REVIEWS. 
451 
have been expected. Common bone is never exposed to the action of the 
external air, nor is it ever subjected to abrasion : on the contrary, when 
two become opposed to each other, a cartilage tips the end of each, to pre- 
vent either friction or concussion. The teeth have one third of their sub- 
stance not only exposed to the action of the air, but are very often brought 
into contact with the hardest bodies. It is not to be wondered at, therefore, 
if some specialty is found in them ; neither ought it to be a matter of sur- 
prise if the free or exposed portions, as the crown and bodies, and the 
covered parts, as the root or fangs, should be seldom formed altogether of 
the same materials : and even where they are in the main similar, yet the 
proportions of their substances vary, or the modes in which they are dis- 
tributed are unlike. Enamel, ivory , and cement , enter the composition of 
the teeth of the horse generally ; but, individually, their distribution varies, 
in the nippers and tushes, the enamel covers the whole of the free portion ; 
in the grinders, instead of covering the surface of the table generally, we 
see that it forms distinct penetrating layers*. Of these dental components, 
the enamel is by far the hardest and densest : it also contains less animal 
matter, and, when examined closely, appears fibrous. It will yield fire with 
steel, like flint, and is hardly to be acted upon by the best tempered files : 
it never occurs alone, but always as a coating to the ivory of the tooth. The 
enamel, chemically examined, appears to be a crystalline compound, from 
gelatine and phosphate of lime, and is secreted from the membrane of the 
pulp- 
“ The ivory is harder than common bone ; it is also fibrous, and is the pro- 
duce of the pulp of the tooth in the early state, which consolidating, forms 
* By comparative anatomy we are frequently enabled to throw great light 
on the functions of particular parts of the human body, by which some of 
the obligation due to the knowledge of the latter are repaid. I have had 
frequent occasions of pointing to this mutual advantage ; perhaps a more 
felicitous one does not occur than the present, in which erroneous notions 
on the subject of the preservative use of the enamel have prevailed, and are 
yet every day insisted on by dentists. Were a preservative quality the prin- 
cipal use of the enamel, the teeth of both man and beast would be seldom 
free from decay. In both the one and the other, parts of the teeth are 
entirely deprived of it, and in neither does decay take place at such parts. 
The depressions on the broad surfaces of the horse molar teeth are, as 
described above, purposely formed from the first without enamel, and on the 
surface of the incisors or nippers it early wears away, and yet caries is 
almost entirely unknown to both. The human incisors also wear from a 
sharp edge to a flattened surface, entirely uncovered by enamel, but on 
which surface caries never commences; on the contrary, when these 
decay, the disease commences at the neck of the tooth where the 
enamel is thickly encrusted over. The first carious spot usually 
seen in the human molar teeth is in the deep depressions on their semi- 
incisive grinding surface, where the enamel can suffer no abrasion : add to 
which, that decay in a tooth may generally be stopped, if the whole of the 
diseased portion be filed away. Some tribes among the Indians, remarked 
for the soundness aud goodness of their teeth, always keep them filed to a 
point ; we therefore learn, by collating these facts, that the enamel of the 
teeth operates little in preserving them from morbid decay ; but that its 
principal use is, by its extreme hardness, so to temper tlfe teeth, as steel 
tempers iron, that they may resist the impressions of constant mastications, 
and make the wear of these organs commensurate with that of the body in 
general. 
