in front. Below it presents the pharyngeal spine for the attachment of the superior constrictor of the pharynx, and impressions for the rectus capitis anticus major and minor muscles.

Describe the cerebral surface of the bone (Fig. 6).

It is deeply concave, divided into four fossa by a crucial grooved ridge, crossing at a prominent internal occipital protuberance; the two upper fosse lodge the occipital lobes of the cerebrum, the two lower, the hemispheres of the cerebellum; the two lateral and superior grooves lodge, respectively, between the layers of the tentorium cerebelli and the falx cerebri, the lateral and part of the superior longitudinal sinuses; the inferior groove, or internal occipital crest, has attached the falx cerebelli, and lodges in two grooves, the occipital sinuses. Between the sinuses is the vermiform fossa. Perforating the base of the condyle downward and outward is the canalis hypoglossi, sometimes double, ending in the anterior condylar foramen transmitting the hypoglossal nerve, emissary veins, and a branch of the ascending pharyngeal artery. The upper surfaces of the jugu lar processes are deeply grooved for the lateral sinuses. The upper surface of the basilar process shows the basilar groove for the medulla, and laterally a half groove for the inferior petrosal sinus.

Angles. The superior angle is the apex of the tabular portion; the inferior angle is the end of the basilar process; the lateral angles correspond to the ends of the superior curved lines.

Borders. The superior borders between the upper and lateral angles form the lambdoid suture, occipito-parietal; the upper part of an inferior border extends from the lateral angle to the jugular process, masto-occipital; the lower part articulates with the petrous portion of the temporal, petro-occipital suture, which presents posteriorly the jugular fossa (foramen lacerum posterius).

Attachment of muscles.

Thirteen pairs, as mentioned above.

With what bones does it articulate?

Two parietal, two temporal, sphenoid, and atlas; six in all.

What centers of ossification has it?

Four in the tabular portion; upper pair in membrane (eighth fetal week), one for the basilar, and one for each condyloid portion in cartilage (seventh or eighth fetal week); in four pieces at birth, in the sixth year it forms one bone.

The Sphenoid Bone.

What are its divisions and where is it situated?

A body, two lesser wings, two greater wings, two pterygoid plates, two spinous processes, and six clinoid processes; it is situated at the base of the cranium, articulating with all the other cranial bones, and five or seven of the face. Named from a Greek word meaning wedge; in the process of development this is its function. It also somewhat resembles a bat with extended wings.

Describe the body of the sphenoid.

It is large, cuboidal, hollowed out (in adult) into the sphenoi dal sinuses (Fig. 8). Superiorly, in front, is a sharp projecting

FIG. 7.-Sphenoid bone, superior surface (Gray).

ethmoidal spine for the ethmoid; behind this, a smooth surface with a longitudinal eminence producing two shallow olfactory grooves, bounding which behind is a transverse optic groove (Fig. 7), lodging the commissure of the optic nerves, terminating in

two optic foramina for the optic nerves and ophthalmic arteries. Behind the groove is the projecting olivary process, forming the anterior boundary of a marked depression, the sella turcica, lodging the pituitary gland and circular sinus. On each side, in front, are two small projecting middle clinoid processes, while behind is a square-shaped plate, dorsum sellæ, terminating at the superior angles by the posterior clinoid processes, and to them are attached portions of the tentorium cerebelli. The sides of this bony plate are notched for the sixth pair of nerves, and the superior surface, or dorsum, with a broad, shallow depression, slopes obliquely downward and backward, becoming continuous with the basilar portion of the occipital bone; it supports the pons Varolii. A broad cavernous groove for the carotid artery and cavernous sinus exists on either side of the body. Posteriorly to the outer side of the groove is the lingula, and internally the petrosal process. The posterior rough quadrilateral surface articulates with the basilar portion of the occipital bone.

A vertical plate, the sphenoidal crest (Fig. 8), projects from the anterior surface of the body, forming part of the nasal septum, having the irregular openings of the sphenoidal sinuses on either side (absent in child), which open either into the posterior ethmoidal cells or directly into the nasal cavities. Two thin curved sphenoidal turbinated bones narrow these orifices to a round opening at their upper part; these plates articulate with the palate and ethmoid bones.

The inferior surface presents in the center the rostrum for the vomer, having on either side a thin plate, the vaginal process, articulating with the alæ of the vomer, and the pterygopalatine grooves, which by articulation with the sphenoidal processes of the palate bones are converted into the pterygopalatine canals for the pterygo-palatine vessels and pharyngeal

nerve.

Describe the greater wings.

Two strong processes arising from the sides of the body, curving upward, outward, and backward. The concave cerebral surface of each wing forms part of the middle fossa of the skull, presenting a circular opening at its antero-internal part, the foramen rotundum for the second branch (superior maxillary) of the fifth pair of nerves; a second larger ovoidal foramen,

the foramen ovale (Figs. 7 and 8), transmitting the third (inferior maxillary) division of the same nerve (sometimes the small petrosal nerve) and small meningeal artery, while piercing the sharp, pointed, posterior extremity of the wing, or spinous process, is seen a short canal (sometimes double), the foramen spinosum, for the passage of the middle meningeal artery, its venæ comites and the nervus spinosus, a recurrent branch of the inferior maxillary nerve. Occasionally the foramen Vesalii, on the inner side of the oval foramen, is present, transmitting a small vein.

The external surface presents the infratemporal ridge dividing the temporal from the zygomatic fossa; at its posterior

part the spinous process (Fig. 8) gives origin to the tensor tympani muscle and the internal lateral ligament of the lower jaw. The anterior or orbital surface, smooth, quadrilateral, forming part of the outer orbital wall, is bounded above by a serrated edge for articulation with the frontal, below by a rounded border which enters into the formation of the spheno-maxillary fissure; internally it aids in the formation of the sphenoidal fissure and has a notch at the upper part of the inner border for a branch of the lacrimal artery; a small spine at its lower part, for part of the origin of the external rectus muscle; ex

ternally it presents a serrated articular margin for the malar bone, and (at times) one or more external orbital foramina. The circumference has been described above.

Describe the lesser wings.

These (processes of Ingrassias) two thin, triangular processes arise from the upper and lateral parts of the body, and project transversely outward, to terminate internally in the anterior clinoid processes (Fig. 7), externally in sharp points. By their anterior borders they articulate with the orbital plates of the frontal bone, while by their posterior free margins they divide the anterior from the middle cerebral fossa. Between their roots on each side is the optic foramen for the optic nerve and ophthalmic artery. Each inferior surface forms on either side the back part of the orbital roof, and the upper boundary of the sphenoidal fissure, or foramen lacerum anterius, which is bounded internally by the sphenoid body, and inferiorly by the orbital margin of the greater wing, externally by the frontal, transmitting the third, fourth, ophthalmic branch of the fifth, and sixth nerves, with the ophthalmic veins, filaments from the cavernous sympathetic plexus, also branches of the lacrimal and middle meningeal arteries, with a process of the dura

mater.

Describe the pterygoid processes.

These, consisting of an internal and external plate separated behind by a notch-the pterygoid fossa for the internal pterygoid muscle-descend on each side perpendicularly from the junction of the greater wing with the body. At the base of the internal plate is the scaphoid fossa, giving origin to the tensor palati muscle, above which is the orifice of the Vidian canal. Internal to these is the pterygoid tubercle, below that a sulcus and a processus tubæ for the Eustachian tube, and then a concave edge for the superior constrictor; this plate terminates by a curved hamular process, around which plays the tendon of the tensor palati muscle. The external pterygoid plate has arising from its outer surface the external pterygoid muscle. The triangular notch below has fitting into it the tuberosity of the palate bone.

Name the bones with which the sphenoid articulates.

The other seven cranial bones and five facial, viz., the two