THE defence mechanisms peculiar to the alimentary tube itself are very limited. For analytical purposes they may be considered under the already mentioned broad classification of mechanical and chemical types of assault.

Mechanical Assault.—The most easily investigated response to a harmful stimulus is a modification of the muscular activity of the wall of the tube. Since the only stimulus capable of evoking visceral pain is stretching leading to an exaggerated muscular activity, visceral pain of a diffuse character usually indicates an irritation of the surface of the mucous membrane rather than a local penetrating disease; in other words, it implies an intact or almost intact muscle coat.

It must not be forgotten, however, that in the digestive segments (Compartments III and IV) of the alimentary tube the specialized secretory glands respond readily to mechanical stimuli. A more energetic ferment activity is aroused and this helps to break up solid masses in the tube; at the same time there is an excessive outpouring of mucous fluid that facilitates the onward movement of unsuitable materials. In the expulsive segment (Compartment V) mechanical stimula- tion of the mucous membrane leads to a considerable increase in the secretion of mucus. As a defensive measure this protects the surface of the membrane, and by lubricating the faecal masses aids in their extrusion.

Other more complex autonomic defence reflexes, e.g., vasomotor changes, reverse peristalsis, the co-related actions of the sphincters, and the combined visceral and somatic muscular movements associated with vomiting and diarrhoea, will be discussed later.

Chemical Assault.—Meanwhile consideration may be given to the defence mechanisms which are called into action to deal with localized chemical processes threatening to damage seriously the bowel wall. The autonomic system is inadequate to deal with this form of assault and the defence is therefore handed over to the somatic system. The actual procedure is that the inflammatory process in the wall extends to the serous coat of the alimentary tube and sets up a localized peritoneal irritation. As the bowel wall is not responsive to ordinary sensory stimuli localized pain is not necessarily experienced during the early stages of this process.
The parietal peritoneum that lines the wall of the abdomen rests on a subserous layer in which there is a rich supply of sensory somatic nerves. This parietal peritoneum has a high threshold for ordinary pressure stimuli, but a low threshold for chemical stimuli such as effusions of lymph or blood. When the inflamed serous surface of the bowel comes in con- tact with the contiguous parietal peritoneum a localized parietal peritoneal irritation is set up. A pain stimulus is excited and this is conveyed to the central nervous system; there it is analysed and localized to the somatic nerve supply- ing the appropriate area of the subserous coat of the parietal peritoneum. The stimulation of the somatic nerve affects not only the sensory distribution to the peritoneum and to the over- lying skin, but also, if the stimulus be sufficiently strong, it calls forth a somatic motor response in the corresponding segments.

The clinical evidence of this somatic response is to be found in hyperalgesia, local tenderness to pressure, and muscular rigidity. These clinical signs are localized on the abdominal wall in the region immediately overlying the inflamed area of bowel. They indicate, therefore, with a considerable degree of accuracy the position of the organic lesion that the somatic defence mechanism is guarding.

Obviously it is impossible to do more than mention that there are in addition much more widely distributed voluntary defences.
It is possible now to summarize the mechanisms by which the alimentary tube responds to abnormal conditions.

ι. When the offending agencies are of a mechanical nature an attempt is made to remove them by an increase in the force of the normal muscular activities. This may be supplemented later by reverse muscular movements. Up to this stage the defensive response is autonomic, involving sympathetic and/or parasympathetic systems. If this proves inadequate, the muscles of the abdominal wall (somatic system) may be called upon to supplement the expulsive efforts. For this purpose the thoracic diaphragm and the pelvic diaphragm may be regarded as abdominal muscles.

The sensory complement of the increased motor activity of the bowel is visceral pain. If the parasympathetic centres are involved the pain is localized vaguely by the patient in and around the midline of the abdominal wall; the zone of the abdominal surface involved corresponds to the physio- logical segment of the bowel implicated. If the sympathetic centres are activated the visceral pain, while still confined to the same abdominal zones, is more definitely located in the areas over the three great sphincters of the alimentary tube.

2. When the offending agencies are of an irritant, e.g., chemical, nature, and not sufficiently potent to produce gross damage to the wall of the alimentary tube, the response is at first largely vasomotor and/or secretory. It results in an out- pouring of a protective and diluting fluid which by its bulk may give rise to muscular activities similar to those described above.

3. If, however, the chemical agencies are sufficiently virulent to induce inflammatory reactions throughout the thickness of the bowel wall, or if mechanical agencies are so severe as seriously to damage or destroy a portion of the wall of the tube, the assistance of the somatic defence system becomes imperative. An inhibition of such abdominal muscles as might press on the affected bowel, or a rigidity of overlying muscle segments to guard the bowel from outside pressure, is the defence reaction.
On the sensory side pain is now somatic, definitely located over the affected part of the bowel, and readily evoked by clinical examination as local tenderness to pressure or as hyperalgesia of the overlying skin.

Two of the additional defence measures, namely, reverse peristalsis and co-ordinated sphincter action, may now be reviewed briefly.
Reverse Peristalsis.— In the alimentary tube under normal conditions there are only two regions in which a reverse peristalsis is described, namely: (i) in the duodenum during the reflux of duodenal contents into the stomach in the later stages of gastric digestion; and (2) a possible some- what sluggish antiperistalsis in the colon.

When, however, abnormal conditions are present in the tube reverse peristalsis may become an important defence measure. In such circumstances the reverse peristaltic propulsive waves may be sufficiently powerful and widespread to carry material all the way from the rectum to the mouth. This is evidenced by the vomiting of materials introduced by a rectal enema. As a rule, however, the length of tube involved is more restricted.
The exciting cause of the reverse peristalsis is usually an obstruction of the tube—of either intrinsic or extrinsic origin. Attempts by the muscular wall to overcome the obstruction in the direction of the anal outlet having failed, fresh efforts are made by the reverse peristalsis to drive some of the offending material towards the mouth (intrinsic obstruction) or to prevent the accumulation of more material behind the immovable obstruction (extrinsic obstruction).
The mechanism of reverse peristalsis does not seem to involve any new factors. If a segment of the bowel contracts on a mass within it the mass will move in the direction of least resistance, distending the bowel in front of it, and thus initiating a travelling impulsive wave. As the route towards the anus is blocked the alternative route towards the mouth is chosen.
Co-ordinated Sphincter Action.—Under the influence of abnormal conditions the normal co-ordinated actions of the three great sphincters — pyloric, ileocaecal, and pelvirectal—are intensified, and assume considerable import- ance as defence measures.

The great sphincters are much more irritable than the rest of the bowel wall. They are* therefore liable to spasmodic contraction when there is any inflammatory or other irritative condition in their vicinity. It has been pointed out {Part II, p. 64) that normally gastro-ileal and gastro-colic reflexes, and their reverse equivalents, ileo-gastric and colico-gastric reflexes, control the distribution of the relative amounts of the contents of the three main physiological segments of the abdominal alimentary tube. If a source of irritation appears in the neighbourhood of one of the sphincters the spasm that results from it may evoke a spasm in one or both of the other two. Thus an irritative spasm of the pylorus may set up a spasm of the ileocaecal sphincter, or, alternatively, an irritative spasm of the ileocaecal sphincter may be accompanied by a spasm of the pylorus, etc.
The importance of this as a defence mechanism is twofold : (1) It may prevent the passage of intestinal contents from a high to a lower segment of the tube; or (2) It may by blocking the tube act as a point d’appui that will determine the develop- ment of reverse peristaltic waves to empty the upper segment.

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