are sometimes found taste-buds, or taste-goblets, which lie imbedded in the epithelium and extend through its entire thickness. The circumvallate papilla occupy a definite region on the upper surface of the tongue, and are arranged in two rows, forming almost a right angle, with the apex directed backward and situated just in front of the foramen cæcum (Morgagni). These papillæ are few in number, about eight to fifteen in all. In shape they are similar to those of the fungiform type, but are much larger (about 1 or 2 mm. in diameter), and sunk so deeply into the mucous membrane that the latter forms a wall around their sides. Here also the mucosa passes up into the papillæ and forms connective-tissue papillæ of its own at the upper surface, while at the sides it merely adheres to the smooth inner surface of the epithelial layer. Taste-buds are found in the epithelium at the sides of the papillæ, and also in that of the ridges surrounding the papillæ. At the sides of the human tongue and near its base are the so-called fimbria lingua. These are irregular folds of mucous membrane,

the sides of which also contain taste-buds. In the rabbit they are more regular in structure and consist of parallel folds of mucous membrane thickly dotted with taste-buds, and are termed the foliate papilla. In place of the circumvallate papillæ, the guinea-pig possesses structures similar to the foliate papillæ of the rabbit.

Into the depressions in which the circumvallate papillæ lie and into those between the folds of the fimbriæ linguæ open the ducts of numerous serous glands, the glands of Ebner (see below).

The Taste-buds.-The gustatory organs in the form of tastebuds are found on the surface of the tongue, principally on the lateral surfaces of the circumvallate papillæ and the fimbria linguæ (foliate papillæ). They are also occasionally met with in the epithelium of the fungiform papillæ and the soft palate, and on the posterior surface of the epiglottis. They always lie imbedded in the epithelium and extend through its entire thickness; they are ovoid in form, with base downward and the smaller pole at the

surface. The whole structure is surrounded by the epithelium of the mucous membrane of the regions in which they occur, except at the attenuated outer end of the taste-bud, where, by means of a small opening, the taste-pore, it communicates with the oral cavity. Most of the cells constituting the taste-buds are elongated, spindle-shaped structures, extending from one end of the organ to the other, with spaces between them. There are four varieties of these cells: (1) The outer sustentacular or tegmental cells, lying at the periphery of the organ with a nucleus in their center, and having a short, cone-shaped cuticular projection; (2) the inner sustentacular or rod-shaped cells, which are more slender structures with basally situated nuclei and without a cuticular projection; between the latter are (3) elongated, spindle-shaped, neuro-epithe

Epithelium.

Taste-buds.

Groove surrounding papilla.

Ebner's gland.

Fig. 187.-Longitudinal section of a human circumvallate papilla; X 20.

lial cells, with the nucleus of each in the thickest portion of the cell, and with slender, stiff processes projecting into the taste-pore; (4) a few broad basal cells, communicating with each other as well as with the sustentacular cells by numerous processes. We have, therefore, in the cells of the first, second, and probably fourth varieties, elements which belong exclusively to the sustentacular apparatus of the organ (Hermann, 85, 88).

Von Ebner found in the taste-buds of the circumvallate papillæ of man, monkey, and cat, as well as of the papillæ foliatæ of the rabbit, an open space situated between the taste-pore and the tip of the taste-bud (Fig. 188). These spaces vary according to the species, and are bounded above by the summits of the tegmental cells and laterally and below by the more centrally situated sus

tentacular cells. The cavities are often 10 μ in depth, and are filled with a fluid apparently in communication with the fluid of the depression into which the circumvallate papillæ are sunk. The processes of the neuro-epithelial cells project into the cavity from its floor and lateral walls, but do not extend as far as the tastepore.

Epithe Nerve-
lium. fibrils.

Process of
neuro-epi
thelial
cell.

Taste-
pore.

The circumvallate papillæ are differentiated from the adjacent. surface of the tongue by the development of a solid encircling epithelial ridge. Numerous taste-buds appear on the surface quite early in the history of the embryo. These, however, disappear completely when the permanent taste buds develop from the basal cells of the epithelial ridge. Similar phenomena occur in the fungiform papillæ (Hermann, 88).

The neural epithelia of the taste-goblets were formerly regarded as directly. connected with the

Tegmental
cell.
Neuro-epithe-
lial cell.

Sustentacular cell.

Terminal branches of nerves.

Fig. 188.-Schematic representation of a taste-goblet (partly after Hermann, 88).

nerve-fibers by means of long processes, but the latest researches have shown that dendrites of sensory neurones (sensory nerves) enter the taste-buds and end free in telodendria. The latter surround the neuro-epithelial and, to some extent, the sustentacular cells, their relations depending upon contact.

The Lymph-follicles of the Tongue (Folliculi linguales) and the Tonsils.-At the root of the tongue, and especially at its sides, are numerous elevations due to the increased quantity of lymphoid tissue found in the mucosa of these regions, the lingual tonsils, or lingual follicles. In the center of each follicle is a cavity communicating with the exterior and caused by an invagination of the epithelium. The lymphoid tissue contains a number of more or less distinctly defined lymph-nodules, some even showing germ centers (vid. p. 178). The whole structure is surrounded by a connective-tissue capsule. The epithelial walls of the follicular cavities often show extensive degenerative changes, which are accompanied by increased migration of leucocytes into the oral cavity. These leucocytes change (according to Stöhr, 84) into the so-called mucous or salivary corpuscles of the saliva. The pharyngeal tonsils may be regarded as clusters of small lymph-follicles,

similar to those found in the tongue. They are covered by a stratified pavement epithelium, resting on a mucosa possessing papillæ folded to form pits or crypts of irregular shape. The adenoid tissue of the tonsil is found in the form of diffuse adenoid tissue and a varying number of more or less clearly defined follicles of adenoid tissue often showing germ centers of Flemming.

Epithelium.

Lymph-follicle..

Epithelial
crypt.

Connective-tissue capsule.

Fig. 189.-Section through tonsil of dog; X 20: At a and at the opposite side the epithelium is composed of a very thin layer of cells.

Fig. 190.-The area designated by a in the previous illustration, shown by a higher magnification; about 150: a, Leucocytes in the epithelium; b, one of the spaces in the epithelium filled with leucocytes and more or less changed epithelial cells; c, blood-vessel; d, normal epithelium; e, basal cell of the same.

The epithelium lining the crypts or cavities of the tonsils shows, as in the lingual follicles, extensive degenerative changes, resulting mainly in the formation of variously shaped, communicating spaces filled with lymphocytes and leucocytes. (See Fig. 190.)

Besides the nerves terminating in the taste-buds, the tongue is

a

b

C

richly supplied with sensory nerves which terminate in free sensory endings, which may be traced into the epithelium, and which are especially numerous in the fungiform and circumvallate papillæ ; or in smaller or larger end-bulbs of Krause found in the mucosa of the fungiform papillæ. The motor nerves of the tongue terminate in motor-endings.

GLANDS OF THE ORAL CAVITY.

Interlobular
duct.

Intralobular
duct.

duct.

The glands of the oral cavity comprise numerous lobular, tubulo-acinous glands situated in the mucosa and submucosa of the lips, cheeks, and tongue, and three pairs of lobar, tubulo - acinous glands the parotid, submaxillary, and sublingual glands. These are classified according to their secretions into those secreting principally mucus (human sublingual and many of the smaller oral glands), and known as mucous glands; those secreting a fluid albuminoid substance containing no mucus, the serous glands (parotid glands and the small glands near the circumvallate papillæ); and those having a mixed secretion, Intermediate mucous and serous glands (human submaxillary). The ducts of all these glands open into the cavity of the mouth. The ducts of the smaller oral glands are, as a rule, short and pass up through the mucosa and the epithelium to open on the free surface. The principal excretory ducts of the salivary glands are Steno's ducts (Stenson's ducts), passing from the parotid glands to the mouth; Wharton's ducts, the ducts of the submaxillary glands, and Bartholin's ducts for the sublingual glands. The salivary glands consist of numerous lobules and small lobes of glandular tissue, surrounded by a thin fibrous-tissue capsule which sends septa and trabeculæ between the lobules and lobes. The duct of each gland on reaching the gland divides into smaller ducts, which penetrate the gland between the lobes, the interlobar ducts; these in turn divide into ducts of the next order, which pass between the lobules, the interlobular ducts. The interlobular ducts pass over into short cylindric tubes which enter the lobules, and are known as intralobular ducts. These are followed by very short, narrow tubules, the intermediate ducts or tubules,

Acinus.

2

Fig. 191.-Scheme of a salivary gland.