The Sense of Taste

by Harry L. Hollingworth; Albert T. Poffenberger

The tongue is made up of seventeen muscles, acting in three planes—vertical, longitudinal, and transverse. Separating these muscles one from another are layers of fatty tissue, enabling the muscles to glide easily over each other. Fifteen of these muscles are extrinsic in the sense that one end of each has its point of attachment outside of the tongue. It is these muscles especially that give the tongue its great motility. By the contraction of single muscles or contraction in various combinations the tongue is protruded or drawn back, the tip raised or lowered, the dorsal surface pressed against the roof of the mouth or withdrawn toward the floor of the mouth, the tongue protruded and turned to one side or the other. In addition to these extrinsic muscles there is a pair of intrinsic muscles, each having both points of attachment within the tongue. By their contraction the sides of the tongue are raised and drawn together.

These muscles are richly supplied with blood vessels and receive an especially large supply of nerve fibers from the hypoglossal, which is known as the motor nerve of the tongue, and some from the lingual branch of the seventh, or facial, nerve.

It is generally believed that the tongue movements serve the sense of taste only as they facilitate contact of the sapid substances with the real taste endings; for instance, by pressure of the tongue against the roof of the mouth, and by its protrusion from the mouth to receive the stimuli upon its surface. However, it has been asserted by some investigators that the movements are of more direct use in the taste process, in that tongue movements in themselves tend to increase the sensitivity of the taste mechanism. But careful experiments in which the tongue was rendered motionless during tasting show that taste sensitivity is just as great as when the tongue is free to move.

By far the most important portion of the tongue is its mucous covering. This varies considerably in character in different regions, being thickest and toughest on the superior surface where It comes into contact with objects taken into the mouth, and thinnest on the inferior surface where it is ordinarily protected from such contact. On the sides and tip it is moderately thick and tough. The tongue covering has a highly complex structure. Two distinct layers, or strata, are commonly described: the more superficial, or epithelial layer, and the deeper layer, called the chorion. The first, or epithelial, layer contains all of the sensory endings concerned in taste upon the tongue, and these will be described later. The second is more complex and consists of connective tissue, a great network of blood vessels, nerve fibers, and numerous glands and their ducts which open upon the surface of the tongue. Upon the more superficial surface of this inner layer there is an extremely large number of slight elevations. These are apparent to the unaided eye upon the tongue surface, since the epithelial layer of the membrane follows very closely the contour of this deeper layer. These elevations are called papillæ. ([Fig. 2.]) They vary in size and shape and are quite unevenly distributed upon the surface of the tongue. On the inferior surface there are none, while on the superior surface they are most numerous. With the aid of the papillæ on this surface the tongue can be divided into two parts, an anterior or horizontal portion and a posterior or vertical portion. The former includes about the forward two-thirds of the tongue and the latter the posterior third. These two parts are separated by a row of relatively large elevations, about ten or twelve in number, and arranged in the shape of a V, with the open portion of the V turned forward. From the apex of the V a furrow passes forward to the tip of the tongue, dividing the anterior portion laterally into two halves. The posterior portion is broken up into a series of folds taking about the same direction as the legs of the V.

Fig. 2.

Classification of Papillæ

The papillæ of the tongue have been described and classified more or less in detail since the middle of the seventeenth century. They can be grouped into four classes ([Fig. 2.]), the circumvallate, the fungiform, the filiform, and the foliate papillæ, each group having certain distinctive characters. A fifth group is sometimes added and comprises the hemispherical, or simple, papillæ.

The circumvallate group comprise the largest and the most important papillæ. There are only from eight to twelve or sixteen of this type, and by their arrangement they form the V-shaped figure on the superior surface of the tongue, mentioned above, and which divides the anterior two-thirds from the posterior third of the tongue. These papillæ are found nowhere else. They rise only slightly above the surface of the tongue from the bottom of a pit or cup-shaped depression, giving the impression of a small mound surrounded by a ditch, whence the name, circumvallate. In a few cases more than one papilla rises from a single pit, but in most cases there is but one. The papilla itself has an average height of 2 millimeters, with a diameter at the top of 1.0 to 1.5 millimeters, and slightly less at the base. The cup-shaped depression averages about 1 to 1.5 millimeters in depth. The largest of these papillæ is that one forming the apex of the V-shaped figure and is called the foramen cæcum. The smallest are those found at the ends of the V.

The fungiform papillæ are so called on account of their resemblance to a toadstool. Each consists of a rather slender stalk capped by a relatively large, rounded head, about .8 to 1.0 millimeter in diameter. The whole papilla has an average height ranging from 1.0 to 1.5 millimeters. These are scattered very irregularly over the superior surface of the tongue, most of them being found on its anterior two-thirds. However, a few are found back of the circumvallate papillæ, but always very close to them. They are most numerous on the sides and the tip of the tongue, where they appear as bright red points upon the paler background of the tongue covering. The total number of this type has been estimated at from 150 to 200.