OCR Text |
Show 1883.] TONGUES O F T H E M A R S U P I A L S . 603 some indication of such a separation in the presence of two kinds nuclei in the bulbs-the one spherical or oval, and the other greatly elongated. The latter must belong to the central cells. To be sure of this point, or indeed of anything in minute structure, the fresh tissue should be examined. Considering, however, that these structures in Halmaturus were not fresh, it would be hardly possible to have obtained tissues in better condition for minute examination. The question of the termination of nerves is better considered after the description of the layers within the papilla. These are shown in fig. xxi. Plate LV., which represents a transverse section through the thickest part of a papilla. In the axis are the nonmedullated nerves, which enter from below. They do not form any distinct ganglion in the papilla (as in Perameles and Phalangista). In a few cases isolated ganglion-cells were seen in the axis of the papilla, in one instance at some considerable height. It is probable that the ganglion-cells, which are always connected with the nerves of special sense, form small ganglia on the nerve-branches near the base of the papilla. The axial nerves are supported by trabecule from the next layer, and large blood-vessels are present, entering with the nerves. The next layer, already mentioned, is derived from the dense mucosa, and is composed of fibrous and possibly smooth muscular elements. Blood-vessels are present in it; and nerve-branches passing from the axial nerves to the subepithelial layer may be seen streaming outward through it. The next subepithelial layer is characteristic, not occurring elsewhere. It represents the unravelled elements of the two other layers united into interpenetrating networks. The importance of the layer is well seen by looking at the tissues underlying the ordinary epithelium, after looking at that beneath the bulbs. Below the limits of the taste-bulbs the subepithelial layer thins away abruptly, but its connective-tissue elements are probably continuous with a delicate layer which lies between the deeper denser part of the mucosa and the lowest layer of the epithelium on the outside of the involution. The subepithelial layer disappears less rapidly above the limits of the bulbs, and in some places its fine fibrils end against the lowest epithelial cells. This is seen with especial clearness in the cells of interpapillary processes ; and it may be that bulbs are arising directly in this region, or (as is more probable) that these masses of epithelial cells with the appearance of nerve-terminations in them represent bulbs that have degenerated into the structures from which they originally arose. This, however, is merely a suggestion. Capillaries are present in this layer. At certain places the subepithelial layer is converted into a tissue resembling adenoid tissue, which may also invade the layer last described, sometimes even reaching the axial nerves. A similar tissue has been described in a corresponding position in the tongue of Ornithorhynchus (see paper on this subject by the present writer in the ' Quarterly Journal of Microscopical Science' for July 1883). When the subepithelial layer is studied under high powers (T x ¥ oil-immersion of Zeiss), fine fibrils are seen to terminate abruptly against the contour of the convex lower surface of the bulb, separated only from the cells by the linear |