TASTE-BUDS.

6. Taste-Buds, or the peripheral organs of taste, occur on the fungiform papilla and lateral surface of the circumvallate, soft palate, posterior surface of the epiglottis, and a few amongst the epithelial cells on the dorsum and sides of the tongue. It is more convenient, however, to study them in the rabbit. On either side of the posterior part of the rabbit's tongue are two oval patches with transverse ridges and intervening furrows, the papillæ foliatæ (fig. 239).

(i.) Cut out these parts and harden them for fourteen days or so in Müller's fluid and then in spirit. Stain in bulk in boraxcarmine or hæmatoxylin and cut in paraffin.

(i.) The excised organ, with as little adherent muscle as possible, is placed for one hour in 1 per cent. osmic acid, or pinned on a cork and exposed for the same time to the vapour of osmic acid. Fine sections are made across the laminæ, and stained with logwood and mounted in balsam.

(a.) (L) Observe the sections of the laminæ, each one with a central papilla or projection of connective tissue (fig. 240, 1). This is covered by many layers of stratified epithelium.

(b.) Between the laminæ a furrow, and embedded in the epithelium, on each side of this furrow, the taste-buds (), which are oval in shape, and composed of epithelial cells, whose bases touch the connective tissue of the mucous membrane, where they receive FIG. 239.-Tongue a branch of the glosso-pharyngeal nerve. The apex has an open mouth-gustatory pore-which communicates with the furrow. The cells composing the bud are arranged somewhat like the staves in a barrel.

of Rabbit. P.F. Papillæ foliatæ.

(c.) Gland-ducts open at the bases of the furrows, and if these ducts be not seen, sections of their acini-serous gland-are sure to be seen deep in the corium (fig. 240, d). The corium has what look like secondary papillæ on it, but they are really septa (l').

(d.) (H) Study a single taste-bud (80 μ long and 40 μ broad) It is composed of two kinds of elongated epithelial cells.

(1.) The sustentacular cells, which are most numerous. They are elongated, flattened, and either of uniform breadth or narrowed at their base. They form a protective covering for the true gustatory cells, which lie between and within them.

(2.) The gustatory cells consist of narrow fusiform nucleated

cells, whose lower pointed end is continuous with a branch of the axial cylinder of a nerve-fibre, while the free end is continued into a fine point or cilium, which projects through the gustatory pore.

If it be desired to study the mode of termination of the nerves in these organs, use the lemon-juice gold chloride method, with subsequent exposure of the tissue to sunlight in water acidulated with acetic acid, Golgi's rapid hardening method, or methylene-blue. For an elaborate research, with beautiful plates showing the terminations of the

nerves in the papillæ foliatæ, see Drasch.1

SOFT PALATE.

1. T.S. Soft Palate.-Harden the soft palate of a rabbit or dog in Müller's fluid, alcohol, or corrosive sublimate. Make transverse sections by freezing, or stain in bulk in borax-carmine and cut in paraffin.

(a.) (L and H) One of the most beautiful methods of staining is that recommended by List, viz., to stain with aniline-green and eosin, and mount in balsam. Even with the naked eye the thick layer of mucous glands can be seen.

(b.) The stratified epithelium and connective tissue are rosy-red, the nuclei blue. The glands are bluish, and are seen to be mucous in character, lined by a single layer of mucous cells without demilunes. In the borax-carmine section, the cells lining the acini of the glands are clear and transparent and show no demilunes, a typical example of a pure mucous gland.

1

"Unters. über d. Pap. fol. et circumvall. d. Kaninchens," Abhand. d. math.-phys. Classe d. K. Sächs. Gesell. d. Wissensch.. Bd. xxiv.

ADDITIONAL EXERCISES.

Glands of Tongue or Palate. —Harden the tongue or soft palate of a rabbit in 3 per cent. nitric acid for 1-2 hours. Wash out all the acid and stain the sections with methylene-blue. Wash out the blue with alcohol until only the glands remain blue. Mount in xylol-balsam.

Terminations of Nerves in the Lingual Papillæ and Glands.-These may be studied by staining small pieces of the tongue of mouse or rat by Golgi's silver nitrate method (p. 78), or by the rapid hardening method (bichromate of potash and osmic acid, Lesson XXVI. 14). The nerve in the papillæ contains nerve-cells which are in connection with the nerve-fibres. The nerve-fibres form a plexus of fibres outside the basement membrane of the serous gland acini-epilemmal plexus-and one within this membrane amongst the secretory cells-hypolemmal plexus (Fusari and Panasci).1

LESSON XXII.

TOOTH-SOPHAGUS.
TOOTH.

THE chief mass of a tooth consists of dentine. It is capped by enamel, and the root or fang is invested by a layer of bone, the crusta petrosa. All three tissues are calcified, and contain calcie phosphate. The enamel, however, is an epithelial structure, and consists of modified and calcified epithelial cells, while the dentine and crusta petrosa belong to the connective tissue group.

Unsoftened Tooth. This is one of the few preparations which had better be bought.

1. Longitudinal Section of a Dry Tooth (fig. 241).

(a.) (L) Observe the crown and fang, and, connecting the two, the neck.

(b) The dentine surrounding the pulp cavity, the enamel covering the dentine of the crown, and the crusta petrosa or cement covering the dentine of the fang. The wavy black lines in the dentine or dentinal tubules are really tubules filled with air, hence they appear black. Note their direction from the pulp cavity towards the outer margin of the dentine. Quite at the apex of the crown of the tooth they run vertically; in the fang they run nearly horizontally, and in the part of the dentine intermediate between

1 Archiv. ital. de Biol., xiv. p. 240, 1891.

both they gradually become more and more oblique from the centre of the crown.

Arched or curved lines-incremental lines or Schreger's lines

-may sometimes be seen crossing the course of the dentine tubules.

(c.) The enamel covering the dentine on the crown of the tooth; somewhat brownish-coloured concentric lines may be seen in it.

(d.) (H) The enamel consists of striated prisms, hexagonal when seen in transverse section (fig. 243).

are

The dentinal tubules lie in a homogeneous matrix, and wavy tubes, which divide diPM. chotomously, and give off lateral branches which anastomose with other lateral branches from adjacent tubules. At the outer part of the dentine are irregular interglobular spaces, which appear black when they contain air (fig. 244).

FIG. 241.-V.S. Tooth in Jaw. E. Enamel;
D. Dentine; P.M. Periodontal mem-

brane; P.C. Pulp cavity; C. Cement;
B. Bone of lower jaw; V. Vein; a.

The crusta petrosa consists of bone-a thin layer-composed of lamellæ and bone-corpuscles, but no Haversian canals (fig. 244).

2. Softened Tooth.-Select the jaw of a small mammal, e.g., a cat, and decalcify a short length-1 inch-of the lower jaw in chromic acid and nitric acid. It will take two or three weeks to remove all the bone-salts, and the decalcifying fluid must be frequently renewed. The tooth is sufficiently soft to be cut when a needle can be pushed into it. Make vertical sections through the whole jaw and a tooth in situ.

Stain one section in picro-carmine, another in osmic acid (twentyfour hours), and mount both in Farrant's solution or in glycerinejelly.

(a.) (L) Observe the tooth in the alveolus or depression of the jaw in which it is fixed. The enamel has disappeared. The bone of the jaw with its periosteum, lining the alveolus and forming there the periodontal membrane (fig. 241, P.M.).

(b.) Next the latter on the fang the cement (C), the dentine,

pulp-cavity and its contents.

If the section passes directly through the middle of the tooth, the orifice in the fang of the tooth may be

seen.

(c.) (H) The dentinal tubules, not so distinct as in the dry tooth.

If they are cut obliquely, they appear merely as tailed dots in a homogeneous matrix.

(d.) The pulp-cavity contains blood-vessels and fine connective tissue, but next the dentine there is a layer of large cubical cellsodontoblasts--which give off fine processes which enter the dentinal tubules the fibres of Tomes. They are best seen, however, in a tooth which has not yet cut the gum.

3. Development of Tooth.-Without entering into all the details of the development of the teeth, the following directions will suffice as to the method of preparing sections so as to show the various stages. What may be called the first stage-that shown in fig. 245-is to be obtained from the lower jaw of a sheep's embryo 7 cm. in length. At this stage only a very little bony matter exists. Harden the whole jaw in corrosive sublimate and decalcify in dilute hydrochloric acid. Stain in bulk in borax-carmine, embed in paraffin, and make T.S. across both rami of the jaw and the tongue. Or harden and decalcify at the same time the jaw of a fœtal kitten by placing small pieces containing embryonic teeth in Flemming's fluid. This yields excellent results, the tissues are thereby sufficiently differentiated and may be cut in paraffin.

The second stage, fig. 246, is obtainable from the upper jaw of an embryo sheep 15 cm. long. It is treated in the same way.

The third stage, fig. 247, is obtained from the lower jaw of a dog six days old or thereabout.