870 



TEETH. 



is shown at p; this is immediately surrounded 

 by the transverse sections of large cylindrical 

 medullary, or pulp-canals of different sizes; 

 and, beyond these, there are smaller and more 

 numerous medullary canals, which are pro- 

 cesses of the central pulp-cavity. In the 

 transverse section these processes are seen 

 to be connected together by a net-work of 

 smaller medullary canals belonging to a coarse 

 osseous texture into which the pulp has been 

 converted, and this structure occupies the 

 middle half of the section. All the medullary 

 canals were filled by the opaque matrix. 

 From the circumference of the central net- 

 work, straight medullary fissures radiate at 

 pretty regular intervals to the periphery of 

 the tooth : most of these canals divide once, 

 rarely twice, in their course ; the division 

 taking place sometimes at their origin, in 

 others at different distances from their termi- 

 nations, and the branches diverge slightly as 

 they proceed. Each of the above medullary 

 fissures is continued from a short process of 

 the central structure, which is connected by a 

 concave line with the adjoining process, so 

 that the whole periphery of the transverse 

 section of the central coarse reticulo-medul- 

 lary body of the tooth presents a crenate out- 

 line. From each ray and its primary dicho- 

 tomous divisions, short branches are sent off 

 at brief intervals, generally at right angles 

 with the trunk, or slightly inclined towards 

 the periphery of the tooth. These subdivide 

 into a few short ramifications, like the branches 

 of a shrub, and terminate in irregular and 

 somewhat anguhr dilatations, simulatingleaves, 

 but which resolve themselves into radiating 

 fasciculi of calcigerous tubes. There are 

 from fifteen to twenty-five or thirty- six of 

 these short and small lateral branches on 

 each side of the medullary rays. 



A third kind of complication is produced 

 by an aggregation of many simple teeth into 

 a single mass. 



The examples of these truly compound 

 teeth* are most common in the class of 

 Fishes, but the illustration here selected is 

 from the Mammalian class. Each tooth of 

 the Cape Ant-eater (Orycteropus) presents a 

 simple form, is deeply set in the jaw, but 

 without dividing into fangs ; its broad and 

 flat base is porous, like the section of a com- 

 mon cane. The canals to which these pores 

 lead contain processes of a vascular pulp, and 

 are the centres of radiation of as many inde- 



* In the " Lemons d'Anatomie Compare'e " of 

 Cuvier, the teeth, in which folds of enamel and 

 cement penetrate the entire substance of the crown, 

 are called " compound : " " Nous appellous ' dent 

 composee ' celle dont les differentes substances 

 torment des replis tellement profonds, que dans 

 quelque sens qu'on coupe la dent, on coupe plusieurs 

 fois ehacune des substances qui la composent : telles 

 sont les dents molaires de r Elephant." The teeth 

 of the " Labyrinthodonts " would come under this 

 definition more truly than those of the elephant, 

 although they differ from them in having no enamel ; 

 for a molar of an elephant might be bisected, verti- 

 cally and transversely, without cutting the tissues 

 across more than once. 



pendent series of dentinal tubules. Each 

 tooth, in fact, consists of a congeries of long 

 and slender prismatic denticles of dentine, 

 which are cemented together by their ossified 

 capsules, the columnar denticles slightly de- 

 creasing in diameter and occasionally bifur- 

 cating as they approach the grinding surface 

 of the tooth. 



A figure of a longitudinal section of the 



o 



molar teeth is given in PL 76, fig. 10. of my 

 " Odontography," and a magnified view of a 

 similar section in PI. 77. ; fig. 555. gives a 

 magnified view of a portion of the transverse 



Fig. 555. 



Part of transverse section of the tooth of the Oryc- 

 teropus. (Magnified.) 



section of the fourth molar, showing c the 

 cement ; d the dentine ; p the pulp-cavity of 

 the denticles ; and d' a section of one or the 

 denticles just beyond its bifurcation. 



The pectinated incisors of the flying Lemur 

 of the Indian Islands (Galeopithecus) are ex- 

 amples of teeth, the crowns of which are 

 composed of denticles consisting of hard den- 

 tine, with a covering of true enamel. The 

 layer of cement over this is too thin to show 

 its characteristic structure, and does not fill 

 up the intervals of the denticles, which stand 

 out as free processes from the base of the 

 crown. Tubular prolongations of the pulp- 

 cavity are continued up the centre of each 

 denticle. 



Fig. 556. exhibits a longitudinal section, 

 magnified, of this kind of compound tooth; 

 d is the dentine; e the enamel; p the pulp- 

 cavity. The originally detached summits of 

 the crown of the human incisor are homo- 

 logous with these columnar processes, or 

 denticles of the incisor of the Galeopithcciis. 



In the compound molars of the great Afri- 

 can wart-hogs (Phacochaerus) the columnar 

 denticles are in three rows, and their inter- 

 spaces are filled up by cement : each denticle 

 consists of a slender column of hard dentine 

 inclosed in a thick sheet of enamel, the whole 

 being bound together by the cement ; and the 



