artery. The lymphatics are not very numerous. see Lesson XVII. 7.

For the nerves

8. Spleen. Tie the blood-vessels at the hilum of the spleen of a cat, so as to keep the blood in the spleen. Cut it out and place it in a large volume of Müller's fluid (two weeks) or 2 per cent. bichromate of potash. Wash it thoroughly in running water for an hour or two. Cut out small pieces and harden them in alcohol. Make transverse sections, i.e., across the long axis of the organ; stain one in logwood, and mount in balsam. Other sections are to be stained in picro-carmine and mounted in Farrant's solution, and a thin set placed in 1 per cent. osmic acid (twenty-four hours), and mounted in Farrant's solution. This sharpens the outlines of the elements. Stain other sections in eosin-logwood or safranin (fortyeight hours).

9. T.S. Spleen (L) (Cat; hæmatoxylin or eosin and hæmatoxylin). -The Malpighian corpuscles are visible as small blue spots to the naked eye.

(a.) Externally the capsule, fibrous, thick, firmly adherent and closely applied to the organ, sends trabeculæ into the spleen, where they branch and anastomose to form a trabecular framework. Some of them will be cut longitudinally, others obliquely, and some transversely. In the larger trabeculæ, sections of large blood-vessels

2

FIG. 232.--Elements of Human Splenic Pulp. 1. Leucocytes; 2. Epithelial cells; 3. Coloured blood-corpuscles; 4. Cells containing pigment-granules.

FIG. 233.-Reticulum of
the Splenic Pulp.

(fig. 231). Note that there is no lymph space between the capsule and the gland substance, as is the case in lymph glands.

(b.) Filling the interstices of this network, the splenic pulp, and in it oval or rounded bodies-Malpighian or splenic corpusclesas blue-stained bodies contrasting with the yellowish brown pulp in which they lie. In the centre of each is a lighter area, the "germcentre" of Flemming. In each corpuscle a section of a small artery lying excentrically in the mass. The splenic corpuscles are

small lob-sided aggregations of lymphoid tissue around branches of the splenic artery. They are relatively more numerous than in the human spleen. The track of the blood in the pulp is mapped out by the yellow blood-corpuscles.

(c.) (H) The capsule and trabeculæ, composed of fibrous tissue, with elastic fibres and smooth muscle. The Malpighian corpuscles, consisting of leucocytes in an adenoid reticulum. The centre is lighter in tint than the circumference, which is more condensed. The lighter centre is due to the larger cells present there. They are undergoing proliferation. The cells formed in the splenic corpuscles pass into the spaces of the pulp and leave the organ by the venous blood stream.

(d.) In the pulp irregular rows of coloured blood-corpuscles— yellow and between these leucocytes and other cells.

The exact structure of the pulp can only be properly studied in a section which is very thin, and especially at the edges of the section, or best of all in a section of a dog's (or cat's) spleen, whose blood-vessels have been washed out, and cleared of all bloodcorpuscles by a warm stream of normal saline solution. The vessels are then injected with a 5 per cent. solution of ammonium bichromate, and the organ hardened in a large quantity of the same fluid, and subsequently in alcohol.

(e.) In a section prepared in this way, or at the edges of a very thin section, the fine reticulum of branching cells may be seen (fig. 233) with the cells of the splenic pulp washed out of it.

In

10. Human Spleen. -Harden this in the same way as 8. Note that, as a rule, the Malpighian corpuscles are less numerous. other respects the general structure is the same.

11. Injected Spleen. It is very difficult to inject the finer splenic blood-vessels. They should be washed out first with normal saline, and preferably a watery solution of Berlin blue, or Berlin blue with gelatine, should be used as the injection. Note that an artery and capillaries exist in the Malpighian corpuscles, but the splenic pulp seems to be infiltrated with a blue mass. The capillaries open into this system of labyrinthine blood-passages. These intermediate blood-passages are merely the spaces amongst the cells of pulp and are not lined by epithelium. The terminations of the capillaries in some situations are surrounded by thick sheaths or collars of tissue, perhaps derived from the cells of the pulp (Bannwarth).1

12. The varieties of Leucocytes in lymph glands are best studied by fixing the gland in Hg Cl, and staining sections with EhrlichBiondi fluid (Hoyer). There are at least four varieties, not including phagocytes.

2

1 Archiv f. mik. Anat., xxxviii. p. 345.

2 Ibid., xxxiv.

LESSON XXI.

TONGUE TASTE-BUDS-SOFT PALATE.

TONGUE.

PLACE small portions in Müller's fluid or 2 per cent. potassic bichromate for fourteen days, and complete the hardening in alcohol, or harden it in mercuric chloride. Make vertical transverse sections. It is well to have the tongue of a small cat or kitten, and parts of the human tongue also-the former because a complete transverse section can be put on a slide. The structure will vary according as the section is made through the anterior or posterior part of the organ, as the latter contains many lymph follicles and mucous glands. The sections may be stained in logwood and mounted in balsam.

1. T.S. Tongue of Cat.-(a.) (L) Observe the papillæ, of various shapes, on the dorsum of the tongue, and covered by stratified epithelium. Under this the connective tissue of the mucous membrane (fig. 234).

(b.) Muscular Fibres.Many cut transversely and arranged in groups under the dorsal mucous membrane and elsewhere; others which run from the vertical mesial plane or septum horizontally outwards, and some which pass vertically. The last may be seen to become conical and end in the connective tissue of the mucous membrane. Some of these fibres branch. (The methods of isolating branched fibres are referred to in Lesson XVI. 3.)

FIG. 234-TS. of One-half of the Tongue of a Cat.

(c) Lingual Papillæ. The dorsum of the tongue is beset with elevations of the mucous membrane covered by stratified epithelium, and constituting three varieties of papillæ.

(1.) Filiform (.7-3 mm long), by far the most numerous, and are placed all over the dorsum. They are conical elevations of the mucous membrane, the upper end of which is beset with five, fifteen,

or thirty secondary papillæ. Each papilla is composed of fibrous tissue with elastic fibres, and covered by many layers of stratified epithelium, the superficial cells of which are often corneous (fig. 235). (2.) Fungiform (0.5-1.5 mm. long), are not nearly so numerous as the foregoing, and are scattered over the dorsum. Each papilla is club-shaped or lenticular, with a constricted base. The apex is beset with secondary papillæ, but the epithelium covering them is thinner than in (1) (fig. 236).

(3.) Circumvallate (1-1.5 mm. high and 1-3 mm. broad), are confined to the posterior part of the tongue, where they 8-15 in number) are arranged in the form of a V, the apex of the V being directed backwards. Each circular elevation is raised above the level of the tongue and surrounded by a circular trench or fossa. Secondary papillæ occur only on their surface. Taste-bulbs occur in the wall of the papilla directed toward the fossa. They are the organs of taste, and are supplied by the glosso-pharyngeal nerve. It may require several sections to obtain views of all three forms of papillæ.

FIG. 235.-Filiform Papillæ, X 30. 1. Primary papilla; 2. Secondary papillæ on its summit; 3. Epithelial process on papilla; 4. Single process, with tangled loose epithelial cells.

en

(d.) (H) The stratified squamous epithelium covering the papillæ

and sides of the tongue.

The superficial cells are

very thin.

(e.) Glands, in the back part mucous glands with clear contents, and it may be also serous glands in which the acini are more granular (fig. 237).

(f) Fat-cells, like padding between the striped muscular fibres

Secondary

papillæ.

Fungiform papilla.

Nerve in its axis.

Simple papilla.

here and there.

Near

FIG. 236.-Fungiform Papilla, x 30.

the lower surface, sections of the lingual artery and nerves. latter ganglionic cells may sometimes be seen.

In the

At the back part of the tongue are little depressions of the

mucous membrane called crypts (fig. 237).

In the walls of these

are spherical masses of adenoid tissue, and into some crypts open the ducts of small mucous glands (fig. 237).

2. The mucous glands occur chiefly at the base of the tongue

and along its edges.

3. The serous glands occur only near the circumvallate papillæ and taste-bulbs. acini are granular and resemble those of the parotid gland in structure (fig. 240, d).

4. T.S. Tongue (Double-Stained).-Stain a section from the posterior part of the organ, first with methyl-green and then with

cosin. Mount in balsam. The connective tissue and papillæ are reddish; the serous glands are reddish also, while the mucous glands have a purplish-green colour.

5. T.S. Injected Tongue (L).This is obtained when the head or whole body is injected. Observe the very vascular muscular portion and the papillæ, each with an artery entering it. If the papillæ be compound, i.e., beset with other smaller secondary papillæ, a small capillary loop passes into each of these secondary papillæ; sections of large blood-vessels in the connective tissue of the mucous coat (fig. 238). If desired, another section may be faintly stained with logwood and mounted as above.