OCR Text |
Show 1876.] MUCOUS MEMBRANE IN KANGAROOS. 175 of the tissue, and the limit of their distribution can be readily out. They are seen to be absent near the orifices of the glands, where the tubes are lined with columnar epithelium. The relation of the peptic cells to the central cells is best seen in the horizontal sections (as in fig. 8, which is taken from the Dorcopsis stomach). Here the peptic cells (p, p) lie immediately outside the central cells (c, c) (which almost fill up the tube, leaving but a very small lumen) and in close contact with them. But in Macropus the contact is not so close ; for the basement membrane of the gland sends horizontal lamellar projections inwards, partially surrounding the spheroidal cells and separating them more from the central ones. It can be clearly made out (both in vertical sections showing the glands along their whole length, and in sections carried obliquely across them so that in different parts of the section different levels of the tubes are cut) that the central cells are directly continuous at the neck of the glands with the gradually shortening columnar cells of the gland-mouth, and resemble, therefore, in this respect the cubical cells which line the greater part of the tubes of the second region*. In general aspect too the central cells resemble those; but they are for the most part, as before mentioned, smaller and more angular and closely packed. This is especially the case at the base of the gland, where the cells almost entirely fill the tube so as to leave little or no lumen (fig. 9). Transition between the Second and Third Regions.-The line of demarcation between these is best marked, as before stated, in Dorcopsis, where there is a slight furrow between them, the mucous membrane increasing rapidly in thickness on the peptic side of the furrow. A section across the line and including a part of each region, is shown in fig 10, as seen under a low power in a preparation stained with aniline blue. The glands of the second region become gradually shorter until opposite the bottom of the furrow, where they are shortest; those beyond rapidly increase in length, but exhibit at first exactly the same structure. At about the third or fourth row, however, a few peptic cells become superadded to the others about the middle of the glands; and these increase in number and occupy a greater length of the gland as we proceed further into the third region, until after a few more tubes they are found throughout the greater part of the length of the glands; so that from a study of the mode in which the two kinds of glands pass into one another, as well as from a comparison of their structure, it is clear that the main parts of the glands of both regions are almost precisely similar and will probably have a similar function, and that the only difference of importance lies in the fact of the superaddition of the peptic cells in the glands of the third region-probably implying the superaddition of some other function in these glands. Whether this, as is generally believed, is the elabo- * Strictly speaking, these cells are not cubical; for although they appear so when the glands are seen longitudinally, they must of course, as seen in a transverse section of the glands, become narrower towards the lumen; so that the shape of each cell is in reality that of a truncated wedge. |