duodenum.
Lymph-follicles occur at intervals in the intestine. In the ileum they are collected into patches (Peyer’s patches), on the side opposite to the line of attachment of the mesentery. They serve both for the supply of phagocytes, which hunt any germs that have penetrated the mucous membrane, and also as stations to which germ-laden phagocytes retreat.
The wall of the intestine is composed of mucous membrane, submucous tissue, and muscle. The mucous membrane is everywhere pitted with tubular glands, termed in the stomach “gastric glands,” and in the intestines, both small and large, “crypts of Lieberkühn.” Their relation to the wall might be exemplified by taking a block of dough about 6 inches thick and pushing a pencil vertically into it almost down to the table on which it rests. The holes should be made as close together as possible, since, especially in the stomach, extremely little tissue intervenes between the tubes of gland-cells. If the piece of dough were placed upon a folded cloth, the cloth would represent the muscularis mucosæ, a layer properly regarded as a constituent of the mucous membrane. The fibres of this coat are disposed in two or three sheets, the fibres of one sheet crossing those of the next. By their contractions they squeeze the ends of the crypts, and probably wobble them about, expelling their secretion. Beneath the muscularis mucosæ is a layer of connective tissue, the submucosa, which contains abundant lymphatic channels, bloodvessels, and nerves. At the pyloric end of the stomach, the tubes of gland-cells tend to pierce the muscularis mucosæ. In the first part of the duodenum, certain tubes, having pierced this layer, branch in the submucosa. A layer of racemose glands is thus formed—the glands of Brunner. Outside the submucosa is the muscular coat proper, composed of plain muscle-fibres, except in the upper part of the œsophagus, where the fibres are striated. It consists of an inner and an outer sheet, the fibres being disposed circularly in the inner, longitudinally in the outer sheet, with a slight departure from this regular arrangement in the wall of the stomach. On its outside the canal is invested by peritoneum, a layer of flattened epithelial cells supported by connective tissue. The abdominal wall also is lined with peritoneum. The smooth moist surface of the peritoneum covering the intestines glides on the peritoneum lining the abdominal wall. Between the two is a “potential” space. In dropsy, fluid accumulates within this space. In a healthy condition the apposed surfaces are merely moist.
The movements of the intestines are of two kinds. At all times they exhibit swaying movements, in the production of which the longitudinal fibres play the chief part, although the circular fibres also contract. The object of this undulation is to thoroughly mix the contents of the gut with its secretions. If pills of subnitrate of bismuth are administered, and their progress observed by the aid of Röntgen rays, they are seen to oscillate backwards and forwards on their way down the canal. The slower vermicular movement which squeezes the contents forwards is called “peristalsis.” It resembles the progressive contraction of an elastic tube which may be effected by drawing it through a ring, but is rather more complicated. At the point at which it is occurring the circular coat is sharply contracted. Above this it is also somewhat contracted; below it is relaxed. The longitudinal fibres, using the constricted portion as a point d’appui, pull up the segment of the intestine which lies immediately below it, drawing it off the contents of the tube as a glove from a finger.
When food is swallowed, it falls down the œsophagus, aided by slight peristalsis. As soon as sufficient has accumulated on the upper surface of the cardiac valve of the stomach, the valve relaxes; at the same time a stronger peristalsis of the lower portion of the œsophagus squeezes its contents into the stomach. Food remains in the stomach until it has reached a certain stage of digestion, the chief object of which is its subdivision into small particles. Until this stage is reached, the pyloric valve is firmly closed. The contractions of the wall of the stomach drive its contents round and round—down the greater and up the lesser curvature—mixing them thoroughly with the gastric juice ([cf. p. 124]). As the acidity of the mixture increases, the peristaltic contractions of the stomach become more vigorous, until, the pyloric valve relaxing, the food is little by little driven into the duodenum.
The alimentary canal has an abundant supply of nerves from the vagus and the sympathetic systems. It contains also within its own wall an enormous quantity of nerve-fibres and nerve-cells. They are disposed as two plexuses, one in the submucosa, the other between the circular and longitudinal muscular coats. In a specimen successfully stained with methylene-blue, they are so abundant as to give the impression that every plain muscle-cell may have its own separate nerve-twig. Nevertheless, the contraction of the muscle-cells may take place independently of all nerve-influence—independently, even, of the local mechanism, the plexus referred to above. Nicotin applied to the wall of the intestine paralyses the local nerves; yet rhythmic contractions still occur. They are, however, no longer progressive. They do not drive the contents of the intestine forwards. Co-ordinated contraction is observed so long as the local mechanism is intact, even though all external nerves have been cut. The intestines have their own nerve cells and fibres, which, acting as a linked system of reflex centres, provide for the harmonious contraction of their walls. External nerves, sympathetic and splanchnic, convey impulses which either intensify the movements or inhibit them, as need may be.
In the matter of its nerve-supply, the alimentary canal stands apart from the other organs of the body. It may be supposed that it presents a more primitive condition. Its muscular fibres have the power of contracting spontaneously. The pressure of the contents of the tube acts as a stimulus. When the fibres are stretched, they contract. When the tube is dilated, its muscles endeavour to restore it to its normal calibre. Such direct action would not, however, provide for the forward passage of its contents. To bring about peristalsis, a nervous mechanism is needed, as abundant and complicated as that which ensures the progress of a slug or a worm. To deal satisfactorily with the various contents of the tube—liquid, solid, gaseous—the mechanism must be capable of complicated adjustments. The dilated portions of the tube—stomach, cæcum coli, rectum—require special arrangements of muscle and nerve. Nor is the canal altogether independent of the rest of the body. To a large extent its work is carried on without regard to the activities of other organs, yet it is not wholly free from the control of the central nervous system. It is regulated by means of both afferent and efferent nerves of the vagus and sympathetic. Even the brain has something to say with regard to the way in which it shall contract. It is a matter of common experience that emotional influences may affect the movements of the stomach and intestines—“His bowels yearned.”
Normally, vomiting is due to irritation of the endings of the vagus nerve in the stomach, although the afferent impulses may have other sources. Touching the upper surface of the epiglottis with the finger will provoke the reflex. So also will stimulation of the olfactory nerves by a foul smell. In this latter case the emotion of disgust to which the odour gives rise brings about the reflex action. A flow of saliva precedes the act of vomiting. A deep inspiration is then taken, in order that for a time the lungs may be independent of a fresh supply of air. The glottis is closed, the diaphragm fixed. Contraction of the abdominal wall presses the stomach against the diaphragm; its cardiac sphincter relaxes, and its contents are squirted into the œsophagus, which undergoes a forcible retrogressive peristalsis.
It is interesting to note the difference between carnivora and herbivora in regard to vomiting. Carnivora swallow fur and other indigestible materials, as well as many unwholesome things which they need to be able to return. A dog can, apparently, vomit at will. Never, while in a state of nature, do herbivora need to return the contents of the stomach. No provision is made for vomiting. A heifer which has strayed into a dewy clover-field is not unlikely to die from the effects of distension of its paunch, if relief be not given by opening it with a knife. In a horse the cardiac sphincter is strong, the pyloric weak. Pressure on the stomach tends to drive its contents through the pyloric valve into the duodenum, not backwards into the œsophagus. The stomach is not so placed as to allow of its being compressed between the wall of the abdomen and the diaphragm. Horses cannot vomit. It is a mistake to suppose that they suffer from sea-sickness. In rough weather they sweat, their limbs tremble, they go off their feed; but these symptoms are probably due to the fatigue which results from excessive anxiety to maintain their balance, and to fear. We can never know their feelings, but there is no reason for supposing that they experience the sensation of nausea.