Part 14

THE FRONTAL BONES

The =frontal bones= (Ossa frontalia) are situated on the limits of the cranium and face, between the parietals behind and the nasal bones in front. Each is irregularly quadrilateral, and consists of frontal, orbital, and temporal parts.

The =frontal part= (Pars naso-frontalis) forms the basis of the forehead. Its =external surface= (Facies frontalis) is nearly flat, and is smooth and subcutaneous; it is separated from the temporal part by the external frontal crest (Crista frontalis externa). At the junction with the orbital part the =supraorbital= or =zygomatic process= (Proc. zygomaticus) curves outward and downward to join the zygomatic arch. The process partially separates the orbit from the temporal fossa; its root is perforated by the =supraorbital foramen=, or presents instead a notch on its anterior border; its upper surface is convex, while the lower or orbital surface is concave and smooth, forming a shallow fossa for the lacrimal gland (Fossa glandulæ lacrimalis). The =internal surface= enters into the formation of the cranial cavity and the frontal sinus. The cranial surface presents digital impressions for the cerebral gyri. The two plates of the bone separate and diverge in front, and thus inclose a large air-space which is part of the =frontal sinus=. The cranial plate curves downward and articulates with the cribriform plate of the ethmoid bone; the facial plate extends forward and joins the nasal and lacrimal bones.

The =orbital part= (Pars orbitalis) forms the major part of the inner wall of the orbital cavity. It is separated from the frontal part by a prominent ridge which is part of the orbital margin. Its =external= or =orbital surface= is concave and smooth, and presents superiorly a small depression (Fovea trochlearis), which is bridged by a small bar of cartilage, around which the superior oblique muscle of the eye is reflected. The lower border concurs with the orbital wing of the sphenoid in the formation of the ethmoidal or internal orbital foramen. The =internal surface= faces into the frontal sinus and gives attachment to the lateral mass of the ethmoid.

The =temporal part= is separated from the orbital part by the deep =sphenoidal notch= (Incisura sphenoidalis), which is occupied by the orbital wing of the sphenoid. Its =external surface= forms part of the inner wall of the temporal fossa. The =internal surface= is largely covered by the orbital wing of the sphenoid in the young subject, but later forms part of the wall of the frontal sinus.

The principal connections of the frontal bone are as follows: (1) The inner border joins its fellow at the =frontal suture=. (2) The anterior border meets the nasal and lacrimal at the =naso-frontal= and =fronto-lacrimal sutures=. (3) Laterally it forms the =spheno-frontal= suture with the orbital wing of the sphenoid, and also joins the palate bone and maxilla. (4) Posteriorly it meets the parietal at the =parieto-frontal= (or coronal) =suture=, and articulates below this with the squamous temporal. (5) The extremity of the supraorbital process unites with the zygomatic process of the temporal bone.

=Development.=—Each ossifies in membrane from one center which appears in the root of the supraorbital process. In the new-born foal there is a slit between the cranial plate and the orbital and temporal plates which receives the cartilaginous margin of the orbital wing of the sphenoid.

THE TEMPORAL BONES

The =temporal bone= (Os temporale) forms the greater part of the lateral wall of the cranium. It is situated between the occipital behind, the parietal above, the frontal in front, and the sphenoid below. It consists of two distinct parts, =squamous= and =petrous=.

1. The =squamous temporal= (Squama temporalis) is a shell-like plate which has two surfaces and four borders.

The =internal surface= (Facies cerebralis) is largely overlapped by the surrounding bones, but its central part is free and presents digital impressions and vascular grooves.

The =external surface= (Facies temporalis) is convex, and enters into the formation of the temporal fossa. From its lower part there springs the =zygomatic process= (Processus zygomaticus), which forms the external boundary of the temporal fossa. It is at first directed outward, and is wide and flattened dorso-ventrally. It then turns forward, becomes narrower, and is twisted so that its surfaces are internal and external. Its anterior end is pointed and joins the zygomatic process of the malar bone, with which it forms the =zygomatic arch= (Arcus zygomaticus). The narrow anterior part has a convex outer surface and a concave inner one. Its upper border has a rough area for articulation with the supraorbital process of the frontal. Its lower border is wide and rough. The wide posterior part presents on its ventral face a surface for articulation with the condyle of the mandible. This surface consists of a transversely elongated =condyle= (Tuberculum articulare), behind which is the =glenoid fossa= (Fossa mandibularis). The fossa is limited behind by the =postglenoid process=, the anterior surface of which is articular. Behind this process is a fossa in which is the external opening of the parieto-temporal canal. The dorsal surface is concave and forms the outer boundary of the temporal fossa. The superior border is sinuous and is continuous behind with the temporal crest.

The =posterior process= (Processus posterior) springs from the posterior part of the squama. Its external surface is crossed by the temporal crest, which forms here the outer limit of the temporal fossa. The internal surface forms the outer boundary of the parieto-temporal canal, and is elsewhere applied to the petrous portion. It divides into two branches, upper and lower; the upper branch unites with the occipital bone, while the lower one curves downward behind the external auditory process and overlaps the mastoid process.

The =superior border= of the squamous temporal articulates with the parietal, forming the =parieto-temporal suture=. The =inferior border= joins the temporal wing of the sphenoid at the =spheno-squamous suture=. The =anterior border= unites with the frontal bone, and the =posterior= with the parietal.

2. The =petrous temporal= (Os petrosum) is placed between the occipital behind and the parietal in front, and is largely overlapped externally by the squamous temporal. It has the form of a four-sided pyramid, the base of which is ventral.

The =external surface= is mainly concealed by the squamous temporal, but two features are visible. A short tube of bone, the =external auditory process=, protrudes from the lowest part through the notch of the squamous temporal. The process is directed outward, upward, and a little forward. It gives attachment to the annular cartilage of the ear. Its lumen, the =external auditory meatus= (Meatus acusticus externus), conducts to the cavity of the middle ear (tympanum) in the dry skull, but is separated from it by the tympanic membrane in the natural state. The =mastoid process= projects ventrally in the interval between the posterior process of the squamous temporal and the root of the paramastoid (or styloid) process of the occipital bone; its outer surface is crossed by a curved groove which leads to the parieto-temporal canal.

The =internal surface= faces into the cerebellar fossa of the cranium. It is concave and smooth but irregular. In its lower part is the entrance to a short canal, the =internal auditory meatus=, which transmits the seventh and eighth cranial nerves.

The fundus of the meatus is divided by a crest into a superior and an inferior fossa. In the superior one is the origin of the =facial canal=, which curves through the bone and opens externally at the stylo-mastoid foramen; it transmits the facial (seventh cranial) nerve. The inferior fossa presents small foramina for the passage of fibers of the auditory (eighth cranial) nerve.

Behind the meatus and near the posterior margin of the surface is the slit-like opening of the aquæductus vestibuli, covered by a scale of bone. Below this is a narrow fissure, the orifice of the aquæductus cochleæ.

The =anterior surface= looks upward and forward. The outer part articulates with the parietal bone and the inner part faces into the cerebral fossa of the cranium. A sharp border, the =petrosal crest= (Crista petrosa), separates this surface from the inner one.

The =posterior surface= joins the lateral part of the occipital bone.

The =base forms= the outer boundary of the foramen lacerum basis cranii. It is very irregular and presents a number of important features. The =hyoid process= is a short rod which projects downward and forward below the base of the external auditory process, inclosed in a bony tube; it is connected by a bar of cartilage with the hyoid bone. The =stylo-mastoid foramen= is situated between the root of the hyoid process and the mastoid process; it is the external opening of the facial canal, through which the facial nerve emerges. The =bulla ossea= is a considerable eminence situated centrally; it is thin-walled and incloses a cavity which is part of the tympanum. The =muscular process= is a sharp spine which projects downward and forward from the anterior part of the base; it gives origin to the tensor and levator palati muscles. External to the root of the preceding is the small =petro-tympanic= or =Glaserian fissure= (Fissura petro-tympanica) for the passage of the chorda tympani nerve. The osseous =Eustachian tube= is a semicanal at the inner side of the root of the muscular process; it leads to the tympanum. At the inner side of the preceding is the slit-like orifice of the petrosal canal, which communicates with the facial canal.

The =apex= projects upward and backward between the squamous temporal and the occipital bone.

=Development.=—The petrous temporal may be regarded as consisting of petro-mastoid and tympanic parts. The latter includes the external auditory process, the bulla ossea, and the muscular process; it is developed in membrane. The petro-mastoid is developed in the cartilaginous ear capsule. Its petrous part consists of very dense bone which contains the labyrinth or internal ear and forms the inner wall of the tympanum.

The =parieto-temporal canal= (Meatus temporalis) is a continuation of the transverse groove which extends outward from the base of the tentorium osseum. It is directed downward, forward, and somewhat outward, and opens externally in front of the root of the auditory process. It is bounded by the squamous temporal externally, the petrous behind, and the parietal in front and internally. Several foramina open from it into the temporal fossa. It contains a large vein (Vena cerebralis dorsalis), the continuation of the transverse sinus of the dura mater.

The =foramen lacerum basis cranii= (Foramen lacerum et jugulare) is a large irregular opening in the cranial base, bounded internally by the basilar part of the occipital bone, externally by the petrous temporal, and in front by the temporal wing of the sphenoid. It consists of a large anterior part (Foramen lacerum anterius), and a narrow posterior part (Foramen lacerum posterius s. jugulare). It transmits the internal carotid artery, the middle meningeal artery, the mandibular, ninth, tenth, and eleventh cranial nerves, and the inferior cerebral vein.

In the fresh state the foramen is occupied by a dense fibrous membrane which is perforated by apertures for the various structures transmitted. Thus there are three openings in front for the internal carotid artery, the mandibular nerve, and the middle meningeal artery; these are (from within outward) the foramen caroticum, ovale, spinosum.

(B) BONES OF THE FACE