The tongue (Lingua)
The tongue is a muscular organ which almost entirely fills the oral cavity. It is covered with mucous membrane and consists of three chief portions: the middle and largest portion, attached to the floor of the mouth, the body; the anterior portion, projecting into the oral cavity, and completely clothed by mucous membrane, the tip; and the posterior portion, attached to the hyoid bone and epiglottis, the root.
In the body of the tongue there may be recognized an inferior attached surface and a convex superior surface or dorsum which is covered throughout its entire length by the oral mucous membrane. The lateral margin of the tongue is rounded; in its anterior part it is free, while posteriorly it is continuous with the soft palate.
When the mouth is closed, the larger anterior portion of the dorsum of the tongue is applied to the palate; its posterior portion borders upon the pharynx at the isthmus of the fauces. The junction of the body with the root of the tongue is indicated upon the dorsum by a depression, the joramen ccecum, which leads into a short blind mucous canal, the lingual duct, an embryonic rudiment which in the adult exhibits only the orifices of a few mucous glands. From the foramen caecum the circumvallate papillae (see below) extend laterally, being arranged like a letter V, the apex of which is directed posteriorly and is formed of the foramen caecum itself. Parallel and just posterior to the circumvallate papillae there is frequently a groove, the sulcus terminalis, which indicates the dividing-line between the body and the root of the tongue.
If this be absent, the row of circumvallate papillae forms the boundary.
The root of the tongue is connected with the epiglottis by three folds of mucous membrane, a single median glosso-epiglottic fold and two lateral glosso-epiglottic folds. Between these folds upon either side of the median line there is a roundish pit which is known as the epiglottic vallecula.
In the tongue two chief constituents, the mucous membrane and the musculature, may be recognized. Upon the under surface of the tongue these two constituents are but loosely attached to one another, but upon the dorsum the attachment is firmer, the terminal prolongations of the muscle fibers inserting directly into the lingual fascia, a layer of firm connective tissue situated immediately beneath the mucous membrane.
Upon the dorsum of the tongue the mucous membrane is sharply divided by the sulcus terminalis or the circumvallate papillae into a portion covering the body and a portion covering the root of the organ, and the two portions differ from each other in such a way that the anterior one may be termed the papillary portion and the posterior the tonsillar.
The anterior portion owes its characteristic velvety appearance to the lingual papillae, which are in general of a conical shape and project above the level of the mucous membrane.
According to their shape, the following varieties may be recognized :
1. The filiform papilla have the form of elongated cones rather than of cylinders, and are present in large numbers over the entire papillary portion of the mucous membrane, and especially at the sides and tip of the tongue, where they attain their greatest length. The great majority of the lingual papillae are of this type.
2. The conical papillce are found scattered among the filiform structures and are not sharply differentiated from them.
3. The fungiform papillce occur scattered among the filiform papillae at the sides and tip of the tongue and are characterized by having the top broader than the base or pedicle. A sub-variety of these constitute:
4. The lenticular papillce, which are low T er than the fungiform but otherwise similar to them.
5. The circumvallate papillce (papillae vallatae), so named because they are surrounded by a circular wall-like elevation of the mucous membrane. They resemble the fungiform in shape, but are larger, and their surfaces are frequently slightly depressed beneath the general surface. In their more minute structure, however, they differ markedly from the fungiform papillae.
They are few in number and are always arranged in a typical manner upon the dorsum of the tongue. Their number varies between seven and twelve, and they are arranged in a V- shaped manner, the apex (of the V) being at the foramen caecum. They may be situated at unequal distances from one another and rarely they are arranged in two rows.
6. The foliate papillce are but rudimentary structures in the human subject. They are arranged in several parallel transverse folds, usually only faintly indicated, upon the lateral margins of the tongue just in front of the glossopalatine arch. Unlike the other transverse folds and wrinkles in the relaxed tongue of the dead subject, the folds or laminae of the foliate papillae are not obliterated by traction.
(For further details concerning the structure of the lingual papillae see the Sobotta-Huber "Atlas and Epitome of Normal Histology," Saunders' Medical Hand-Atlases.)
The posterior tonsillar portion of the lingual mucous membrane is markedly different from the anterior papillary portion. It is characterized by the presence of lymphatic structures, the lingual follicles, which together form a diffuse tonsillar structure, the lingual tonsil, and it is also particularly rich in mucous glands. Each lingual follicle forms a small rounded elevation with a fine central opening; at the root of the tongue they form a dense compact mass, while toward the epiglottis and the adjacent palatine tonsil they are more scattered.
The sublingual mucous membrane is smooth and thin and exhibits the ordinary characteristics of the oral mucous membrane. In the median line, beneath the tip of the tongue, it presents a fold, the frenulum, to either side of which is found a plica fimbriaia, which is always well developed in the newborn and less distinct, though rarely entirely absent, in adult life. This fold is always lobulated in the newborn, and usually so in the adult, and gradually disappears as it runs backward and outward from the anterior extremity of the frenulum. In the floor of the mouth just beside the anterior portion of the lateral margin of the tongue, and running obliquely from behind forward and inward, is a fold, the sublingual fold, which is produced by the underlying secretory duct of the sub-maxillary gland and usually contains the orifices of the lesser sublingual ducts. The two folds converge toward the posterior extremity of the frenulum, in the immediate proximity of which they terminate in a small elevation, the sublingual caruncle, which marks the orifice of the submaxillary duct.