CHAPTER XI.

THE ALIMENTARY CANAL.

THE TONGUE; THE TEETH; THE SALIVARY GLANDS.

GENERAL PLAN OF CONSTRUCTION OF THE CANAL.

Terminates at the anus.
THE

HE alimentary canal commences at the mouth, and Its epithelium, with the exception of that lining the mouth, the oesophagus, and the termination of · the rectum, is wholly of hypoblastic origin. The large glands (the salivary glands, the liver, and pancreas) are special outgrowths of the epithelial layer, surrounded with connective tissue and blood-vessels. Before describing the structure of the various parts of the canal it will be well to mention, to some extent in detail, its general plan of construction. internally by epithelium, which differs in character in different regions, according to the function which it has to subserve. In the mouth and œsophagus, where this is largely mechanical, it is of the nature of stratified squamous epithelium; in the stomach and intestine, where its function is absorptive and secretory, it consists of a layer of "glandular cells" of the columnar type; and the same holds good with the large intestine, and the rectum to the margin of the anus, where it becomes protective in function, and here again it is of the nature of stratified squamous epithelium. Bencath the epithelium, throughout the whole of the canal, is a layer of connective tissue, which is modified in the various parts with a view to the function of these parts. In the mouth, cesophagus and rectum, it is of the nature of ordinary connective tissue; that is, it consists of white fibres, with a certain proportion of elastic fibres and connective tissue cells. In the

stomach, the small and the large intestine, it is more of the nature of adenoid tissue. Throughout, this connective tissue contains blood capillaries, which are developed to an especial extent in those parts of the tract lined by active glandular epithelium. It contains also in some parts special muscular and lymphatic arrangements, which will be dealt with when the individual parts are more fully described. At the surface of this connective tissue, forming in sections a line between it and the epithelium covering it, there is a "basement membrane"; in this case consisting of a thin layer of condensed tissue-a layer made up partly of connective tissue and partly of flattened connective tissue corpuscles. The membrane is thus not a complete one, but composed merely of a surface condensation of connective tissue. Beneath the layer of connective tissue there is one of non-striped muscle, a narrow layer, composed for the most part of fibres arranged circularly and longitudinally, the internal ones circularly and the external, longitudinally. This layer is absent in the mouth, and imperfect in the œsophagus, but fairly complete throughout the rest of the canal. These three parts-the epithelium, the connective tissue, and the muscularis mucosa—constitute together the mucosa of the alimentary canal.

Outside the mucosa is found the sub-mucous layer of connective tissue, which consists throughout of typical areolar tissue. It is a broad layer, loosely arranged, containing large blood-vessels, nerves, and lymphatics, and connects the mucous coat with the subjacent muscular one. It is distinctly loose, and admits of free movement within certain limits between the mucosa and the muscular coat; so free that the former readily forms folds upon the lumen of the canal, when the latter is at all contracted. It corresponds with the deeper part of the cutis vera of the skin, which allows of free movement of the epithelium and more condensed fibrous tissue beneath it, upon the subjacent structures.

Outside the submucosa we have the muscular wall of the canal. Except in part of the oesophagus, this consists throughout of non-striped muscle, which is specially developed at certain points to form sphincters. It is arranged in two layers, a thicker internal circular coat, and an outer thinner longitudinal one. Between the two is a small amount of connective tissue, containing Auerbach's nerve plexus.

Surrounding the canal throughout the greater part of its extent we have the peritoneal covering, consisting of a delicate

layer of connective tissue, invested with simple squamous epithelium.

Fig. 97 shows the relations of these parts, throughout the whole of the canal, diagrammatically.

Secretion and absorption by the alimentary canal.-Secretory epithelium, in its simplest form, consists of a layer of cells with a stratum of vascular connective tissue immediately beneath it. The cells withdraw from the lymph, with which the tissue is bathed, those substances necessary for the formation of the secretion which they pour out upon their free surface. Any of the serous sacs will serve as an illustration of this simple arrangement. Usually, however, when the secretion is at all a special one, the arrangement is much more complicated. The first differentiation consists in an inpushing of the epithelium in the form of a tubular or saccular process into the tissue beneath it. Instances of these simple tubular or saccular glands are found in Lieberkühn's follicles of the intestine of mammals, and in the simple glands of the frog's skin respectively. These are termed simple tubular and saccular glands, the name being descriptive of the shape of the invagination. They are represented diagrammatically in Fig. G. When the secreting epithelium has advanced to this stage of specialisation, its cells usually possess a more or less cubical or columnar shape, and it is termed, glandular epithelium. The next step towards complexity consists in the division of the simple tube or saccule, so that a compound An instance of a compound gland of the tubular variety, in its early stage of development, is found in

gland is the result.

Fig. G.-Diagram of different classes of glands: A-Simple: B-Commencing complexity; C-Compound; a-Tubular; 6-Saccular.

the glands of the stomach, to be shortly described. Here, the lower part of the tube becomes divided, and constitutes the secreting part of the gland, the upper part remaining single and

being termed the duct. A later stage of development is shown in the case of the testis, where we have the duct dividing again and again into smaller subdivisions before the terminal secreting portion of the gland is reached. An instance of a compound gland of the saccular variety is found in one of the salivary glands, e.g., the submaxillary. Here we have again a repeatedly subdividing duct, the ultimate subdivisions of which terminate in clusters of small saccules or alveoli. These various kinds of glands will be described in greater detail as they occur; at present it will be sufficient to draw attention to the general principle of construction which is common to all. They all consist of an inpushing of epithelium (or in the case of glands of the alimentary canal an outpushing); this inpushing may remain simple, as it frequently does when the glands are spread over a large surface, as in the case of the intestine, or it may become compound when a considerable surface is required, but little space is available, as in the case of the salivary glands. Whether the gland is simple or compound, its epithelium rests upon a basement membrane, separating it from the connective tissue around it. The basement membrane is, however, usually of the nature merely of a surface condensation of this tissue. The surrounding connective tissue supports a blood capillary network. The substances required for the production of the secretion of the gland pass from the blood capillaries into the lymph spaces of the connective tissue, through the basement membrane, into the gland cells. Here, they are sometimes combined in a new form, ie., rearranged, or, without such rearrangement, they may be passed on into the lumen of the gland tubule or saccule, as the case may be, and thence poured out upon the surface either directly in the case of simple glands, or, indirectly, through the duct or ducts in the case of compound glands.

The stomach, small and large intestines, are absorptive as well as secretory, and the process of absorption is performed by that part of the epithelium between the invaginations which constitute the glands. These intervening portions of epithelium are in the small intestine specially developed in connection with the connective tissue beneath them to form villi. This absorptive epithelium is of the columnar glandular variety. In this case, the stream passing through the epithelial cells is in exactly the opposite direction to that which is taken in the case of a secreting gland.

Here, some of the contents of the alimentary canal enter the free end of the cell, traverse it, and pass through the basement membrane beneath, into the lymph spaces in the connective tissue, and thence into the blood capillaries, or, in the case of fat absorbed in the small intestine, into lacteal vessels.

The structure of the alimentary canal and the glands opening into it may be considered in the following sections: The tongue; the teeth; the salivary glands; the œsophagus; the stomach; the duodenum; the pancreas; the liver; the ileum and jejunum; the large intestine; the rectum.

THE TONGUE.

The tongue is composed chiefly of a series of muscles of the voluntary striated variety, so arranged as to facilitate the various movements required. These muscles are separated from each other, and supported, as usual, by connective tissue, continuous with that surrounding the organ generally; and the whole is enveloped with a layer of stratified squamous epithelium, continuous with that lining the rest of the cavity of the mouth. Over the whole of the superior surface of the tongue this epithelial layer is raised into projecting papillæ, which have a corresponding core of vascular connective tissue. These papillæ are of three kinds : filiform, fungiform, and circumvallate. In each the fibrous tissue core is split up at its extremity into a series of secondary papillæ, which may vary in number from five to thirty. The epithelium bears corresponding secondary projections, in the case of the filiform papillæ. The superficial surface of the fungiform and circumvallate papillæ is, however, smooth (Fig. 99).

The filiform papillæ are the most numerous, and are found covering the dorsum of the tongue, with the exception of the edges and tip. They are conical in shape. They are shown in vertical section in Fig. 101. The epithelium covering them is divisible, like that of the mouth generally, into two layers, a deeper softer one, the stratum Malpighii, and a superficial harder one, the stratum corneum. It is prolonged at the apex of the papilla into pointed processes corresponding with the subdivisions of the connective tissue core it covers.

The fungiform papillæ are found especially at the tip of the tongue and along the edges of the upper surface. They are not narrow and pointed like the preceding, but thicker, broader,