418 ON THE GROWTH AND STRUCTURE 
made at a short distance from the anterior opening towards 
the base of the tooth, there will be seen, first, the central 
osseous canal, the future channel for the poison, but which 
is in the present stage filled with its nourishing and forma- 
tive pulp or mould ; ScZ, A circular space surrounding the 
poison-canal, inclosed by the external parietes of the tooth, 
and containing also its nourishing pulp ; Sd, A vertical os- 
seous or horny lamina, proceeding from the convex side of 
the tooth to the poison-canal, and dividing this cavity, supe- 
riorly, into two equal parts. But immediately behind the 
anterior aperture, the convex part of the poison-canal has 
united so firmly and closely with the parietes of the tooth 
itself, that the vertical lamina is scarcely discernible. The 
reason of this is, simply because the poison-canal presents 
throughout a calibre of nearly equal dimensions, whilst the 
tooth itself is constantly becoming more and more tapered 
tow^ards the point ; the parietes, consequently, of the respec- 
tive canals soon approach each other, and at last are united ; 
the cavity, also, for containing the nourishing pulp of the 
tooth, becomes gradually less and less, and is more confined 
to the concave side of the tooth. If the section be made 
near the base of the tooth, that is, where the enamel has 
not become consolidated, the vertical lamina is quite carti- 
laginous, and very thin, but always exists ; so that the ex- 
ternal riourishing pulp at no period of the growth, unin- 
terruptedly surrounds the poison-canal. Moreover, it is 
here to be remarked, that the poison-canal is never open 
on its convex surface ; but that, on the contrary, its parie- 
tes form at ail times a completely circular canal, contain- 
ing at an early period its formative pulp, and, afterwards, 
poisonous fluid itself. The fissure which, at a very early 
period of the growth of the tooth, may be seen on its con- 
vex side, is quite closed, even in the rudimentary fangs, 
long previous to the formation of the aperture at the base ; 
