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ME. G-EOKG-E W. CALLENDEE ON THE FOEMATION AND 
The Inferior Maxilla. 
This bone is developed partly in Meckel’s cartilages, partly in membrane which covers 
them. In the human foetus the cartilages are prolonged to the middle line in the first 
visceral arch, where the two are loosely connected by membrane. These outgrowths 
of cartilage taper as they pass forward, but expand behind the incisor portion of the jaw 
into a lobed extremity (Plate XIII. fig. 6 (2)), the front lower surface of which, unlike 
in this respect the remainder of the cartilage, is not covered by bone in a foetus 1*5 
(Plate XIII. fig- 6 (1)). Ossification commences in a foetus - 9 by deposits along the lower 
middle border of the maxilla on either side, and extends thence in all directions in the 
membrane which covers Meckel’s cartilage, so that the bone is well defined in a foetus 
IT, and is then one-tenth of an inch long. 
Each half of the maxilla may be said to grow from four centres, formed (1) by the 
cartilage which tips the condyloid extremity, (2) by the layer of membrane in front of 
Meckel’s cartilage, (3) by the ossification of the anterior extremity of Meckel’s cartilage, 
(4) by deposits of bone in the perichondrium of the anterior and middle thirds of the 
same cartilage from which is derived the plate of bone which forms the base of the 
dental canal. 
In a foetus 2'3 the outer surface of the bone has a uniformly smooth appearance, 
marked only by the opening for the mental nerve. The ascending ramus rises from the 
horizontal at an obtuse angle. The angle formed by the anterior (coronoid) border 
jmeasures 169° in a foetus 2-3, 156° in a foetus 3-5, 129° in a foetus 8 inches long. The 
angle formed by the posterior (condyloid) border measures 165° in a foetus 2*3, 153° in 
one 3'5, and 144° in a foetus 8 inches long. In a foetus 1'5 the angles formed between 
the rami are scarcely appreciable (Plate XIII. fig. 6 (1)). 
The alveoli are deepened by deposits in the membrane from which the gums are also 
formed ; the canine socket, which is open on its outer side in a foetus 9 inches long, is 
separated by a posterior septum in a foetus 2’3, and from the lateral incisor in one 3‘5, 
and the last-named from the middle incisor in a foetus 3*7, in which also a partition is 
beginning to form between the sets of teeth behind the canine socket. 
Whilst little is to be observed in the growth of the outer surface of either half of the 
inferior maxilla, the inner surface presents for examination, (1) the formation of the 
floor of the dental canal, (2) the changes in the anterior extremity of Meckel’s cartilage. 
In the immediate covering of the anterior half, or thereabouts, of the cartilage of 
Meckel a plate of bone begins to be formed at a very early period. It stands out, in a 
foetus 1-5, from the inner surface of the horizontal ramus, and at first, as it grows, it 
follows the curve of the cartilage beneath, and is consequently convex on its upper 
surface, upon which the dental nerve is situated. This plate, or ridge, extends from the 
future position of the dental foramen, and, in front, turns up to between the canine and 
incisor sockets. Gradually this ridge becomes concave on its outer surface, grows up 
to constitute the boundary of the dental canal and part of the posterior or inner wall 
of the sockets for the teeth, and in a foetus 8 inches long its lower border is recog- 
