ON THE TEETH. 
371 
pleted , at four years and a half the corner incisors are replaced as 
before, first above, then below. Deviations from these rules are, of 
course, of frequent occurrence, and those most frequently occurring 
at the first change ; and I have frequently known that not a tooth 
has been shed until nearly three years old. These deviations are 
of most frequent occurrence amongst forest, Welch, or common 
ponies ; this may be accounted for by their being late foals, as also 
by their being hardly kept during early life. The later changes 
are much more regular. 
The process of displacement of the temporary molar teeth is 
not so much by absorption of the fangs as by compression against 
them, and wearing away from the crown. The new tooth, in its 
formation, presses upon the thin and weak extremities of the fangs; 
these are partially absorbed : but the pressure from above pre- 
venting the rising up of the body of the tooth, except as the face is 
worn down, causes a greater consolidation of the fangs, which take 
the impress of the crown of the new tooth ; while those portions of 
the outer layer which come opposite to the furrows upon the sides 
of the new tooth, not being subject to this perpendicular pressure, 
but only to lateral pressure, become prolonged into these furrows, 
which enable this shell of the temporary tooth to be adherent. 
After the crown of the new tooth has risen above the alveolar 
cavity, the temporary tooth is still farther fitted for this change 
by not being so dense generally in its structure, but as it wears 
down lower and lower, the substance is less and less dense ; and it 
is not by any means uncommon to find that fragments only of the 
shell drop away, so complete has been the apposition between the 
surfaces. After the pulp and membranes of the tooth are ab- 
sorbed — that is, the organic parts — no absorption, can take place ; 
but the process is then simply that of wearing away. This is a 
very wonderful adaptation of means to an end, as by it the masti- 
catory organs are never unfitted for the due performance of their 
office; and, in fact, the opposing ranges of teeth are always in 
contact: the upper range being thrown off the first, the lower, 
from not having so great an opposition as before, are worn less, 
and, being pushed on at the same time, meet the new upper mo- 
lars, and thus the surface for grinding is not diminished, except for 
the short period which elapses between the throwing off of the 
shell from the lower molar. The space, T have shewn before, is 
often not more than the one-sixteenth of an inch ; and it must be 
borne in mind that though, unlike the carnivora, the teeth of her- 
bivorous animals are always growing, yet they grow with greatly 
increased rapidity during the period assigned for their develop- 
ment. 
The shedding of the incisor teeth is now in accordance with the 
