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The tactile hairs—as, for instance, those on the upper side of the proboscis of the fly-are delicate, hollow, tapering, pointed organs, inserted on a chitinous ring, and connected with a nerve which immediately below the skin swells into a multicellular ganglion.
Fig. 20.-Right half of eighth segment of the body of the larva of a gnat (Corethra
plumicornis); after Graber. E, G, Ganglion; n, nerve; g, auditory ganglion; gb, auditory ligament; Ch, auditory rods; a, auditory nerve; e, attachment of auditory organ to the skin; b, attachment of auditory ligament; hn, hn', termination of skin-nerve; tb, plumose tactile hair; h, simple hair; tg, ganglion of tactile hair ; lm, longitudinal muscle.
The terminations of the nerves and their connection with the sensitive hairs are also beautifully shown in some of the transparent water-insects. Fig. 20 represents part of one segment of the glassy larva of a gnat (Corethra plumicornis), showing the tactile hairs (Fig. 20, h, tb), and the nerves connecting them with the central ganglion (Fig. 20, EG).
THE SENSE OF TASTE.
WHILE the organs of touch are spread more or less over the whole surface, and those of sight and of hearing may be, and in fact are, situated in very different parts of the body in different animals, the sense of taste is naturally confined to the mouth or its immediate neighbourhood.
In the case of Man, it resides especially in the tip, the edges of the upper surface, and the back part of the tongue, and (probably) the inferior portion of the soft palate. The actual mode of termination of the nerves of taste has, however, only recently been discovered.
Loven and Schwalbe detected, independently and almost simultaneously, in the epithelium of the papillæ of the tongue, many small budlike groups of cells (Fig. 21) which are probably connected with the ultimate fibres of the glosso-pharyngeal nerves. These have been supposed to be the special seats of the sense of taste, and thence termed “taste-buds;" they are in man shaped like a flask, in some other animals they are more slender. In the dog, they are :072 of a millimeter in length, and .03 in breadth.
In the pig, the number is estimated at 9500; in the sheep, at 9600; in the rabbit, at 1500; in the cow, at
TASTE-ORGANS OF MAN.
35,000. In man they almost touch each other on some parts of the tongue, and their number is very great.
Fig. 21.–Taste-Buds of the rabbit (after Engelmann in Stricker's “Hand book "), X 450,
The "taste-buds" consist of from fifteen to thirty long narrow cells, arranged almost like a circular bundle. Those on the outside lie in close contact with the walls of the cavity. The cells appear to be of two kinds :
Fig. 22.—a, Isolated taste-cells from the mouth of rabbit; b, two cover-cells and a
taste-cell in their natural position (after Engelmann), x 600.
the outer ones do not differ markedly in appearanceat least, with our present magnifying powers—from ordinary epithelial cells, and have not been shown to be connected with nerves.
Those in the centre are
more highly organized. Each consists of an ellipsoidal nucleus surrounded by a thin layer of protoplasm, continued downwards into a fine fibril, which sometimes branches, and which—though this is not clear-probably joins the nervous fibres. The upper process of the protoplasm is a narrow cylinder, in some cases prolonged at the end into a very delicate hair or rod.
Schwalbe thought he could distinguish in man and the sheep, two kinds of taste-cells—firstly, needle cells, in which the cell appears to terminate in a narrow, brilliant needle, abruptly cut off at the end; and, secondly, staff cells, which are less numerous, shorter, of uniform breadth, and without any terminating needle. It is still unknown whether there are different classes of taste-cells for different tastes, and whether one taste-bud can distinguish more than one taste.
I know of no detailed description of the organs of taste in birds and reptiles. In the frog the taste-organs are not flasklike, but are flat disks. They occur in hundreds on the tongue and soft palate. These taste-disks are composed of Fig. 23. — several forms of cells. Those which are supposed to be espe
(after Engelmann), X 600. cially connected with the sense of taste terminate in a fork, sometimes, though rarely, of three prongs. The taste-organs of fishes are shaped like beakers.
Termination of the nerves of taste in the frog, showing the ramifications of the nerve-fibres and their connection with the cells of taste
TASTE-ORGANS OF THE LOWER ANIMALS.
It will be observed that these structures give us no help to realize in what actually consists the sense of taste. We know that we possess it ourselves. We perceive that other animals can select, and appear to enjoy, their food, and hence we ascribe to them a similar faculty. We know that in our own case this sense resides in the mouth, and we assume that it must do so in other animals; we find in the mouth certain structures, and we infer that to them is due the sensation of taste. Even in our own case the inferences are, perhaps, not very clear, and certainly the facts, as yet known, aid us but little in framing any definite idea of the process.
But if our knowledge is so imperfect in the case of the higher animals, it becomes much more so in the
In the Mollusca, Annelida, and lower groups, we know scarcely anything of the organ of taste, though we
can hardly doubt that such exists.
Medusa (jelly-fishes) are very sensitive to any change in the composition of the seawater; for instance, they sink below as soon as it begins to rain. It is difficult, however,
to say which sense is affected. Fig. 24.—Inner layer of the skin of the proboscis of Asterope can In Asterope (a marine worm
400 (after Greef). a, Cuticle terminal nerve belonging to the Alciopidæ), organs; c, ganglionic cells; d, longitudinal muscle;
Greef has described, in the
skin of the proboscis, certain peculiar club-shaped, ringed bodies, which taper into a thread connected with a nucleated cell. These he