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. [Illustration: 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.