2 A MANUAL OF DENTAL ANATOMY. 



been mastered ; still a few words may even at the outset be 

 devoted to the elucidation of their real nature. 

 . The mucous membrane which lines the alimentary canal 

 is continuous with is, indeed, a part of the external skin, 

 with which it blends at the lips. Now if a young dog-fish, 

 just about to be hatched, be examined, it will be found that 

 it has no distinct under lip, but that its skin turns in over 

 its rounded jaw without interruption. The skin outside 

 carries spines (placoid scales)^ 1 ) and these spines are con- 

 tinued over that part of it which enters the mouth and 

 bends over the jaws ; only they are a little larger in this 

 latter position. If the growth of the dog-fish be followed, 

 these spines of the skin which cover the jaws become deve- 

 loped to a far greater size than those outside, and the identity 

 and continuity of the two become to some extent masked. 

 No one can doubt, whether from the comparison of adult 

 forms or from a study of the development of the parts, 

 that the teeth of the shark correspond to the teeth of other 

 fish, and these again to those of reptiles and mammals ; it 

 may be clearly demonstrated that the teeth of the shark 

 are nothing more than highly developed spines of the skin, 

 and therefore we infer that all teeth bear a similar relation 

 to the skin. This is what is meant when teeth are called 

 " dermal appendages," and are said to be perfectly distinct 

 from the internal bony skeleton of the animal ; the teeth of 

 the shark (and of many other creatures) remain imbedded in 

 tough mucous membrane, and never acquire any connection 

 with the bone. Indeed, all teeth alike are developed from a 

 part of the mucous membrane, and any connection which 

 they may ultimately get with the bone is a secondary 

 matter. As it has been well expressed by Dr. Harrison Allen 



( J ) "The placoid scale has the structure of dentine ; is covered by 

 enamel, and is continued at its base into a plate formed of osseous tissue. " 

 Gegenbaur's Comparative Anatomy, translated by F. Jeffery Bell, p. 424. 



