of the soft palate, and on the epiglottis. But they are most easily studied in the papillæ foliate of the rabbit, two small oval areas lying on either side of the back of the tongue and marked transversely with a number of small ridges or lamina with intervening furrows (see

P

FIG. 185.-TONGUE OF RABBIT, SHOWING THE SITUATION OF THE
PAPILLE FOLIATE, p.

figs. 185 and 186). Sections across the ridges show numerous tastebuds embedded in the thick epithelium which clothes their sides.

The taste-buds are ovoid clusters of epithelium-cells which lie in cavities in the stratified epithelium (fig. 187). The base of the taste

p, central lamina of the corium; v, section across a vein, which traverses the whole length of the folia; p', lateral lamina in which the nerve-fibres run; g, taste-bud; n, sections of nerve-bundles; a, serous gland.

bud rests upon the corium of the mucous membrane, and receives a branch of the glosso-pharyngeal nerve; the apex is narrow and communicates with the cavity of the mouth by a small pore in the superficial epithelium (gustatory pore, fig. 187, p).

The cells which compose the taste-bud are of two kinds, viz.: 1. The gustatory cells (fig. 188, a), which are delicate fusiform or bipolar cells composed of the cell-body or nucleated enlargement, and of two processes, one distal, the other proximal. The distal process is

FIG. 187.-SECTION THROUGH THE MIDDLE OF A TASTE-BUD. (Ranvier.) p, gustatory pore; s, gustatory cell; r, sustentacular cell; m, lymph cell, containing fatty granules; e, superficial cells of the stratified epithelium; n, nerve-fibres.

nearly straight, and passes towards the apex of the taste-bud, where it terminates in a small, highly refracting cilium-like appendage, which projects into the pore above mentioned. The proximal process is more

α

FIG. 188.-VARIOUS CELLS FROM TASTE-BUD OF RABBIT. (Engelmann.)

a, four gustatory cells from central part; b, two sustentacular cells, and one gustatory cell, in connection; c, three sustentacular cells.

The

delicate than the other, and is often branched and varicose. nerve-fibres take origin in ramifications amongst the gustatory cells

(Retzius). 2. The sustentacular cells (fig. 188, c). These are elongated cells, mostly flattened, and pointed at their ends; they lie between the gustatory cells, which they thus appear to support, and in addition they form a sort of envelope or covering to the taste-bud. Between the cells of the taste bud lymph-corpuscles are often seen, having probably wandered hither from the subjacent mucous membrane.

LESSON XXIX.

THE SALIVARY GLANDS.

1. STUDY carefully sections of the submaxillary gland of a dog. The gland may be hardened in alcohol and stained with hæmatoxylin. Notice the acini filled with clear cells, the nuclei of which usually lie near the basement-membrane. Notice here and there, outside the clear cells, demilunes or crescents of small darkly stained granular-looking cells. Observe also the sections of the ducts with their striated columnar epithelium. Try and find a place where one of the ducts is passing into the alveoli. Sketch under a high

power.

2. Study sections of the parotid gland prepared in a similar way.

3. Examine small pieces of both submaxillary and parotid gland of the dog fresh in 2 per cent. salt solution. In the submaxillary gland notice that the alveolar cells are swollen out with large granules or droplets of mucigen, which swell up in water to form large clear vacuoles. Dilute acids and alkalies produce a similar change. The cells of the parotid gland are also filled with granules (zymogen), but they are smaller, and simply dissolve in watery fluids.1 Make a sketch from each preparation under a high power.

4. Prepare a transverse section of the oesophagus. Notice the thick muscular coat partly containing cross-striated fibres and the mucous membrane with its papillæ and stratified epithelium. Look for mucous glands in the areolar coat. Sketch under a low power.

The salivary glands may be looked upon as typical of secreting glands in general. They are composed of a number of lobules bound together loosely by connective tissue. Each small lobule is formed of a group of saccular or somewhat tubular alveoli or acini (fig. 189) from which a duct passes, and this, after uniting with other ducts to form larger and larger tubes, eventually leaves the gland to open upon the surface of the mucous membrane of the mouth.

The alveoli are inclosed by a basement membrane, which is reticular (fig. 190). This basement-membrane is continued along the ducts. Within it is the epithelium, which in the alveoli is composed of polyhedral cells (fig. 191, a), but in the ducts is regularly columnar,

1 To study the changes which the alveolar cells undergo during secretion, pilocarpine is injected subcutaneously into an animal in sufficient amount to produce copious salivation; after which the animal is killed and its salivary glands are examined as in preparation 3. The granules are not seen in preparations that have been in alcohol, but osmic acid preserves them; they are best seen in the fresh tissue.

L