156 THE ESSENTIALS OF HISTOLOGY. 



milk-teeth ; these are the permanent molars. They are developed from 

 an extension backwards of the original epithelial thickening or common 

 enamel-germ and the downgrowth from this into the corium of three 

 successive special enamel -germs at comparatively long intervals of time. 

 Within these the tissues of the permanent molars become formed in a 

 manner exactly similar to that in which the milk-teeth are developed. 



THE TONGUE. 



The tongue is mainly composed of striated muscular fibres, running, 

 some longitudinally, and others transversely. It is covered by a mucous 

 membrane, the epithelium of which, like that of the rest of the mouth, 

 is thick and stratified, and conceals microscopic papillae (fig. 181) like 

 those of the skin. Besides these, the upper surface of the organ is 

 covered with larger papillae, which give it a rough appearance. These, 

 which are termed the lingual papillce, are of three kinds : (1) About twelve 

 or thirteen comparatively large circular projections, each of which is 

 surrounded by a narrow groove (fossa), external to which the mucous 

 membrane is raised above the general level (vallum) (fig. 182). These 

 papillae form a V-shaped line towards the back of the tongue ; they 

 receive filaments of the glosso-pharyngeal nerve, and have taste-buds in 

 the epithelium which covers their sides, and in that of the side of the 

 vallum. They are known as the circumvallate papillce. (2) All the rest 

 of the papillary surface of the tongue is covered by conical papillce, so 

 named from the conical pointed cap of epithelium which is borne by 

 each ; sometimes this cap is fringed with fine epithelial filaments, when 

 they are termed filiform (fig. 184). (3) Scattered here and there 

 amongst the conical papillae are other larger papillae, the fungiform 

 (fig. 183). These are very vascular, and lie partly embedded in little 

 depressions of the mucous membrane. 



Small tubular glands may be seen between the superficial muscular 

 fibres sending their ducts to the surface. Most of them secrete mucus, 

 but those which open into the trenches of the circumvallate papillae, 

 and a few others elsewhere, yield a serous secretion (glands of Ebner). 



The mucous membrane at the back of the tongue contains a large 

 amount of lymphoid tissue. 



The taste-buds. The minute gustatory organs which are known as 

 taste-buds may be seen in sections which pass through the papillae 

 vallatae or the papillae fungiformes; they are also present here and 

 there in the epithelium of the general mucous membrane of the tongue, 

 especially at the back and sides, and occur also upon the under surface 



