The internal structure of the face appears to be very complex, presenting several cavities and divisions which give it at the same time strength and lightness. The arrangement of these parts may be understood by observing, 1. the perpendicular septum formed by the ethmoid and vomer, which divides the upper part of the face into two equal halves; 2. in each half three horizontal divisions, viz. an upper or frontal, which separates the cranium from the orbit ; a middle or maxillary, placed between the orbit and the cavity of the nose, and an inferior or palatine situated between the nose and mouth; 3. three outer divisions, viz. an upper or spheno-jugal, forming the outer wall of the orbit, and separating that cavity from the temporal fossa; a middle, formed by the maxillary tuberosity which separates the cavity of the nose from the spbcno-maxillary and zygomatic fossm; an inferior, formed by the ramus of the jaw ; 4. above and at the centre the ethmoid and lachrymal bones sepa rate the orbits from each other and from the cavities of the nose.
The principal cavities of the face arc the orbits, the nasal fossm, and the mouth ; and with these all the rest are more or less con nected. These cavities will be described under the several articles, OaIIIT, NOSE, Mourn.
.Mechanisms of the J'acc.—The face forms a structure which combines both strength and lightness; the former quality is owing to the arched form of its exterior and to the strong pillars of supports (to be presently described) which connect its different parts to each other and to the cranium. The lightness of the face depends upon the thinness of some of its bones, and the large cavities which it com prises. The two upper maxillary bones form by their alveolar border and palatine arch a strong platform, from which ascend five osseous pillars; one median, formed by the vomer and the perpendicular plate of the ethmoid; two at the sides of the nose, formed by the nasal process of the superior maxilla ; and at the lateral parts of the face two others, formed by the malar processes of the upper jaw and the malar bones. All these pillars connect the upper jaw with the bones of the cranium, and contribute by their form, strength, or extent of articulation to resist or diffuse the concussion of violent blows applied to the face. The strength of the lower jaw depends upon its arched form and upon its mobility, but, from its exposed situation, it is notwithstand ing frequently broken.
Development of the fuce.—The development of the face consists not merely in its general increase, but in the relative proportion of its several parts at different periods of life. As
the face contains the organs of sight, smell, and taste, together with those of mastication, we shall not expect to find it much deve loped in the fcetus and infant while these parts are scarcely called into action ; accord ingly, we observe the vertical diameter of the face (strictly so called) to be very short, which is owing to the slight elevation of the ethmoid, the lachrymal, the upper and the lower maxil lary bones, consequent on the imperfect deve lopment of the nasal cavities, the maxillary sinuses, and the teeth ; see fig. 131. The orbits, indeed, arc remarkably large, but this depends upon the great development of the cranium and the breadth of the orbitar plates of the frontal bones, for in their vertical dia meters the orbits are not remarkable at this period of life.
The transverse diameter of the face in the fcetus is considerable across the orbits, but below these it is narrower in proportion than in the adult. The other chief peculiarities of the festal face are, the small size of the nasal cavi ties, the absence of the canine fosses, depend ing partly on the small vertical diameter of the upper jaw, and partly upon the teeth being still lodged within it; the prominence and shortness of the alveolar borders of both jaws, the vertical direction of the symphysis menti, which even inclines from above downwards and a little backwards; the remarkable con verity of the maxillary tuberosities, owing to the teeth being lodged within them ; and the great obliquity from above downwards and forwards of the posterior apertures of the nose, arising from the smallness of the maxillary sinuses; the small antero-posterior diameter of the palatine arch, which depends upon the same cause; and, finally, the oblique direction of the rami of the lower jaw : see fig. 377, vol. i. p. 742.
In the adult, as the ethmoid and turbinated bones together with the maxillary sinuses become developed, the nasal cavities enlarge, especially in their vertical diameter; above, they communicate with the frontal sinuses, which are now fully formed and projecting; the jaws have become deeper from the protru sion of the teeth, which cause a considerable addition to the vertical diameter of the face; below, the palatine arch has extended back wards with the development of the maxillary sinuses, and the posterior apertures of the nose have become in consequence nearly vertical : the rami of the lower jaw form also nearly a right angle with its body.