THE most prominent feature of the mucous membrane lining the alimentary tube is the great collection of glands related to it. The majority of these glands are embedded in the membrane. They are small (microscopic) single tubes lined by secretory cells and opening directly on the free surface of the mucous membrane. The material secreted may be of a simple nature as in the mucous glands, but in other glands, e.g., seromucous glands, certain stomach glands, and follicles of Lieberkühn, more than one substance may be produced in each tube. Modifications of the simple form are intro- duced by the branching of the tubules, and later by the aggregation of these branched tubules into composite glands (racemose glands) of macroscopic size still embedded in the wall of the alimentary tube, and opening into its interior. In more elaborate forms large composite glands are with- drawn from the wall, but their secretions still drain into the interior of the alimentary tube by long ducts that pierce the wall. This latter type is represented in the large salivary glands—parotid, submaxillary, and sublingual.
The most complicated glands directly associated with the alimentary tube are the liver, the pancreas, and the spleen.
The liver is connected with the interior of the tube by the common bile-duct; many of the substances, however, elaborated by the liver do not enter the alimentary tube, but are transferred directly into the blood-stream through the hepatic veins.
The pancreas has a large duct that opens into the duodenum beside the common bile-duct ; through this pancreatic duct the greater part of the secretion of the pancreas enters the alimentary tube. There are, however, portions of the pancreas that do not link up with this drainage; the cells of these portions secrete a substance that passes directly into the blood-stream.
The third gland, the spleen, is a ductless gland, i.e., it has no duct opening into the alimentary tube; all the materials elaborated in it are taken directly into the circulating blood.
Some details of the modifications in form and structure of the mucous membrane in the several compartments may be noted.
In Compartment I the important features are the following:—
- a. Teeth.—The maxillae and mandible are fitted with teeth of various shapes; those in the maxillae are immobile, but those in the mandible can be moved with the bone in various directions. For clinical purposes it is essential to remember that the teeth are living structures, taking from and delivering to the blood-stream certain materials.
- b. Glands.—Many small mucous and some serous glands are widely distributed in the mucous membrane of the lips, cheeks, palate, and dorsum of the tongue. In addition to these isolated glands, three large composite salivary glands on either side—parotid, submaxillary, and sublingual— open into Compartment I. The parotid gland, situated in a recess below the ear, is drained by the parotid duct, which opens opposite the second upper molar tooth. The sub- maxillary gland lies under cover of the mandible in the floor of the mouth; its duct opens at the side of the frenulum linguae. The sublingual gland is under the plica sublingualis in the floor of the mouth and its ducts (18-20) open on the plica. A special collection of mixed serous and mucous glands (Blandin or Nuhn) is situated in the mucous membrane of the under surface of the tongue just behind the tip. All the large salivary glands are surrounded by muscles that are capable of compressing them.
- c. Taste Buds.—These are found mainly on the dorsum of the tongue in relations to the papillae there. They are also present on the oral surface of the palate and on the posterior surface of the epiglottis.
In the mucous membrane covering the isthmus of the fauces there is a thick ring of lymphoid tissue. O n either side, between the anterior and posterior pillars of the fauces, this tissue forms a large mass—the palatine tonsil. O n the surface of the base of the tongue is a smaller mass described as the lingual tonsil.
In Compartment II scattered mucous glands are present in the pharyngeal wall. Lymphoid tissue is prominent and a mass of it in the roof and posterior wall is termed the pharyn- geal tonsil. In the mucous membrane of the oesophagus are many large mucous glands and also other glands com- parable with those found in the cardiac end of the stomach. Owing to the loose nature of the tunica mucosa, the lining of the empty oesophageal tube is thrown into a series of longitudinal folds.
The mucous membrane of Compartment III presents a variety of glands and an interesting disposition of the surface layers.
In the stomach there are three varieties of specialized glands (Fig. 12). (a) Cardiac glands (few in number) are situated close to the oesophageal opening; they comprise (i) single tubules with short columnar granular cells, and (ii) compound racemose glands. The cardiac glands secrete mucus in an alkaline medium, (b) Fundus glands are found in the fundus and body of the stomach ; each has one or more tubules lined by short columnar cells termed chief or central cells. At intervals between these cells and the basement membrane are larger spheroidal cells called parietal or oxyntic cells. The cells lining the necks of the fundus glands, although apparently similar in appearance to those in the bodies of the tubules, are physiologically different; they secrete mucus, (c) Pyloric glands, in the pyloric part of the stomach, are characterized by short ducts draining groups of short but tortuous gland tubules. The lining cells resemble the neck-cells of the fundus glands; their secretions, which consist entirely of mucus, are alkaline in reaction.
In the first half of the duodenum are Brunner’s glands that form an almost continuous layer up to the opening of the bile-duct; beyond this they diminish progressively. Amongst them are intestinal glands and nodules of lymphoid tissue. Brunner’s glands resemble the pyloric glands in structure; their compound tubules, however, are disposed in the sub- mucous tissue.
The HCl of the gastric juice is secreted by the parietal or oxyntic cells; it is important, therefore, to note that these are found almost exclusively in the fundus glands of the fundus and body of the stomach. Attention is also drawn to the fact that, in addition to simple goblet mucous cells in the lining membrane of the stomach, all the specialized glands, cardiac, fundus, and pyloric, contribute a profuse mucous secretion which is of great physiological importance.
The inner surface of the stomach is marked by numerous prominent folds or rugae projecting into the interior. Some of them are continuations of the longitudinal folds of the oesophagus and are disposed along the lesser curvature of the stomach as far as the pylorus. Over the rest of the stomach the folds are mostly longitudinal in direction, and are largest and most numerous in the greater curvature; there is, however, considerable irregularity and cross branching in their arrangement. They gradually become more nearly parallel and converge towards the pylorus. Longitudinal folds resembling those of the stomach are found in the duodenal cap.
The whole of the surface of the stomach mucous membrane is marked by small polygonal areas occupied almost entirely by the large openings of the stomach glands—an arrangement designed more for the free exit of the gland secretions than for the absorption of stomach contents.
In the first half of the duodenum the surface is covered by small villi; they begin at the pylorus as short flat pro- jections, but gradually longer and narrower ones make their appearance farther down the bowel. At a distance of 1-2 in. from the pylorus plicae circulares begin to replace the longitudinal rugae of the duodenal cap. These are perma- nent transverse shelf-like folds of the tunica mucosa; they are not obliterated or dispersed even when the bowel is fully distended. At first they are small, irregular, and scattered; lower down they are larger, and at the level of the opening of the common bile-duct are distinct and prominent.
The proximal part of Compartment IV consists mainly of the second half of the duodenum, together with the jejunum and ileum. The whole surface of its mucous membrane is covered by an enormous number of small villi—finger-like projections of the tunica mucosa. These villi make their first appearance in the upper part of the duodenum; they are present as far down as the ileocaecal junction. The intestinal glands (Lieberkühn) open on the surface of the mucous membrane between the bases of the villi.
Lymphatic nodules project on the surface of the mucous membrane. Most of them are small and are widely distri- buted. Larger aggregations of lymphoid tissue (Peyer’s patches) may be noticed in the jejunum, but it is in the lower part of the ileum that they are progressively largest and most
numerous down to the ileocaecal junction.
The most striking and characteristic features of this part of Compartment IV are the plicae circulares (valvulae conni- ventes). As mentioned above, these are noticeable first in the upper part of the duodenum. They reach their fullest development in the lower part of the duodenum and in the upper half of the jejunum. Thence they decrease in size and number and are practically absent below the middle of the ileum. They are covered with villi, and intestinal glands open on their surfaces. Forming, as they do, a series of transverse shelves, they serve to increase greatly the surface area of the mucous membrane and tend also to delay the progress of the intestinal contents.
In the more distal parts of Compartment IV, i.e., caecum, ascending colon, and right half of the transverse colon, the mucous membrane differs markedly from that of the small intestine. It is devoid of villi, and exhibits a smooth surface marked with the openings of large glands. There are numerous lymph-nodules, particularly in the appendix, where the submucous layer is almost entirely occupied by lymphoid tissue. The lumen of the final part of Compartment IV exhibits a sacculated form with transverse projecting shelves at intervals; these shelves, as already noted, consist of almost the entire thickness of the bowel wall.
In the greater part of Compartment V, namely, the left half of the transverse colon, the descending colon, the iliac colon, and the pelvic colon, the arrangement of the mucous mem- brane is very similar to that of the terminal part of Compartment IV. In the rectum, when the tube is empty or nearly so, the mucous membrane is arranged in numerous irregular rugae. When the tube is distended three transverse folds appear ; these are the rectal valves already described (p. 11).
In Compartment VI the mucous membrane is disposed in permanent longitudinal folds—the rectal columns (Morgagni). It contains very few glands or lymph-nodules.
It will be noted that some of the folds of the mucous mem- brane, e.g., plicae circulares, villi, etc., are described as permanent. Other folds, e.g., rugae, while present in definite regions, exhibit patterns that vary from time to time. All these folds are obviously of physiological importance, even those that vary in position, and it has been suggested that the musculares mucosae may assist in preventing their obliteration during the filling of the tube.