The centrum of the frontal vertebra is represented by that primitively distinct portion of the sphenoid-bone, termed presphenoid; which, including the processus olivaris, lies anterior to the pituitary fossa, and between the alae minores, or the processes of Ingrassias. Although the presphenoid may, in the full-grown foetus, be separated as a bony tubercle the size of a pea, from the basi-sphenoid behind it, Soemmering describes the two as a single piece: "Basis, sive continuatio partis basilaris occipitis crassissima est, et os ethmoideum tangit."* It is distinct from the processes of Ingrassias, or alisphe-noids, and united to the vomer, or the centrum of the nasal vertebra, by that form of articulation called Schyndylesis. Soon after birth it becomes confluent with the orbito-sphenoids, but it. remains solid until the eighteenth year, when it is hollowed out to form the sphenoidal sinuses, two generally unequal cavities, separated by an irregular, and often imperfect septum. The sphenoidal sinuses are shut in below by two bones of the splanchnic skeleton, the cornua sphenoidalia, which, coalescing with the presphenoid and approaching in the mesial line, seem to give origin to the descending exogenous spine, the rostrum. It is by the cornua sphenoidalia that the union is effected between the sphenoidal sinuses and the posterior division of the olfactory sense-capsule, the lateral masses of the ethmoid bone.
* De corporis humani fabrica, t. i., p. 107, cxx.
From the upper surface of the presphenoid the neurapophysis (processes of Ingrassias, orbito-sphe-noids) take their rise. A median longitudinal ridge is prolonged forwards into a spine, which articulates with the cribriform plate of the aethmoid bone, at the base of the eminence termed crista galli. (On either side of this is a shallow groove, in which is lodged the commissural chords of fibrous and vesicular nervous matter, which connect the thin encephalic olfactory ganglia with the prosencephalon, or the cerebral hemispheres.) At the junction of the orbito-sphenoids with the presphenoid is the foramen opticum, directed forwards and outwards, and transmitting the optic nerve and the ophthalmic artery. The orbito-sphenoids are flattened and of triangular form, the apex directed outwards: the posterior internal angle projects over the pituitary fossa, and is named the anterior clinoid process : the internal carotid artery passes under it, sometimes through a complete ring of bone, as it emerges from the sphenoidal sinus to reach the base of the brain.
The thin anterior border of the orbito-sphenoids articulates with the frontal bone, their proper neural spine: the inferior surface is separated by an interval from the alisphenoids, or the neurapophyses of the preceding vertebra. The cranial walls, therefore, are imperfect in this situation, in the same manner as if the ligamenta subflava were separated from the neurapophyses of any two adjoining trunk vertebrae : the fissure which results, termed sphenoidal, opens into the orbit, and transmits the third, fourth, ophthalmic division of the fifth, and the sixth nerves, filaments of the sympathetic nerve, and the ophthalmic vein : there is occasionally a small branch from the middle meningeal artery.
The neural arch is completed by the os frontis, the primitively divided halves of which are developed from single points of ossification, commencing immediately above the supra-orbital ridge. Meckel correctly described this bone as corresponding to the broad and expanded portion of the occipital bone; but he went too far when he included in his comparison the "condyloid portions of the occiput" (ex-occipitals), which are serially homological with the orbito-sphenoids. In the ethmoidal fissure he recognised the repetition of the neural arches and of the foramen magnum. "We observe in the place corresponding to the large occipital foramen an analogous opening, which, however, is not closed behind, because the frontal bone has no basilar process. Perhaps this last is represented by the body of the sphenoid bone."* The bone, divided into two portions, frontal and orbital, by the supra-orbital ridge, covers the front surface of the highly-developed cerebral hemispheres, which extend forwards and backwards so as to occupy the whole upper part of the vault of the cranium. The smooth glabella separates the prominences, indicating the position of the frontal sinuses, which, formed by the separation of the outer and inner tables, communicate with the anterior ethmoidal cells, through which they open into the middle meatus of the nose. The two halves of the frontal bone, though usually confluent after the first few years, sometimes remain distinct through life, united by serrated suture, as are the two halves forming the parietal spine. The frontal plates form great part of the roof of the orbits : they are separated by a fissure (a continuation of the vertebral foramen of frontal vertebra) occupied by the olfactory sense capsule (ethmoidal cells), shut in by the cribriform lamella. The ethmoid bone, retracted in man under the projecting frontal spine, in connection with the relatively diminished proportion of the face to the head, will be described in conjunction with the nasal vertebra, with which it is intimately associated. The parapophyses, or the post frontals, are known in human anatomy by the name of external orbital processes, and are confluent with the rest of the bone at an early age. In the monkey, or in the sheep, they bring to mind more strongly than in man the distinct out-standing bones in the cranium of the crocodile. They articulate with the malar bone (or the haemapophysial appendage of the nasal vertebra), to assist in the completion of the firm ring of bone which encircles the orbit, and forms the external boundary of that fissure, between the superior maxillary bone (nasal haemapophysis) and the base of the skull, termed spheno-maxillary.
* Meckel's Manual of Descriptive Anatomy.
The frontal pleurapophysis, or rib, is no longer the cylindrical bone observed in the cranium of serpents, or the chain of bones connected with the post frontals, which may be seen in the cranium of the codfish. It is represented by the thin ring of bone, called the external auditory process, which, though confluent in the adult skull, with the assemblage of elements constituting the "temporal bone," is developed from a distinct centre of ossification in the foetus. It is separated, as in Mammalia generally, from its own parapophysis (post-frontal) by the whole length of the temporal fossa : placed in front tof the parietal parapophysis (mastoid), and pleurapophysis (styloid), it surrounds the meatus audi-torius externus, and forms, in conjunction with the horizontal portion of the squamous bone, a concavity, which receives the articular extremity of its own haemapophysis, the mandible, or the inferior maxilla.