1895.] The Formation and Structure of Dental Enamel. 181 



paper, that quaternion methods may be regarded practically as a par- 

 ticular case of octonion methods. 



When the motion of the rigid body is always in the neighbourhood 

 of an absolutely stable position we get the theory of " Screws." The 

 more o-eneral theorems in connection with this are given in a some- 

 what different form from Sir Robert Ball's. Thus, for instance, we 

 have a " generalised force motor," which in a particular case becomes 

 the " reduced wrench." It is pointed out that the latter is not always 

 intelligible. Some form of the former is always intelligible, as also 

 is a particular form of it called the " virtual force motor." The cor- 

 responding "virtual impulse motor" is the impulse motor due to the 

 external impulses and the reactions of the constraints, but a similar 

 statement does not hold for the virtual force motor 



IV. *' On the Formation and Structure of Dental Enamel." By 

 J. Leon Williams, D.D.S., L.D.S. Communicated by 

 Professor SchaFER, F.R.S. Received December 4, 1894. 



(Abstract.) 



The special points in the formation and structure of enamel which 

 1 have attempted to elucidate in this paper may be summarised as 

 follows : — 



1st. The existence of a very thin membrane, or a structure of 

 membrane-like appearance, lying between the ameloblasts and the 

 forming enamel, and also between these cells and those of the stratum 

 intermedium. I have also, in many specimens, seen a similar mem- 

 brane covering the odontoblasts. 



2nd. The formation of enamel by deposit and not by cell calcifica- 

 tion. This deposit probably consists of two distinct cell products — a 

 granular plasm and spherules of calcoglobulin. 



3rd. The relation of the cells of the stratum intermedium to true 

 secreting tissue ; this relation being especially marked in the enamel 

 organs of the rat and mouse. 



4th. An intricate vascular network in the stratum intermedium. 

 I should also mention that I have seen a free distribution of blood 

 vessels in the odontoblastic layer of cells in the mouse, rat, and calf, 

 as well as in human embryos, thus conclusively proving that these 

 cells are not calcified. 



5th. The fibrous character of enamel in many of the lower animals, 

 and the change of these fibres into more or less regularly arranged 

 granules in the monkey and in man. 



6th. That the varicosities of the enamel rods are not caused by 

 acids (although of ten rendered more clear to view by acid treatment) 



