290 



HISTOLOGY 



Each tooth (Fig. 252) arises from this basis of which it is a part and 

 projects into the stratified epithelial papilla, which it conforms to in 

 shape, but does not quite fill. It is a hollow shell of basket work, ir- 

 regularly reticular, so that but few fenestrae can be seen in its sides. It 

 is narrow at the base, with thick sides that arise parallel and diverge 

 as they pass distally until they arch over and meet at the top. This 

 top is thus broad and blunt. The whole structure has much the shape 

 of a balloon or, still more, that of an ordinary incandescent electric- 

 light bulb. Its walls remain at a little distance from the stratified 

 epithelium, being kept separate by a layer of lax connective tissue. The 

 inside of the tooth is filled with a connective tissue that contains much 

 lymphatic tissue in its meshes as well as an abundant blood supply. 



As so far described the structure could not act as a tooth at all, being 

 covered by a thick, stratified epithelium. It possesses, however, on its 

 upper, outer surface a series of strong, hollow spines which project 

 through the stratified epithelium and come in contact with the food. 

 At the point where they pierce the epithelium the basement membrane 

 is reflected distally along the spines, and the epithelium sends a close- 

 fitting layer a short distance proximally around the spike. Thus a 

 gum is formed. The interior of the spike is filled with very active cells, 

 which line its interior and through whose agency its walls are formed 

 and maintained. 



This tooth is not used to grind with, but its sharp spines, reaching a 

 short distance into the lumen of the digestive tube, tear and grate the 

 flesh of the animals which are eaten by the harvest fish, and thus prepare 

 them for digestion. In some other fishes there is a gizzard with a similar 



structure, except that the teeth, 

 instead of lying inside the gum 

 with only the tips of their spikes 

 projecting through its surface, 

 stand bodily out from the gum and 

 are hard and thick with a white, 

 polished surface. They have 

 shorter, thicker spikes and are 

 used for direct mastication. 



The radula of the snail, Helix, 

 is composed of numerous chiti- 

 FIG. 3S3 . -Epithelial fold in which the radula nous processes which are directed 



or t-oUhed tongue of Helex pomatia is devel- posteriorly. 



oped, r., teeth of radula. 

 SCHNABEL.) 



X 350. (After 



These are produced 

 in a certain epithelial fold of the 

 mouth by a particular set of epi- 

 thelial cells (Fig. 253). They may be regarded as parts of the cuticle 

 which these cells form. As they are formed they are pushed anteri- 



