The surface of the stomach is covered by a single layer of cylindrical epithelial cells which also line the orifices of the numerous glands with which the mucous membrane is thickly studded. This single layer of cylindrical cells commences abruptly at the cardiac orifice of the stomach, and is marked off from the stratified squamous cells lining the oesophagus by a sharp line of demarcation. The glands of the stomach are tubes with conical orifices which often divide into two or three tubular prolongations. The outlet or orifice is covered by the common cylindrical epithelium of the surface of the stomach, and the fundus is filled with specific granular cells. The glands dip down to the delicate submucous tissue, the branching tubes lying parallel and exceedingly close together. A dense network of capillary blood vessels may be demonstrated by injection to surround the tubes and closely invest the thin basement membrane which forms the boundary of the glands and the basis of attachment of the glandular cells. A close-meshed network of absorbent vessels also surrounds the tubules of the glands, and leads to the larger vessels in the submucous tissue.
In the cardiac end of the stomach two distinct kinds of cells are found in the deeper part of the gland tubes. Much the more numerous are small, pale, spheroidal cells, which occupy the lumen of the gland and form the regular cell lining of its cavity. These cells have been called the "chief cells " (Haupt-zellen), "central " or spheroidal cells.
Fig.68. Diagram of a Section of the Wall of the Stomach.
a. Orifices of glands with cylindrical epithelium.
b. Fundus of glands with spherical and oval epithelium.
c. Tunica muscularis mucosae.
d. Submucous tissue containing blood vessels, etc.
e. Circular, (f) oblique, and (g) longitudinal muscle coats. h. Serous membrane.
The cells of the other form are comparatively few, being altogether wanting in some of the glands. They are larger and more striking than the central or spheroidal cells between which and the basement membrane they lie scattered here and there over the fundus of the gland, making the delicate membrane bulge. They stain more easily, and have darker granules than the central cells. On account of their position they have been called "parietal," "marginal or border cells" (Belegzellen), and from their oval shape, which equally well distinguishes them from the other, "ovoid cells." (See Fig. 69).
Fig. 69. Diagram showing the relation of the ultimate twigs of the blood vessels (V and A), and of the absorbent radicals (L) to the glands of the stomach, and the different kinds of epithelium, viz., above cylindrical cells; small, pale cells in the lumen, outside of which are the dark ovoid cells.
There is a different class of glands, the so-called mucous, found chiefly near the pyloric end of the stomach, in which there is but one kind of cell throughout, and this seems to differ in character from both the varieties in the other glands, resembling rather the cylindrical epithelium covering the surface of the stomach and dipping into the conical orifices which lead to the glands.
The difference between the two kinds of glands found in the stomach, both as regards their distribution and way of branching, and the cells which line the deeper parts of the tubes, is found to vary in different animals. The difficulty of obtaining fresh specimens of the human stomach makes it still uncertain whether the same differences exist in the human subject. The varieties of opinion and drawings published suggest that various stages of gradation from one kind of gland to another are met with in the stomach of even the same animal.
Experimental research does not show decisively that the anatomical differences denote differences of function.