398 
ME. S. J. A. SALTEE ON THE STEHCTUEE AND 
scopist to form an approximate idea of their position : a fracture of the tooth where it 
has become coherent and at the same time not absolutely consolidated, where one series 
of the tooth-elements may be detached from those imbricated upon it without their dis- 
placement, will determine their position definitively. The upper plates in figure 5, 
Plate VI. are very nearly shown in that position of obliquity which is normal, and which 
they maintain when in combination with the other elements of the tooth. The degree 
of imbrication of the plates, the amount covered and the amount left uncovered by each 
plate as it lies imposed upon its neighbour, will be best understood by examining their 
dorsal aspect when in situ. This is seen in Plate VII. fig. 1, where they are little 
disturbed ; and it should be borne in mind, as the uncovered portion in the dorsal aspect 
is the exact limit of the attachment of one of the secondary elements (the enamel fibres) 
to the primary plates, which will hereafter be shown. 
I have never succeeded in getting a view of a fractured plate edgewise so as to ascer- 
tain its thickness : it is, however, very thin, and its area increases rapidly in proportion 
to the accumulating density in its progressive growth. While the outline of the plates, 
when entire, is marked by curves, their fracture is always angular and crystalline. Por- 
tions of the plates, and indeed all the elements of the tooth, break up with a rhoni- 
boidal fracture, and, when the plates are disintegrated under the microscope, they pre- 
sent the appearance of a multitude of the rhombic plate-crystals of cholesterine. 
Secondary Plates . — When the primary plates have advanced considerably in their 
development, and already number some two hundred and fifty or three hundred from 
the growing extremity, a slight thickening is seen on the outer edge about midway 
between the superior and outer angles : this thickening does not consist of a further 
growth upon the extreme edge itself, but a bulging of the margin towards the internal 
or enteric aspect of the plate. Upon examining the plates some five or ten further in 
advance, more developed and older, this projection will be seen to have grown into a 
small valve-like lappet folding inwards towards the enteric region. This is the com- 
mencement of the secondary plates. As the development of these supplemental plates 
progresses — the growth consisting of a marginal enlargement — them form alters and 
their attachment to the outer border of the primary plates extends itself. The growth 
of these plates is not rapid, and in contrasting them at short intervals of their advance- 
ment little change is seen. In Plate VII. fig. 2, a series of these growths is represented 
at internals of fives, each succeeding plate from below upwards being the fifth from the 
previous one. By examining this illustration, the slow progress of development (in 
some instances not distinguishable between the contiguous fifth plate) will be seen and 
the altered form they gradually assume. The most remarkable feature of this altered 
form, when it has arrived at maturity, is the mamilliform process which grows from the 
middle of the free edge, and is ultimately much longer than any represented in this 
plate. This form is so characteristic that the existence and position of the secondary 
plates may always be recognized in the most advanced and complicated condition in 
which the tooth is susceptible of examination by transmitted light. 
