196 THE ANATOMY OF THE HORSE. 



thick and of a three-sided form. The entire organ is invested by the 

 mucous membrane of the mouth, -which, in passing on to the tongue, 

 forms certain folds, or doublings. Thus, if the free portion of the tongue 

 be raised from its position on the floor of the mouth, there will be seen 

 on the middle line a double fold of mucous membrane termed the fraenum 

 linguoB. At the extreme upper part of the mouth, again, the mucous mem- 

 brane, in passing between the root of the tongue and the soft 'palate, 

 forms on each side a fold termed the anterior pillar of the fauces. 



The mucous membrane of the tongue has its surface raised into the 

 form of papillae of which there are three varieties : — 



1. The Filiform Pajnllce are the most numerous, and are found all 

 over the tongue. In shape they are conical, having a tapering summit 

 either simple or bearing secondary papillae. They are largest on the 

 upper half of the dorsum (anterior or upper surface), to which they 

 give a distinct pile. 



2. The Fungiform Fapillce are mushroom-shaped, being expanded at 

 the summit, which bears secondary papillae. They are scattered along 

 the dorsum and sides of the tongue, being most numerous in its middle 

 portion. 



3. The Circumvallate Papilla are generally two in number, and are 

 placed on the dorsvmi, one on each side of the middle line, about five 

 inches from the epiglottis. Sometimes there is a third and smaller 

 papilla, placed on the middle line, about three-quarters of an inch above 

 the other two. Each is isolated by a circular trench, and is terminated 

 by a flat summit, which is level with the surrounding surface, and 

 bears numerous secondary papillae. They contain the peculiar gustatory 

 bodies, to which fibres of the glosso-pharyngeal nerve are distributed. 



The Sublingual Ridge. — This is a longitudinal elevation of the mucous 

 membrane at the floor of the mouth, on each side of the fixed portion of 

 the tongue. It is caused by the underlying sublingual salivary gland, 

 whose ducts open on the summits of little tubercles which stud the 

 ridge. 



The Barbs. — These are two flattened, leaf-like papillae situated on the 

 inferior part of the floor of the mouth, one on each side of the fraenum 

 linguae. Wharton's duct — the duct of the submaxillary gland — discharges 

 itself by a minute opening on the summit of the barb. 



Structube of THE ToNGUE. The tongue possesses a mucous covering, 

 a collection of mucous glands, a median fibrous cord, muscles, nerves, 

 and connective-tissue. 



Mucous Membrane of the Tongue. — This has already been partly noticed. 

 Like the rest of the lining membrane of the mouth, it has a stratified 

 squamous epithelium. It is intimately adherent to the subjacent structures. 

 It is thickest on the dorsum of the tongue, where it is harsh and wrinkled. 

 On the sides and posterior surface of the tongue it is thin and smooth. 



