The external sur'face of the fr,ontal portion in its upper two-thirds is smooth, of an equa ble convexity and directed backwards ; its inferior third is more vertical, and its convexity is interrupted by. prominences. On the me dian line it exhibits evidence of its original division into two parts, and this generally by a slight ridge, although in some instances there is a linear depression of equal indistinctness. This line is terminated by the nasal prominence, which has immediately above it a smooth tri angular surface (glabella), and below it a rough notch for the articulation of the nasal and superior maxillary bones. From the centre of this notch there is a projection ( pro cessus nasalis), on the fore part of which are fixed the nasal bones, and to its back part, which is grooved, the ethmoid bone is ap plied.
On either side of the median line there is, at about the distance of an inch where the middle joins with the lowest third of the bone, the frontal eminence (eminentia.frontalis, pro cessus primi genii), which marks the centre of ossification, and the prominence of which is inversely as to the age of the subject. Be low this eminence, bounding the glabella, and inclining downwards and inwards towards the nasal prominence (with which, in fact, it is ultimately confounded), is a pyramidal protu bemnce, varying very much in distinctness in The posterior or cerebral surface of the frontal bone is concave, is marked by depres sions which correspond with the convolutions of the brain, and by sulci for the lodgement of the arteries of the dura mater, and is conti nuous inferiorly with the orbitar portion ; cor responding to the eminentim frontales there are two depressions, and on the median line there is a sulcus (sukus longitudinalis) for the re ception of the longitudinal sinus, on the edges of which sulcus may sometimes be seen the fossm Pacchionii for the glands of the same name. This sulcus as it descends is generally replaced by a dense crest, which projects con siderably into the cavity of the cranium ; to it and to the edges of the sulcus, the falx cerebri is attached ; and at its lowest point it is bifid, so that, by its being applied against a similar bifurcation of the processus cristatus of the ethmoid bone, it contributes to form the fiwa men ca'cum.
The orbitar portion by its upper surface supports the anterior lobes of the brain, and its under surface forms the root of the orbits.
It is divided into two processes by a longitu dinal notch, which corresponds to the roof of the nose.
The orbitar process of either side is convex in both directions on its upper sill:face, and the mammillary eminences and digital im pressions formed by the intergyral spaces and convolutions of the brain are of a decided character. On its under surface it is concave and triangular, the base being directed for wards; at its anterior and outer part there is a. fossa ( fossa lachrymalis) for the lachrymal gland, and which is overhung by the external orbital. process; at its anterior and inner part, near to the internal orbitar process, and be tween it and the foramen supra-orbitarium, there is a. small pit (pssa trochlearis) to which is fixed the cartilaginous pulley in which plays the tendon of the superior oblique muscle of the eye; at the middle of its inner edge there is a notch, which, applied to a similar notch of the ethmoid bone, constitutes the foranien orbitarium internunt anticum, through which pass the ethmoidal twig of the ophthalmic branch of the fifth pair of nerves, and the an terior ethmoidal branch of the ophthalmic artery; and a little behind this there is another notch, which by a like contrivance forms a hole (the foramen orbitarium internum posticum) for the passage of the posterior ethmoidal branch of the ophthalmic artery and corres ponding vein.
The notch which is between the orbitar pro cesses is the hiatus ethmoidalis (incisura ethmoidalis), and in the cranium it is filled up by the eribriform plate of the ethmoid bone. Its longitudinal is twice the length of its trans-. verse diameter; anteriorly, it is bounded by the notch which, in part, forms the foramen ccecum and the posterior surface of the nasal process; posteriorly, it is open ; and its sides are bounded by the commutual edges of the orbitar processes, the tables of which are sepa rated in such a manner as to communicate with the ethmoidal cells and close them at the upper part, and at the anterior part of the notch to communicate also with the frontal sinuses.
The frontal:sinus is formed by the separation of the two tables of which the bone is com posed, and by the absorption of the diploe ; they are usually separated by a septum, and they communicate .on each side with the mid dle meatus of the nose in the manner indi cated above.