168 DIGESTION. [CHAP. xxin. 



simple of the human teeth, these grinders are greatly surpassed in 

 complexity of form and structure by the corresponding teeth of her- 

 bivorous animals, such as the ox, the horse, and the elephant. 



The internal structure of the teeth, like that of bone, has been 

 much illustrated by those modern microscopic investigations which 

 have introduced a new era in the sciences of anatomy and physi- 

 ology. The researches on this subject, opened by Purkinje, Fraenkel, 

 and Retzius, and subsequently pursued with more or less originality 

 and extent by Miiller, Schwann, Tomes, Nasmyth, and especially 

 by Professor Owen, have confirmed the almost forgotten discoveries 

 of Leeuwenhoek, and brought the whole subject of dental structure 

 and development into clear and consistent light. We shall now give 

 a short summary of the facts as they have appeared to our own 

 minds, and refer, once for all, to the works quoted at the end of the 

 present chapter, for information as to the share each inquirer has 

 had in the general and very satisfactory result. 



The three constituent substances, dentine or ivory, enamel, and 

 tooth-bone or crusta petrosa, are found in all the higher and more 

 perfect forms of teeth ; and their several conditions in the range of 

 animals have been greatly instrumental in leading to our present 

 knowledge of them in the human teeth : our design, however, will 

 allow us to speak of their character in the latter only, except in 

 the way of illustration. 



Taking a simple tooth as an example (fig. 149), we find the great 

 bulk to consist of dentine, a term used by Mr. Owen to distinguish 

 this substance from the rest, in preference to that of ivory or tooth- 

 substance. The dentine gives the general form, size, and hardness 

 to the tooth, both root and crown, and in its central part is the 

 cavity containing the papillary substance or pulp, supplying the 

 vessels and nerves of the organ. 



Dentine is manifestly a modification of the osseous tissue. Like 

 bone, it may be seen, in favourable specimens, to present a finely 

 granulated ultimate texture, and, like bone, it is perforated by a 

 series of minute channels, opening on the one hand on a vascular sur- 

 face (that of the pulp-cavity, which corresponds with the Haversian 

 surface of bone: vol. i. p.lll), and on the other sparingly branch- 

 ing, so as to permeate every portion of the tissue. The peculiarity 

 of dentine consists chiefly in these internal channels of nutrition ; 

 and, as we have before shewn the Haversian canals and systems 

 of lamellae in bone to be arranged in constant subservience to me- 

 chanical ends, so the corresponding parts in dentine, and especially 

 those parts which answer to the lacunse and canaliculi of bone, ap- 



