798 



THE TEETH. 



secondary cells with a single one in a deeper layer of the pulp ; and the constricted or 

 moniliform appearance of the tubuli already mentioned as having been seen by some 

 observers in growing or even in mature teeth, is thought to depend on an imperfect 



Fig. 556. 



Fig. 556. VERTICAL SECTION THROUGH THE POINT OF A HUMAN FOETAL MILK TOOTH, IN 



WHICH THE FORMATION OF THE DENTINE AND ENAMEL HAS RECENTLY COMMENCED (from 



Kolliker after Lent). ^ 



a, dental pulp with blood-vessels ; 5, the dentine-cells upon its surface ; c, the cap of 

 dentine which has been formed on the summit, the tubuli being shown as prolongations 

 from the tapering extremities of the dentine-cells ; d, the enamel begun to be deposited ; 

 c, membranous layer, mernbrana prseformativa of Huxley. 



coalescence of the nuclei. In the teeth of young animals, Tomes has noticed the 

 division of the cells and their subsequent coalescence to form the tubes, but he has 

 failed to recognise the existence of primary cells including secondary ones. Lent 

 finds that the superficial elongated cells of the dentinal pulp send off from their 

 free ends long slender processes which form the tubes of the dentine, and which divide 

 into branches, and anastomose together in the same manner as the tubes. Kolliker, 

 who confirms Lent's observations, thinks it probable that a single cell may generate a 

 tube in its whole length ; at the same time a cell is sometimes constricted or incom- 

 pletely divided into two, the more superficial of which becomes narrowed and length- 

 ened into the dentinal tube. 



With respect to the actual formation of the hard substance of the tooth, two views 

 have been entertained ; Kolliker conceiving it to proceed from the calcification of a 

 soft matrix excreted from the dentinal cells and their thin prolongations already 

 referred to ; whereas Waldeyer, who denies the existence of a preformative mem- 

 brane, maintains that the formation of the dentine consists in the conversion of a part 

 of the protoplasm of the dentinal cells into a collagenous substance, which is subse- 

 quently calcified, while the remaining part of the cell-protoplasm continues in the 

 form of soft fibres to occupy the interior of the tube surrounded by the calcified sub- 

 stance. (Op. cit. p. 189.) When the cap of dentine is examined in the newly formed 

 state, besides the ordinary dentine, globules are commonly observed ; but, if diluted 

 hydrochloric acid be added, the globules disappear. Hence Czermak concludes that 

 earthy impregnation proceeds for a time in a globular form, and that the after- 

 presence of globular dentine is the result of arrested development; perfect develop- 

 ment leading to the filling up of the spaces between the globules, and to the pro- 

 duction of an uniformly compact tissue. 



The enamel appears in the form of prismatic fibres which, until the point was con- 

 tested by Huxley, have been generally supposed to be produced by calcification of the 

 cells of the enamel-membrane, with which they correspond in figure. An enamel 

 fibre may be formed by a single cell growing in length, while its previously formed 



