Give the boundaries and the foramina of the middle fossa.

In front it is bounded by the posterior margins of the lesser wings of the sphenoid, the anterior clinoid processes, and the anterior margin of the optic groove; behind, by the upper borders of the petrous portions of the temporal bones and dorsum sellæ; externally by the squamous plates of the temporals and anterior inferior angles of the parietals. The foramina are on each side, the optic for the optic nerve and the ophthalmic artery. The sphenoidal fissure transmits the third, fourth, three branches of the ophthalmic division of the fifth and sixth cranial nerves, and filaments of the sympathetic, the ophthalmic veins, branches of the lacrimal and middle meningeal arteries with a process of the dura mater; immediately behind this, the foramen rotundum, for the superior maxillary division of the fifth cranial nerve; more posteriorly, the foramen ovale, for the inferior maxillary division of the fifth nerve, the small meningeal artery, and the small petrosal nerve; between the two, internally, the foramen Vesalii (often absent), for a small vein; piercing the posterior inferior angle of the greater sphenoidal wing, the foramen spinosum, for the middle meningeal artery, meningeal veins, nervus spinosus, a recurrent branch from the inferior maxillary nerve, and the sympathetic branches from the cavernous plexus; on the inner side of the oval foramen the foramen lacerum (medium) (filled in below by a plate of cartilage in the fresh state), for the internal carotid artery, carotid sympathetic plexus, Vidian nerve, and a small meningeal branch from the ascending pharyngeal artery; on the anterior surface of the petrous portion of the temporal is seen the eminence caused by the superior semicircular canal, eminentia arcuata; outside this the roof of the middle ear, tegmen tympani; the groove leading to the hiatus Fallopii; a smaller groove and foramen for the small superficial petrosal nerve; a depression for the Gasserian ganglion; and the orifice of the

carotid canal.

Describe the posterior fossa.

It is deeply concave and formed by the occipital, petrous, and mastoid portions of the temporal, the posterior inferior angle of the parietal, and part of the body of the sphenoid. It presents the meatus auditorius internus for the facial and auditory nerves and the auditory artery; the aquæductus vesti

buli for a small artery and vein; the subarcuate fossa; the foramen lacerum posterius transmitting the glosso-pharyngeal, pneumogastric, and spinal accessory nerves, the inferior petrosal and lateral sinuses, and the meningeal branches of the ascending pharyngeal and occipital arteries; the mastoid foramen for a vein and artery; the posterior condyloid foramen (inconstant) for a vein; the anterior condyloid foramen for the hypoglossal nerve, and a meningeal branch from the ascending pharyngeal artery and in the floor of the fossa in the median line is the foramen magnum, accommodating the medulla oblongata and membranes, the spinal accessory nerves, and the vertebral arteries.

What additional foramina or openings are found at either side of the base of the cranium (Fig. 31)?

The orifice of the Eustachian tube, admitting air to the middle ear. The canal for the tensor tympani muscle above the former, separated from it by the septum tuba, the posterior end of which is the processus cochleariformis. The posterior orifice of the Vidian canal, for the Vidian nerve and vessels. Glaserian fissure for the processus gracilis of the malleus, and the tympanic branch of the internal maxillary artery. The orifice of the canal of Huguier, transmitting the chorda tympani nerve. The canal for Jacobson's nerve, the tympanic branch of the glosso-pharyngeal. The aquæductus cochleæ, for a small artery and vein running to and from the cochlea. The canal for Arnold's nerve, the auricular branch of the pneumogastric. The auricular fissure, for the exit of Arnold's nerve. The stylo-mastoid foramen, for the exit of the facial nerve and the entrance of the stylo-mastoid artery.

The anterior and the posterior foramina of the palatal region have already been sufficiently described, and this region does not properly belong to the base of the cranium; while those opening externally at the base and upon the face have been described sufficiently under the temporal bone and the superior and inferior maxillary bones.

The Orbital Cavities.

Describe them (Figs. 32 and 33).

They are two quadrilateral pyramidal cavities, with bases outward, their long axes directed from the front inward and

backward, in such directions that if prolonged they would meet about the sella turcica of the sphenoid. Seven bones contribute to the formation of each, viz., the frontal', the eth

moid27, the sphenoid 23, the lacrimal 24, the superior maxillary 6, the palate 25, and the malar22; each communicates with the cranial cavity behind by the optic foramen 18 and the sphenoidal fissure 23, with the nasal fossa through the canal of the nasal duct, and below externally by the spheno-maxillary fissure 25 with the temporal, zygomatic, and spheno-maxillary fossa.

What bones compose the roof, and what is its form?

The orbital plate of the frontal anteriorly, the lesser sphenoidal wing behind; it is concave and directed downward and forward, presenting internally a depression 28 for the fibro-cartilaginous pulley of the superior oblique muscle of the eye, and externally the lacrimal fossa1 for the lacrimal gland.

Describe the floor.

Nearly flat, formed chiefly by the orbital plate of the superior maxillary, and to a less extent by the orbital processes of the malar and palate bones; it presents just external to the lacrimal canal a depression for the inferior oblique muscle ; externally, the malo-maxillary suture; near the middle, the infra-orbital groove; and posteriorly, the palato-maxillary

suture.

Name the bones forming, and the points of interest upon the inner wall.

It is flattened, formed by the nasal process of the superior maxillary, the lacrimal", os planum of the ethmoid", and the sphenoidal body. It presents in front the lacrimal groove 20, bounded behind by the lacrimal crest; further back respectively the lacrimo-ethmoidal and ethmo-sphenoidal sutures.

What forms the outer wall?

The orbital plate of the malar and the greater wing of the sphenoid, and on it are seen the openings of one or two malar canals 19 and the spheno-malar suture.

Describe the superior external angle of the orbit.

Posteriorly, the sphenoidal fissure (foramen lacerum anterius) 23 for the entrance of the third, fourth, three branches of the ophthalmic division of the fifth and sixth nerves, and branches of the middle meningeal artery, a process of the dura mater, sympathetic nerve filaments, and the exit of the ophthalmic veins, and a recurrent branch from the lacrimal artery. This angle also shows the fronto-malar and the fronto-sphenoidal sutures.

What points does the superior internal angle present?

The suture between the lacrimal, ethmoid, and frontal bones; between the junction of the two latter bones the anterior ethmoidal foramen for the anterior ethmoidal vessels and nasal nerve, and the posterior ethmoidal foramen" for the posterior ethmoidal vessels.

What points does the inferior external angle present?

The spheno-maxillary fissure 26 transmitting the infra-orbital vessels and superior maxillary nerve, the ascending branches