Compartment IV is the final digestive and absorptive chamber of the alimentary tube, and its vital centre is the distal half of the duodenum and the whole of the jejunum.
From its situation this compartment is very difficult to investigate. Materials introduced into it, either from above or below, have had to traverse long stretches of the alimentary tube before reaching it, and may be completely altered in composition before arriving. Changes taking place in the more active parts of it are so rapid and intricate that it is almost impossible to get good radiographs of them. Investi- gators, therefore, are compelled to fall back on animal experiments, or on human experiments that are rendered possible by injuries or pathological lesions. Naturally these
cannot be relied on to give an accurate picture of the normal happenings in this part of the tube.
The main activities of Compartment IV may be analysed under the headings of: Chemical actions; Absorption] Muscular movements. The three types of activity are, however, closely interwoven and directed towards a common end; they are,
for all practical purposes, taking place simultaneously.
CHEMICAL ACTIONS
The largest gland providing digestive juice is the pancreas, but most of the glands of the wall of the small intestine contribute digestive fluids. In addition, bile from the liver, although not containing any digestive ferments, is added to the digestive mixture and must be considered in its chemical
processes. While the greater part of the chemical action on the chyme takes place in the interior of the tube, a certain amount of it is carried out actually in the cells of the intestinal gland tubules. All the digestive reagents of this compartment are most active in an alkaline medium.
It will simplify the discussion of the various constituents of the digestive juice to consider separately the contributions of the three main sources, namely, pancreas, intestinal glands, and liver.
Pancreas.—The secretion of the pancreas is stimulated partly by nervous impulses through the vagi, and partly by a hormone—secretin—derived from cells of a large stretch of the intestinal wall.
There is a distinct difference in composition between the pancreatic juice stimulated by nervous activity and that stimulated by the hormone. Pancreatic juice resulting from stimulation of the vagi is rich in ferments but poor in alkali. The juice produced by chemical action has the opposite characteristics—it is rich in alkali but poor in ferments. Both factors—nervous and chemical—are used to initiate the normal flow of pancreatic juice and both come quickly into action. It is probable (and has actually been observed) that the
swallowing of food, or even the sight of it, is followed very rapidly by a (nervous) secretion of pancreatic juice. Also, the entry of acid chyme into the duodenum initiates almost at once a (chemical) secretion of the juice. The relation of this chemically stimulated pancreatic juice to the later stages of gastric digestion is worthy of note; in the fluid regurgitated through the pylorus, neutralization òf the stomach acid rather than digestive action is the dominant requirement, and for this the chemically stimulated juice would seem to be the more suitable.
Pancreatic juice is an alkaline fluid containing several digestive ferments, the chief of which are: Trypsinogen; Lipase; Amylase.
Before it can effect any digestive action trypsinogen must be converted into trypsin. This is brought about mainly by a substance—enterokinase—which is elaborated by the intestinal glands, although some other materials, e.g., bile, are also able to convert some of the trypsinogen into trypsin.
In the prepared chyme that reaches Compartment IV a large part of the protein content of the meal has been converted into peptones by the pepsin and H C l of the stomach. By the action of trypsin and alkali this process is continued; protein is converted into alkali metaprotein, proteoses, and peptone, but the cleavage of the peptone is taken further and some of it is broken down into groups of amino-acids termed peptids.
Lipase is responsible for dealing with the greater part of the fats of the diet. In a mixed diet there are usually some free fatty acids, and the HCl of the stomach may hydrolyse a small amount of neutral fat. The chief digestive change, however, is effected by lipase. By the action of this enzyme, neutral fat is split up into fatty acids and glycerol. Some of the fatty acids thus split off enter into combination with the alkalis of the digestive juices and form soaps. The soaps form thin coverings round the oil globules and convert the remaining fat into a fine emulsion. This process reduces the surface tension between the watery solution of lipase and the oil globules, and enables the ferment to carry out and complete the splitting of the fat.
The ferment amylase acts on all forms of starch, differing in this respect from ptyalin, which can act only on cooked starch. Starch is converted by amylase into maltose.
Intestinal Glands.—These glands secrete a fluid—succus entericus—which contains a variety of ferments, and a substance already mentioned, enterokinase, that converts trypsinogen into trypsin. A group of ferments—peptidases —were originally described as a single ferment, erepsin. These continue the action of trypsin and convert the inter- mediate products of peptic and tryptic digestion into the final amino-acids. It is probable that some of the peptidases are not secreted into the succus entericus, but carry out their actions within the epithelial cells of the intestinal wall.
The other important ferments of the succus entericus are a group—invertase (sucrase), maltase, and lactase—which complete the conversion of the starches and sugars into glucose.
The final result, therefore, of the digestion of the main constituents of the original meal is:—
Proteins—into amino-acids.
Fats—into fatty acids and glycerol.
Carbohydrates—into glucose.
All these final products of digestion are in forms suitable
for absorption into the bowel wall.
Liver.—The secretion of bile is only one of the many
activities of the liver. Bile enters into the middle part of the duodenum through the common bile-duct by an orifice common to it and the pancreatic duct. It is an alkaline fluid (due to the presence of N a 2 C 0 3 ) and contains several substances, none of which has any digestive properties and some of which might be considered as excretions rather than secretions. In the digestive fluids with which they intermix, the most important constituents of the bile are the bile-salts, sodium glycocholate and sodium taurocholate. There is no direct evidence as to where normally these are formed, but the amount of them is increased by fats and proteins, and lowered by sugars, in the diet. They pass through a curious cycle; they are reabsorbed from the bowel, carried in the blood-stream to the liver, and then returned through the bile-ducts to the duodenum. Bile-salts have the power of facilitating the actions of trypsin and amylase. In associa- tion with the alkali of the bile they increase very greatly the activity of lipase in the manner explained above.
The secretion of bile by the liver is practically a continu- ous process, but the discharge of this bile into the duodenum is intermittent and is related to the digestive processes in the bowel. In order, therefore, to regulate this entry of bile, a cistern—the gall-bladder—is provided in the course of the efferent duct of the liver. In the gall-bladder water is absorbed, and cholesterol and mucin are added to the bile, so that the ultimate fluid is more concentrated and more mixed than the original liver bile. The discharge of bile into the duodenum is closely synchronized with that of the pancreatic
juice, and some of the factors controlling the secretion of the pancreas have a similar effect on bile production.
ABSORPTION
In the preceding description references have been made only to those constituents of the ordinary diet that are affected by the chemical actions of the digestive ferments. It must be realized, however, that a certain amount of the material in the diet is not altered by those ferments, e.g., the woody fibre of vegetables, etc., and other elements of the diet are not completely disintegrated and/or digested. There is, there- fore, a residue of material simply passed on down the bowel
for ultimate rejection. The greater part of a well-chosen diet, however, is reduced to forms that can be absorbed into the system, and this process of absorption may now be reviewed briefly.
It will have been appreciated that the greatest quantity of digestive juice is discharged into the interior of the alimen- tary tube in the distal part of the duodenum and in the
jejunum. Correspondingly it is in these portions that the greatest amount of the products of digestion is absorbed. Absorption at a reduced rate, however, is carried on along the greater part of the ileum, and probably a slight amount of digested material is absorbed in the first part of the great intestine.
The structure of the individual parts of the alimentary tube gives a clear indication of.this. The two chief require- ments for maximum absorptive activity are ( ι ) a large surface, and (2) a profuse vascular supply. It has been shown already that a very great extension of the surface of the mucous membrane is obtained by the formation of the plicae circulares, and this is increased further by the enormous number of intestinal villi that cover the plicae and the intervening
mucous membrane. The relative disposition of these in the jejunum and ileum has been described.
In addition to extending the surface the plicae function in some degree as delaying barriers to ensure that sufficient time is given to the digestive and absorptive processes. Each villus has its loops of artery and vein which are fed and drained by rich arcades of the superior mesenteric vessels. In addition, lacteal vessels starting in the villi carry fatty products back to the cisterna chyli and ultimately into the general circulation.
The end-products of protein digestion, namely, the amino- acids, and the end-product of carbohydrate digestion, namely, glucose, are absorbed directly into the blood-stream through the wall of the alimentary tube, and carried via the portal vein to the liver. With regard to fats, the free fatty acids, glycerol, and soaps into which they have been resolved pass into the cells lining the bowel wall and are there built up again into fat globules. Some of these globules enter the lacteals directly, others are taken up by white corpuscles and are carried by them into the lacteals.
The absorption of fats is helped greatly by the presence of the bile-salts in the intestine. The physical effect of these salts in lowering surface tension renders possible the transit of the fatty products into the watery fluids of the epithelial cells. T w o other constituents of bile—Cholesterin and lecithin —contribute by helping to dissolve fatty acids and soaps.
An important addendum is that fat-soluble vitamins are probably absorbed by the same route as the fats.
Although mineral salts are* present only in minute amounts in a diet, they are of very great importance. Several inor- ganic metallic salts are absolutely essential for the needs of the body; the chief ones are compounds of calcium, phos- phorus, and iron, but others (e.g., compounds of copper, manganese, etc.) also are necessary.
As mentioned under gastric digestion, some of the insoluble mineral salts of the diet are converted into soluble chlorides by the HCl of the gastric juice and are mixed with the acid chyme that enters the duodenum. Here, in company with other soluble salts, they are absorbed directly into the blood- stream. The greater amount of those that are still unabsorbed before the distal duodenum is reached are precipitated by the alkali of the small intestine; complete alkalization of the intestinal contents is accomplished in the upper part of the jejunum.
The fate of the calcium thus thrown out of solution requires particular notice. A proportion of it combines with phosphorus to form insoluble calcium phosphate, while another moiety of it enters into combination with fatty acids to form insoluble soaps. This soapy mass entangles some unsplit fats and some undigested proteins and constitutes a considerable part of the residue that has been noted as passing through the small intestine for ultimate rejection. From a purely mechanical point of view the unabsorbed residue serves a useful purpose in slowing the rates of digestion and absorption.
The absorption of mineral salts is influenced by the vita- mins of the diet, but the mechanism of this is somewhat obscure.
MUSCULAR MOVEMENTS
The most active muscular movements taking place in Compartment IV are those in the stretch of small intestine from the middle of the duodenum to the ileocolic junction. The peak of this activity is reached in the distal half of the duodenum and in the jejunum; there is a definite progressive decline in the intensity of activity from the jejunum along the ileum to the ileocolic junction.
In the terminal part of Compartment IV, i.e., in the caecum and ascending colon, movements are very irregular, and, on the whole, sluggish.
Three types of movement have been demonstrated in the small intestine, namely: segmentation; pendulum or sway- ing movements; peristaltic rushes. Of these, only the peri- staltic movements are propulsive. The others are concerned mainly in thoroughly mixing the intestinal contents with the digestive juices and bringing them into contact with the absorbing villi ; they are therefore confined largely to the duo- denum and jejunum.
In the segmentation movements a segment of the tube becomes constricted off by two circular contractions a short distance apart. The segment thus marked off is distended, and under the influence of this stimulation other bands of alternating constriction and relaxation appear; in this manner the con- tents of the segment are thoroughly mixed and distributed.
Several of these segments may appear simultaneously in serial contact and may merge into one another to form new segments.
The pendulum movements involve somewhat longer loops marked off by circular constrictions of the bowel. The contents of the loop are thrown from one end to the other and the whole loop undergoes a general swaying movement from which its descriptive name is derived.
Peristaltic waves are the main factors in propelling the bowel contents down to the end of the ileum. Liquid materials, however, can travel for long distances down the tube without the aid of peristalsis, suggesting that there may be another available propelling force, probably in the nature of suction.
Authorities are not agreed on the reason why propulsive waves normally travel downwards along the alimentary tube. There are two main theories advanced. The older theory states that ” i f cerebrospinal reflexes be excluded excitation at any point of the gut excites contraction above and inhibi- tion below.” According to the later theory, “stimulation at any point of the gut leads to the holding back of material coming down from above and the hurrying onward of material already below “.
The second theory is supported by much experimental work on the existence of various gradients of excitability, metabolic activity, etc., in the wall of the alimentary tube; the progressive reduction of the steepness of the gradients from the mouth to the anus determines the aboral flow of the intestinal contents.
Without unduly stressing the similarity, it is interesting to compare in this connexion the nerve impulses and the peristaltic waves. Under normal conditions nerve impulses will travel only in one direction, namely, from the dendrites through the nerve-cell to the axon terminations. Likewise the peristaltic waves under normal conditions will travel only in one direction, namely, from proximal to distal segments of the alimentary tube.
Peristaltic waves may start in any part of the small intestine —frequently as high up as the proximal part of the duodenum.
Individual waves, also, may stop at any part of the tube; occasionally they carry some of the intestinal contents for a short distance, then pass over them and pick up the contents of other portions farther down the bowel.
The peristaltic rushes produced by the waves have no regular relation to either the segmentation or the pendulum movements, but they do appear to be influenced by the state to which the intestinal contents have been reduced by these other movements. The contents of a stretch of bowel, for instance, that have been undergoing mixing for a time may be picked up suddenly by a peristaltic wave and carried onward in a peristaltic rush.
In the ileum peristalsis is very infrequent. Much of the absorbable part of the diet has been removed in the upper part of the small intestine and the propulsive force is expended, therefore, in carrying the unabsorbed residue to the lower end of the ileum, where its further progress is barred by the ileocaecal sphincter.
In this region material accumulates and there is a consider- able delay before it proceeds farther. Thereafter the intes- tinal contents pass through the ileocaecal orifice at frequent intervals and gradually fill up the caecum; the extruded material issues in small jets through the ileocaecal orifice, which relaxes in response to contractions of the terminal ileum. Probably the passage of this material through the orifice is assisted occasionally by some of the peristaltic waves that after travelling down the ileum carry on over the ileocolic
junction to the caecum.
There is a marked synchronization of the opening up of
the pyloric and ileocaecal valves. In the preceding section it was noted that both valves are supplied by the vagus nerves (see Fig. 10); the opening of them is controlled, therefore, by the parasympathetic cerebral outflow. The effect of this is that two reflexes are established. In one—the gastro-ileal —impulses affecting the pylorus are referred down the vagus and determine that when the pyloric valve opens there is a corresponding opening of the ileocaecal valve; in the other —the ileogastric—impulses from the ileocolic region are conveyed through the vagus to produce the reverse effect, i.e., when the ileocaecal valve opens the pyloric follows suit. The ultimate object of these reflexes seems to be the estab- lishment of a mechanism by which the quantity and/or quality of the contents of the small intestine is fixed at a definite level. Stated simply—when food material enters the duodenum through the pylorus a regulated amount of waste material is extruded from the small intestine into the
caecum.
The rate of extrusion into the caecum is therefore somewhat
irregular, but it continues until the caecum is filled. This filling is controlled to some extent by the feeble sphincter (caecocolic) present at the junction of the caecum with the ascending colon. No wave motion is demonstrable in the caecum; emptying of its contents is probably effected by a slow steady contraction of its muscular walls.
In the ascending colon and right colic (hepatic) flexure there is no evidence of any regular peristaltic propulsion. The great intestine—as already described—exhibits a series of sacculi into and along which the contents travel slowly. The chief function of this part of the alimentary tube is the absorption of water; the slow squeezing of the contents in the individual sacculi greatly aids in this process, so that by the time the transverse colon is reached the semi-liquid material from the caecum is converted into a semi-solid mass. The profuse mucous secretion from the glands in this part of the tube is useful in aiding the transit of the firmer material.
There is no direct evidence as to how progression along the colon is effected. It is possible, however, that each sacculus acts like a compressible rubber ball with two open ends. When such a ball is compressed the contents may be driven out in both directions. If, however, one opening is closed or narrowed the compressing force will tend to drive the bulk of the material through the other, i.e., in the direction of least resistance. Anatomical condi- tions in the ascending colon suggest that such a mechanism is provided. The full, contracting caecum and the caecocolic sphincter will act as barriers to the backward flow of material and determine that the contraction of the sacculi shall propel their contents in the direction of the transverse colon. It has been suggested that there is some sphincteric arrangement in the course of the transverse colon; there is no anatomical evidence of this, and any physiological sphincter must be feeble and must occupy an appreciable length of the bowel.
At this stage it is interesting and instructive to compare the two important valves—pyloric and ileocaecal—that play such a vital role in the processes of digestion and absorption. The differences in anatomical structure of the valves have already been pointed out. Their differences in physiological activity may now be reviewed.
ι. Material passing from the stomach through the pyloric valve is necessary for the nutrition of the individual.
Material passing through the ileocecal valve is devoid of most of its nutritional value.
2. On the proximal side of the pyloric valve is a sac (the stomach) in which much digestive activity takes place before the valve opens freely.
On the proximal side of the ileocecal valve material simply accumulates in the lower end of the ileum; the rhythmic opening of the valve is determined by contractions of the ileum, but the rate and degree of these may be influenced from the pylorus.
3. On the distal side of each valve is a saccular dilatation.
In the case of the pyloric valve the material allowed to enter this sac receives such treatment as will render it innocuous if passed back into the stomach.
In the case of the ileocecal valve, the material passing through is, after mixture with the caecal contents, definitely deleterious to the small intestine, and the lower end of the ileum is therefore specially reinforced with lymphoid tissue to deal with any backward leakage. The statement that “apparently the ideal which our vital mechanisms strive to attain is sterility of the alimentary tract above the ileocaecal valve with very free bacterial growth below that barrier”, emphasizes this point.
4. The pyloric valve acts primarily as a guard against unsuitable material leaving the stomach in the early stages of digestion—but it is also designed to allow a reflux of material from the duodenum into the stomach in the later stages of the digestive processes.
The ileocecal valve acts as a guard against material leaving the small intestine too rapidly, i.e., before sufficient time has been given for absorption. It also acts as a guard against unsuitable material entering the small intestine from below and is therefore designed to prevent any reflux.
5. There is a striking difference in mechanism between the two valves.
At the beginning of digestion the pyloric valve (except possibly for a minute aperture) is tightly closed, but during the process of digestion the pyloric orifice gradually enlarges until it is relatively of large size.
The ileocecal valve opens at intervals to permit the passage of material and then immediately closes again.
6. The muscular coat of the pyloric end of the stomach (except for a few longitudinal fibres) stops abruptly at the pyloric valve; related to this is the fact that the peristaltic waves of the stomach end at the pylorus and are not con-
tinued over the duodenal cap.
The muscular coat of the ileum is continued over the
ileocecal valve on to the caecum; peristaltic waves travelling down the ileum sometimes continue over the ileocaecalvalve to the wall of the caecum.