The posterior and tipper border of the bone as far down as the posterior extremity of the inferior margin of the fossa temporalis, is arti culated to the parietal bones; and it will be remarked that rather more than the middle third of it advances upon and secures those bones at the expense of their outer table, while the inferior portions of it are in their turn grasped by each parietal bone respectively, the outer table of the latter advancing, at this part, upon the inner table of' the former.
Behind the external angular process, be tween the temporal fossa on the one hand and the orbitar process on the other, there is a. triangular rough surface which is implanted on a similarly-disposed surface of the great wing of the sphenoid bone. The posterior margin of this surface is in apposition with the edge of the thin extremity of the small wing of the sphendid, to which also• is articulated the re maining portion of the posterior border of the orbitar process ; but with this difference, that, while in the former instance the edges are plain and simply applied to each other, in the latter the margins are denticulated, the sphenoid overlapping the frontal so as to render the roof of the orbit secure.
, Thus the frontal bone articulates by the pos terior borders of its two portions, with the parietal and sphenoid ; by the inner edges of its orbitar processes, with the ethmoid ; by its nasal process, with the nasal ; by its internal angular process, with the lachrymal ; by the surface between the nasal and internal angular processes, with the superior maxillary ; and by its external angular process, with the malar bones.
This bone in the fcetus, and for nearly two years after birth, consists of two pieces, the first deposit in each being at the prominence already indicated. From this point the ossific matter radiates, and approaching that from the opposite side, the two combine so as to form on the median line a suture which is speedily effaced. Nevertheless it occasionally happens that complete union does not take place, and then the suture persists through life.
The ethnioid bone (1101h0Eavn,A.coc, cribrum, os ethmoidcum; Germ. Ethmoid a l-knochen ) com pletes that portion of the base of the cranium, anterior to the sphenoid, which -is not supplied by the frontal. It is however devoted less to
the skull titan to the face, with many of the bones of which it is connected ; and it con tributes greatly to form the nostrils and their septum, as well as both of the orbits.
As an element of the cranium it is very simple, being merely a plate connecting the two orbitar processes of the frontal bone, and ba.ving on its median line a ridge, which joins the frontal spine before, to the body of the sphenoid bone behind. This plate is the cri brifbrm plate or process ; it is notched poste riorly where it receives the ethmoidal process of the sphenoid bone, the apex of which pro cess is applied to the posterior extremity of the central ridge. Advancing forwards, this ridge quickly springs upwards as a pyramidal pro cess (the crista galli, or processus cristatus), to which the falx cerebri is attached ; its pos terior edge is long and oblique, its anterior is shorter, more vertical, and it terminates in feriorly in two slightly divergent plate's, so as to form by their articufation with the frontal bone the foranicn ccecunz. On each side of the crista galli, more especially towards the fore part, the cribriform plate is channelled for the reception of the olfactory nerves, and each channel is perforated by numerous foramina for the transmission of the ramifications of the olfactory nerves (foranzina cribrosa). These -openings are variable in their number, and differ from each other in their size and mod-es of termination ; those nearest the crista galli are the largest, and of them one or two of the anterior ones are very considerable; the smallest are situated on the outer edge of the eribriforrn plate, and both of these sets are the orifices of canals which terminate, the former about the root and upon the sides of the septum, the latter on the outer wall of the nose; those which are intermediate and in the centre of the channel, are complete foramina, and open on the opposite surface of the plate. Immediately in front of the inner set of fora mina, there is, between the crista galli and cribriform plate, a fissure which gives passage to the ethmoidal nerve and vessels.