THE contents of Compartment I are under voluntary selection and voluntary control. Many considerations may be in- volved in making a selection from the materials offered before food is actually taken into the mouth. After the materials chosen have entered the mouth a further selection is possible. T h e chief factors in the final selection are the taste-buds (flavours), the teeth, and the muscular walls (consistence). The substances ultimately selected are then subjected to the mechanical and chemical arrangements of the compartment. Fluids are passed through with little change into Compartment II. Solid materials, however, are gathered into a mass or bolus. The muscles of the tongue, cheeks, and lips bring this into contact with the teeth; these are specially shaped for cutting, tearing, and grinding, and are activated by the muscles of the mandible. At the same time, the bolus is mixed thoroughly with the watery secretions of the serous glands and with the thicker lubricating material of the mucous glands.
Chemical action in the bolus is effected chiefly by the secre- tions from the great salivary glands, namely, parotid, sub- maxillary, and sublingual, which contribute a ferment—ptyalin—to mingle with the mass. Ptyalin acts on the cooked starch of the food, converting it into sugars; it is most potent in a dilute alkaline medium and is capable of effecting a very appreciable amount of chemical change.
In addition to the local mechanical and chemical changes that are being carried out in the mouth, there are some remote effects that have a very important bearing on the digestive processes in other parts of the alimentary tube. The sight or smell of appetizing food, the effect of such food on the taste-buds, and even the mechanism of chewing it, induce a flow of gastric secretions—so-called psychic and appetite juices. This is a reflex phenomenon and the efferent impulses are conveyed by the vagus nerves. When the distribution of the vagus nerves to the whole of Compartments III and IV is recalled, it seems a reasonable assumption that this reflex stimulus will extend to the glands connected with the small and part of the large intestine. It might almost be regarded as a cget ready’ signal to the chemical apparatus of the digestive portions of the tube.
When it is voluntarily decided that sufficient time has been given to the preparation of the food in the mouth, the bolus is swallowed, i.e., passed through the faucial isthmus into Compartment II. Since a few taste-buds are to be found on the back of the epiglottis it is obvious that there still exists a final voluntary control before the food is passed into the pharynx.
The large circle of lymphoid tissue in the faucial isthmus merits some attention. It can have little effect on the micro-organisms within the bolus; its role rather is to deal with fragments remaining after the main bolus has passed. An oral secretion of watery saliva is practically continuous; it acts as a cleansing agent in the mouth and at frequent intervals is swallowed into the pharynx. The lymphoid tissue has thus an opportunity to act on organisms in the course of this more or less slow, steady flow. The accumulation of lymphoid tissue in the upper part of the pharynx can deal with organisms introduced through the nasal cavities.
Having reached the pharynx the food bolus is propelled by striated muscles into the upper part of the oesophagus and is taken over entirely by unstriated muscles at the junction of the upper and middle thirds of this tube. Its passage through the oesophagus to the stomach is extremely rapid, although occasionally it may be halted for a very short interval in the last inch of the oesophagus at the so-called cardiac sphincter. There is very little confirmation of an anatomical sphincter at this part, but a sphincteric action may be demonstrated occasionally by radiography. Under normal conditions there are no peristaltic waves along the oesophagus; a generalized contraction of the muscles and an associated inhibition of the sphincter suffice to convey the foodstuffs into the stomach. It is possible that there is also, particularly in the case of liquids, a suction action which aids in the transit of the materials. The whole movement is under the control of the vagus nerves.
There is no absorption of foodstuffs from Compartments I and //.
Compartment III may be described as the final preparatory chamber, since it is here that the swallowed foodstuffs under- go chemical and mechanical changes that will fit them for presentation to the chamber in which the terminal stages of the digestive processes are carried out.
It is important to consider the work of this compartment in its entirety, i.e., from the cardiac orifice of the stomach to the middle region of the duodenum. It simplifies description, however, to discuss separately the three portions that consti- tute the compartment: (a) The stomach; (b) The duodenal cap; and (c) The proximal part of the duodenum. In the stomach the food materials are held up by a definite strong barrier—the pyloric valve—until certain changes have been effected. The altered substances are then passed into the duodenal cap, which is a somewhat sluggish sac. Here they are met by the active agents from the part of the duodenum immediately distal and are passed on for further preparatory changes before entering Compartment IV. A more detailed study of the work allotted to these three portions may now be undertaken.
The Stomach.—In the first instance the main chemical changes for which the stomach is responsible may be con- sidered. Gastric juice is an acid fluid containing one import- ant ferment—pepsin—and a secondary one—rennin. Even in the fasting stomach there is a secretion of fluid from the gastric glands, and (as already noted) this is increased by the addition of the psychic and appetite juices when food is visualized or taken into the mouth : but the greatest stimula- tion of gastric juice is effected when the food materials actually enter the stomach; the various classes of foodstuffs, however, differ in their powers of exciting the flow of gastric juices. It is generally agreed that water and carbohydrate materials stimulate gastric secretions by simple contact with the mucous membrane; this action is not very vigorous. Fats exert a delaying action on the secretions. Since the point has some clinical importance, it is interesting to note that the presence of fat in the duodenum liberates a hormone —enterogastrone ; this is absorbed into the blood-stream and its action is to inhibit not only the gastric secretions, but also gastric motility. Nitrogenous compounds—e.g., meat, meat extracts, coffee, etc.—and fruit juices act as powerful stimulants, and it is believed that most of this is effected through the agency of a hormone. The early pro- ducts of gastric digestion are also very active stimulants of gastric secretion. A useful pointer to the comparison of the foodstuffs in this connexion is provided by noting that as a result of long practical experience the sequence adopted in an ordinary simple dinner, is soup, meat, carbohydrates,
fruit, and coffee.
For practical purposes the two most important contents of the gastric juice are pepsin and HCl. H C l has definite powers as an antiseptic and can reduce considerably the number of micro-organisms that may have entered the stomach. It has, in addition, the function of converting into a soluble form many of the mineral constituents of the diet. Since the great digestive ferments of the small intestine are active only in an alkaline medium, it is essential that in the stomach a ferment should be provided that can act in an acid medium. This ferment is pepsin, and its action is necessarily local and much more limited than that of the intestinal ferments; it breaks down the large molecules of the proteins into peptones, but the molecules of these are still too large to be suitable for absorption into the circulation.
Summarized, the action of the gastric juice on the various constituents of the ordinary foodstuffs is:—
Proteins.—By the action of pepsin and H C l proteins are converted into acid metaprotein, proteoses, and finally peptones.
A special note may be made of the effect on milk. The caseinogen of milk is acted on by the milk-curdling ferment (rennin) and is converted into soluble casein; this is precipi- tated by calcium salts, and the calcium paracasein so formed is digested to peptones by the pepsin and HCl.
Carbohydrates.—Practically no change.
Although the gastric juice has no action on the carbo- hydrates a reference back will show that when food arrives in the stomach it is mixed with the ptyalin ferment of the saliva. This ferment can act in an alkaline, neutral, or feebly acid solution, so its action continues in the stomach until the acid of the gastric juice is sufficiently mixed with the food materials to alter the reaction and bring the action of the ptyalin to a standstill. In the interval ptyalin has converted some of the cooked starch into dextrin and maltose.
Fats.—Digestion of the nitrogenous envelopes—practically no action on fats.
Mineral Salts.—Conversion of many of the insoluble salts into soluble chlorides.
Micro-organisms.—Destroyed.
It may be noted in passing that the chemical actions have also a mechanical influence. Their effect on the larger masses is a softening-up process that enables these masses to be broken up more readily by muscular action. Also as ancil- laries may be reckoned the secretions of the mucous glands.
The chemical activities of the stomach are necessarily supplemented by its motor activities. Before considering these, it may be well to review some of the properties of involuntary muscle that apply not only to the stomach, but to the whole of the part of the alimentary tube activated by involuntary muscles. One of the most important properties of involuntary muscle is its tone—a property which it has in
common with voluntary muscle.
Tonus.—Since there exists considerable confusion regarding the actual condition implied by the term tonus (or tone), an explanation of its precise significance is important. Every living cell is in a state of constant chemical activity, but the amount and nature of this naturally vary in the individual cells of the different tissues. Changes in the activity of the cell are brought about by stimuli of many types, and it is the degree of the efficiency of the responses to these stimuli that provides a measure of the functional efficiency of the cell. Stated briefly, the property of the living cell that enables it to control the responses to suitable stimuli is termed its tone. A normal tone may be postulated for an individual cell; if a cell of the same type responds more rapidly and strongly to stimuli it is described as hypertonic; if, on the other hand, the responses are more sluggish and feebler, the cell is termed hypotonic. This conception of cellular tone has been extended gradually to tissues and organs; these, therefore, are classified between the ranges of hypertonus and hypotonus according as their activities exceed or fall short of an average standard.
Finally, by a further extension the terms become applicable to the body as a whole. Individuals of sturdy physique and quick reactions are described as hypertonic (or hypersthenic), while frail individuals with slow responses and more sluggish metabolism are termed hypotonic (or hyposthenic). Tonus or tone in this way becomes an expression of the general body habitus. With the wider application of the term, however, complications may be introduced. It is possible, for instance, for certain organs or tissues in the hypertonic individual to be in a state of hypotonus, or vice versa. This may be due either to general causes (e.g., selective toxaemias) or to local disturbances (e.g., circulatory obstructions, etc.).
The application of these facts to involuntary muscle may be considered briefly. Amongst several theories advanced, the simplest is that which takes the view that each muscle-fibre contains two contractile elements. When a muscle is stimu- lated one of these elements responds with a quick contraction that produces a change of form or posture of the muscle and of the part on which the muscle is acting. This con- traction is followed almost immediately by a relaxation, and were it not for the action of the second element the original form of the part would be restored. To the second element, however, is allotted the function of fixing and preserving the new form or posture resulting from the contraction of the first element. Unlike the first element, therefore, this element does not relax until a different form is projected; the second element thus does not produce movement—it controls it.
It is unfortunate that the terms muscle tone or postural tonus should be applied to the results of the activity of the second element, as there is a basic difference between this and cellular tone; the older term active posture or, alterna- tively, postural adaptability expresses more clearly the main characteristic of the phenomenon. A comparison of cellular tone with postural tone shows that: (i) Cellular tone is inherent in every living cell and is dependent only remotely on nervous influence; it determines a state of preparedness for dealing with any alterations in the activity of the cell or tissue; (2) Postural tone is confined to muscle tissue and is dependent on a muscle contraction in a localized region; it is controlled, therefore, directly by intrinsic and extrinsic nervous influences, and its effect is to stabilize a new form or posture.
It will be noted that both types are always present in a living muscle and both exhibit a degree of resistance to any change in the existing form or posture. At the same time, the extremely close connexion between the two types of tonus will be evident. The state of preparedness (cellular tone) of a muscle must be a dominating factor in the character of the contraction that leads to postural tone.
The main object of the present discussion is to show the part played by postural tone in the normal mechanism of the alimentary tube. Under ordinary conditions it is very rare to find a hollow tube (or viscus) completely empty. When the contents are reduced to a minimum the established postural tone suffices to keep the walls practically in apposi- tion. When food or chyme enters the tube the stretching of the walls would be the adequate stimulus for the contraction of the muscular tunic, but since such a contraction would prevent the entry of the food or chyme, the contraction is inhibited (receptive relaxation) and the tube distends. T h e inhibition affects mainly the first contractile element. A contraction of the element of postural tone continues suffi- ciently to keep the wall of the tube closely applied to the new contents. As soon as the entry of food or chyme ceases the inhibition is removed and both elements contract normally with a force proportional to the degree of distension.
When it is remembered that in the living alimentary tube the contents are being subjected to various digestive and absorptive processes, it will be realized that the stretching of the walls and the related muscular contractions are subject to constant variations. T h e contractions of the second element keeping pace with those of the first stabilize the new positions as they arise; an extremely apt analogy compares their effects to those ofc 4 an infinitely fine and varied ratchet “.
This is a very economical arrangement, as the contraction of the second element can be maintained for an almost indefi- nite period with an extremely small expenditure of energy.
The practical result of the postural mechanism is to convert the wall of the tube (or hollow viscus) into a semi-rigid structure that will resist pressure from either within or without. The great importance of this to the alimentary tube, which is moving freely amongst other viscera during frequent changes of intra-abdominal pressure, will be suffi- ciently obvious ; by its influence there is a minimum of inter- ference with the processes taking place within the walls and cavity of the* tube.
One other important point may be noted. As already stated the contraction of the second (postural) element does not produce movement and has, therefore, no compression force on the contents of the tube; in the case of the stomach, for example, it does not affect the intra-gastric pressure.
Another important property of involuntary muscle is that of rhythmic contraction. As stated above, the adequate stimulus for the contraction of involuntary muscle is the stretching of its fibres. When the fibre is stretched it con- tracts, not with a single twitch, but with a series of definite rhythmic contractions and relaxations. This is a property of the muscle’ itself, but may be controlled by nerve influences.
The rhythmic rate and the amplitude of the contractions vary considerably in different parts of the alimentary tube and may be modified by external influences.
A third property of involuntary muscle is its ability to initiate and transmit waves of contraction—peristaltic waves —along segments of the alimentary tube. These waves have a much slower vibration rate and are usually of greater amplitude than the rhythmic waves. They differ also from the ordinary static rhythmic waves in that they travel fairly rapidly along the walls of the tube. The peristaltic waves are superposed on the rhythmic waves, and as there is no proportional timing of them, may clash with the rhythmic waves at irregular intervals.
In all the hollow tubes (or viscera) the state of (relative) emptiness may be regarded, for practical purposes, as that in which the length of the muscle-fibres is initial. The intro- duction of any material into these tubes stretches the walls and provides the stimulus for the contraction of the muscular tunic. During the filling of the tube, as already explained, the actual contraction is inhibited, but when the inhibition is removed, the muscle contraction comes into play with a force that (within certain limits) is proportional to the amount of stretching. This contraction is directed towards the emptying of the tubes and is continued until the tube is empty and the initial length of the fibres is regained.
It is believed that not all the fibres contract simultaneously —relays of them come into action and thus energy is econo- mized by the resting periods provided for groups of fibres.
The three chief results of the activities of the gastric muscles are: (i) A thorough mixing of the foodstuffs with the various secretions of the stomach; (2) A mechanical reduction of the larger masses in the foodstuffs; and (3) The propulsion of the altered food materials towards and through the pyloric valve.
As already noted, the stomach is never completely empty, and, because of the tone of its muscles, is never completely relaxed.
Into the closed stomach the food is driven by the muscles of the pharynx and oesophagus. The stomach musculature is inhibited (receptive relaxation) and the viscus gradually distends. The first part of the food that enters from the oesophagus makes its way towards the pyloric end (see Fig. 3), where its progress is stayed by the thicker and tougher muscle of the pyloric region. The later instalments of the meal, therefore, tend to fill the saccular portion of the stomach which distends towards the left. In this saccular portion are to be found the chief accumulations of the acid-producing cells (see Fig. 12), so that here the foodstuffs may be most thoroughly saturated with pepsin and HCl. The filling of the stomach is associated with a reflex relaxation of the abdominal muscles.
In the early stages some of the fluids find their way along the tubular portion of the stomach, but probably until the intervals between the rugae of the mucosa are filled, fluids will pass also with the more solid parts of the meal into the saccular portion.
Note particularly that, as explained above, the inhibition of the gastric muscles is not absolute; i.e., the muscles are not completely out of action; throughout the whole period of the distension they are maintaining their postural tone and are thus keeping a uniform steady grasp on the gastric contents.
Another point of importance is that the inhibition of the muscles is contemporaneous with the stimulation of the secretory apparatus of the stomach through the vagus nerves.
An ordinary meal may consist of only one course, but usually there is more than one. At the end of the meal, or in the intervals between the courses, the inhibition of the gastric muscles is removed and the stretched muscles then react with steady rhythmic contractions, the vigour of the response depending on the amount of the stretching. The first effect of this is to keep pressing into the tubular portion the food materials that have accumulated in the saccular portion. Gradually this compression force acquires a pro- pulsive factor. The direction in which the food is propelled is decided by the fact that there is a graded excitability of the gastric muscles from above downwards. The result is that the food materials are pushed slowly in the direction of least resistance, i.e., from the cardiac to the pyloric end of the stomach. Naturally, the more fluid parts of the diet are most easily mixed with and acted on by the gastric juices, and they reach the pyloric end at an early stage in digestion, while materials of greater consistence lag behind for more thorough treatment.
The transit of food through and out of the stomach is aided by a series of peristaltic waves; these begin about the middle of the stomach and travel downwards towards the pylorus at the rate of about three a minute. The arrival of some of these peristaltic waves at the pylorus is associated with an opening of the pyloric valve and the expulsion of some of the gastric contents into the duodenum. In many cases, however, the waves stop short of the pyloric ring, and some may initiate a systole of the muscles of the pyloric canal. When the opening of the pyloric valve does not take place, or is insufficient, the gastric contents driven against it recoil to mingle again with the materials in the pyloric canal and pyloric antrum. By this mechanism a thorough mixing of gastric contents is obtained.
It is doubtful if the pyloric orifice is ever completely closed. Very shortly after food enters the stomach liquids begin to find their way through a small pyloric opening into the duodenal cap. The orifice, however, remains small for a considerable time and does not permit the passage of the more solid parts of the food; these, as mentioned, are from time to time reflected back into the stomach for further treat- ment. As digestion proceeds, the pyloric opening increases in diameter, not steadily, but at irregular intervals to permit the passage of more and more thick chyme, i.e., the products of stomach digestion. The exact manner in which this
enlargement of the pyloric orifice takes place, and the factors that are involved in bringing it about, are still matters of much controversy. The ultimate result, however, is that at the end of the gastric digestive processes the pyloric orifice is extremely patulous and distensible and will permit the passage of even large objects, some of which may be com- pletely resistant to the activities of the gastric juices.
The Duodenal Cap.—Further interest now centres on the duodenal cap into which the chyme is discharged. This, as has been described, is a somewhat sluggish reservoir; its muscular tunic contains no circular fibres and consists merely of some longtiudinal fibres carried on from the stomach, and its blood-supply is poor. As it merges into the rest of the duodenum, circular fibres and more longitudinal fibres gradually appear in the wall.
In the earliest stages of digestion the liquid contents of the stomach pass with comparative freedom through the small pyloric orifice into the duodenal cap. A little later stronger propulsive waves from the pyloric canal probably carry chyme straight through the duodenal cap into the adjoining more active part of the duodenum.
The Proximal Duodenum.—This active portion of the duodenum, unlike the pyloric part of the stomach, does not exhibit a sharply defined sphincter, but it does interpose a zone of defences to prevent the chyme entering Compartment IV before it has undergone further treatment. This defence zone has at its disposal both mechanical and nervous factors. Mechanical barriers are provided by the plicae circulares and the very active muscular wall; the nervous block is exhibited by a reflex that evokes the closure of the pylorus whenever any liquid or chyme is present in the duodenum. Little time is lost, however, in dealing with the chyme in the duodenum, and it is swept rapidly onwards, leaving the tube ready for further supplies from the stomach. This is effected by a special nervous mechanism which ensures that expulsion of stomach contents and emptying of the duodenum are synchronized.
No peristaltic waves traverse the duodenal cap; the peri- staltic waves of the stomach are completely blocked at the pylorus. When, however, a gastric peristaltic wave arrives at the pylorus, a peristaltic wave begins near the distal end of the duodenal cap and travels down the duodenum. The duodenal cap is emptied by a general contraction of the thin layer of longitudinal fibres that constitutes its muscular wall. In the early stages the contraction of these is not sufficiently strong to force materials through the small pyloric aperture into the stomach, so the contents of the cap, following the line of least resistance, are driven into the more active part of the duodenum. Here they mix with bile, pancreatic secretions, and alkalis. When the adjacent duodenum contracts, the cap relaxes and some of this mixture re-enters the cap ; the cap thus becomes a mixing chamber in which the acid gastric moiety is partly neutralized by the alkaline duo- denal secretions. As digestion proceeds the pyloric orifice increases in diameter ; the emptying stomach diminishes in size and its secretory activity lessens. The stronger alkaline mixture from the duodenum meets and neutralizes the less active acid mixture from the stomach, and, with the opening of the pylorus, contractions of the duodenal cap and adjacent duodenum now drive an alkaline digestive mixture into the stomach. This is returned by the gastric muscles and a to- and-fro movement is carried on until the stomach has emptied.
Meanwhile, in the duodenum distal to the cap and almost as far as the opening of the common bile-duct, much pre- paratory activity is going on. Bile, pancreatic juice, and other intestinal secretions find their way into this* part and fragmentation of the larger particles and admixture of the chyme with the contained juices is carried out with con- siderable vigour. The forms of movement involved in these
processes will be considered with those of the rest of the small intestine. It will suffice to emphasize here that the process of breaking up the larger fragments of the chyme is a very thorough one, and it reduces the chyme to a very complete pulp. When this stage is completed the chyme is ready to be presented to Compartment IV for its final digestive disposal.
It is interesting to compare, from the point of view of structure, the mucous lining of the proximal half of the duodenum with that of the distal; this reveals the marked difference between a portion which is still engaged in preparatory work and one in which the more delicate opera- tions of digestion and absorption are involved. In the proximal half, by comparison, the plicae are fewer and less regular, and the villi are much less numerous, shorter, and more strongly fashioned.
There is practically no absorption of food products from Compartment III. Possibly a little water and some glucose may be absorbed, but the amount is negligible. Since, however, the venous drainage of the compartment passes into the liver, a certain amount of absorption may be pre- sumed. The materials absorbed are not necessarily products of gastric digestion; it is believed, for instance, that the enzyme necessary to convert the primitive blood-cells of the bone-marrow into normal blood-cells is formed in Brunner’s glands, and possibly also in the somewhat similar glands of the pyloric part of the stomach, and carried by the blood- stream to be stored in the liver. Also it is held that a sub- stance—gastric secretin—which stimulates a secretion of pepsin in the later stages of digestion is absorbed from the wall of the pyloric part of the stomach.
In the wall of the first part of the duodenum, a hormone —secretin—is elaborated. This hormone is absorbed into the blood-stream and returns to stimulate pancreatic secre- tion.
APPETITE AND HUNGER
The motor activities of the stomach have sometimes been credited with giving rise in the healthy stomach to certain general sensations; the chief of these are appetite and hunger. In everyday speech the expressions “to have a good appetite” and uto be hungry” are frequently used to denote the same conditions. Actually, however, they should, for practical purposes, be carefully differentiated, although both may be present at the same time.
Appetite is selective, and the materials capable of evoking it are peculiar to the individual. The actual stimulation may come from substances visualized, smelled, or tasted; even the memory of such substances may suffice. It is largely independent of the stage of distension of the stomach to the point of overloading; it varies, therefore, during the various courses of a meal, and certain materials given before or with a meal are credited with powers of increasing it. Its psycho- logical effect is supposed to be a definite increase in the secretory and/or motor functions of the stomach.
Hunger, like thirst, is a systemic demand. Under normal conditions the demand is very slight and is aimed at keeping up a certain reserve of nutritive material in Compartment IV. To conform to social requirements, the demand is ignored to a great extent in the fixing of meal hours. When, however, there is a danger of the normal reserve being so depleted that insufficient nourishment may be supplied to the body, a more imperative demand reaches the sensorium and a definite feeling of hunger is aroused. The response to the demand is a call on Compartment III and later, if necessary, on Com- partments I and II. This response is predominantly motor and has been observed most carefully in the stomach. Experi- ments have shown that in the empty stomach a series of peristaltic waves—hunger contractions—are demonstrable. These waves are not simply exaggerations of the ordinary digestive peristaltic waves—they are superposed and are obviously part of an additional reserve mechanism. They will be discussed more fully when abnormal conditions in the alimentary tube are considered.
The sensation of hunger is thus dependent upon the lack (relative or absolute) of sufficient nourishing materials in the upper reaches of the alimentary tube. Its exciting stimulus is the stretching of the nerves in the muscular tunic, in contra- distinction to the sensation of appetite, where the exciting stimuli are derived mainly from the sensory distance-receptor organs. In both sensations a desire for food is implied, but it will be obvious that, whereas hunger demands the supplying of food, appetite simply prepares the way for the enjoyment of it. Since so many of the responses to the sensations of appetite and hunger take place in the stomach, it is natural
for the individual to relate those sensations to this viscus. It is not possible, however, to refer them strictly to the stomach, as the actual position of this in relation to the surface of the abdomen is so extremely variable. The location of the sensations (if any) is a very diffuse area corresponding to the nervous representation of the whole of Compartment III on the abdominal wall.