188 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



immediately preceding the conversion of the pulp into dentine, the cells 

 are seen to enlarge, arid to become arranged in lines, nearly parallel to 

 each other, and perpendicular to the coronal surface of the tooth ; they 

 then elongate in such a manner, that their extremities come into appo- 

 sition; and they finally coalesce, so that each row of cells forms a 

 continuous tube. Whilst this change is in progress, the gelatinous 

 intercellular substance is becoming consolidated by calcareous deposit ; 

 which also hardens the thick walls of the tubes, so that only their inte- 

 rior, formed by the coalescence of the original cell cavities, remains 

 uncondensed, thus constituting the dentinal tubuli. This process takes 

 place first on the surface of the pulp, and gradually extends inwards. 

 As the more external and larger cells become hardened, the inner ones 

 increase in size, assume the linear arrangement, and in their turn become 

 converted, with the intercellular substance, into tubular dentine ; until 

 at last the great bulk of the pulp is thus transformed, leaving only a 

 comparatively small portion, which, with its nerves and blood-vessels, 

 occupies the central cavity of the tooth. 



314. Thus the substance of the outer portion of the pulp is actually 

 converted into dentine, and does not form it by a process of excretion, 

 as was formerly supposed. Sometimes it happens that the normal 

 changes are interrupted, and that some of the original meshwork remains 

 persistent ; and it is probably to this that the appearance of large cells, 

 not unfrequently seen in Human teeth (Fig. 53) is due. Although in 

 the most characteristic form of Dentine, no blood-vessels exist, yet there 

 are certain species, both among Mammals, Reptiles, and Fishes, in which 

 the Dentine is traversed by cylindrical prolongations of the central cavity, 

 conveying blood-vessels into its substance ; and the presence of these 

 medullary canals, giving to the Dentine a vascular character, thus 

 increases its resemblance to bone. The central portion of the pulp is 

 sometimes converted into a substance still more nearly resembling bone, 

 having its stellate lacunae as well as its vascular canals. This change 

 is normal or regular in certain animals, as in the extinct Iguanodon and 

 Icthyosaurus, and in the Cachalot or Sperm-whale; and the ossified 

 pulp bears a close resemblance to the bones of the respective animals 

 although it is not formed in continuity with them. A similar change 

 occurs in the Human tooth ; sometimes, it would appear, rapidly, as 

 the result of disease ; but in general more slowly, increasing gradually 

 with the advance of age. 



315. It is not easy to ascertain the amount of nutritive change that 

 takes place in the substance of Dentine, when it is once fully formed. 

 When young animals are fed with colouring-matter, it is found to tinge 

 their teeth, as well as their bones ; and if the tooth be in process of 

 rapid formation at the time of the experiment, the progressive calcifica- 

 tion of the pulp from without inwards, is marked by a series of concen- 

 tric lines. Even in the adult, some tinge will result from a prolonged 

 use of this substance ; and it has been noticed that the teeth of persons 

 who have long suffered from Jaundice sometimes acquire a tinge of bile. 

 These facts show that, even after the complete consolidation of the 

 Dentine, it is still pervious to fluids : and that in this manner it may 

 draw into itself, from the vascular lining of the pulp-cavity, a substance 



