446 THE HUMAN BODY 



minute hexagonal prisms, closely packed, and set on vertically to 

 the surface of the subjacent dentine. It is thickest over the free 

 end of the crown, until worn away by use. Covering the enamel in 

 unworn teeth is a thin structureless horny layer, the enamel cuticle. 



The Tongue (Fig. 127) is a muscular organ covered by mucous 

 membrane, extremely mobile, and endowed not only with a deli- 

 cate tactile sensibility but with the terminal organs of the special 

 sense of taste; it is attached by its root to the hyoid bone. On itn 

 upper surface are numerous small eminences or papillae, such an' 

 are found more highly developed on the tongue of a cat, where they 

 may be readily felt. On the human tongue there are three forma 

 of papillae, the circumvallate, the fungiform, and the filiform. The 

 circumvallate papillae, 1 and 2 (Fig. 127), the largest and least 

 numerous, are from seven to twelve in number and lie near the 

 root of the tongue arranged in the form of a V with its open angle 

 turned forwards. Each is an elevation of the mucous membrane, 

 covered by epithelium, and surrounded by a trench. On the sides 

 of these papillae, embedded in the epithelium, are many small oval, 

 bodies richly supplied with nerves and supposed to be concerned 

 in the sense of taste, and hence called the taste-buds (Chap. XIV). 

 The fungiform papillae, 3, are rounded elevations attached by 

 somewhat narrowed stalks, and found all over the middle and fore 

 part of the upper surface of the tongue. They are easily -recog- 

 nized on the living tongue by their bright red color. The filiform 

 papillae, 4, most numerous and smallest, are scattered all over the 

 dorsum of the tongue except near its base. Each is a conical 

 eminence covered by a thick horny layer of epithelium. It is these 

 papillae which are so highly developed on the tongues of Carnivora, 

 and serve them to scrape bones clean of even such tough structures 

 as ligaments. 



In health the surface of the tongue is moist, covered by little 

 "fur," and in childhood of a red color. In adult life the natural 

 color of the tongue is less red, except around the edges and tip; a 

 bright-red glistening tongue being then, usually a symptom of 

 disease. When the digestive organs are deranged the tongue is 

 commonly covered with a thick yellowish coat, composed of a little 

 mucus, some cells of epithelium shed from the surface, and 

 numerous microscopic organisms known as bacteria; and there 

 is frequently a "bad taste in the mouth." The whole alimen^ 



