Part 13

=Development.=—The sphenoid is ossified in cartilage, and consists in early life of two distinct parts, the presphenoid and postsphenoid. The former develops from two centers, one in each wing; the latter has three centers, one for the body and one for each wing. The pterygoid processes ossify from the centers of the temporal wings.

=Variation.=—The dorsal border of the orbital wing may come to the surface through a defect in the frontal bone at the place where the horn process is situated in animals which have frontal horns.

THE ETHMOID BONE

The =ethmoid= (Os ethmoidale) is situated in front of the body and orbital wings of the sphenoid. It projects forward between the orbital plates of the frontal bones and enters into the formation of the cranial, nasal, and paranasal cavities. It consists of four parts—the cribriform plate, two lateral masses, and the perpendicular plate.

The =cribriform plate= (Lamina cribrosa) is a sieve-like partition between the cranial and nasal cavities. Its margin joins the orbital wings of the sphenoid laterally, and the cranial plate of the frontal bones dorsally. Its =cranial surface= is divided into two parts by a median ridge, the =crista galli=, which is the intracranial portion of the perpendicular plate. Each half forms a deep oval cavity, the =ethmoidal= or =olfactory fossa=, which lodges the olfactory bulb. The plate is perforated by numerous small foramina for the passage of the olfactory nerve filaments, and on either side is the much larger =ethmoidal foramen.= The =nasal surface= is convex, and has the lateral masses attached to it.

The =lateral masses= or =labyrinth= project forward from the cribriform plate into the posterior part of the nasal cavity, which they nearly fill. Each mass is somewhat conical in shape, with the base attached to half of the cribriform plate. The =inner surface= is separated by a narrow space from the perpendicular plate. The =outer surface= is convex and faces chiefly into the frontal and maxillary sinuses, but is attached behind to the inner wall of the orbital cavity; it is covered by a very thin layer of bone, the =lamina papyracea.= The mass consists of a large number of delicate, scroll-like plates of bone, termed =ethmoturbinals= or =ethmoidal cells.= These are attached to the lamina papyracea, and are separated by narrow intervals termed =ethmoidal meatuses=, which communicate with the nasal cavity. In the living animal the ethmoturbinals are covered with mucous membrane.

FIG. 29.—CROSS-SECTION OF LATERAL MASS OF ETHMOID BONE OF HORSE. ]

The lateral mass is a very complex structure, the arrangement of which may be studied on cross-sections of decalcified specimens with the mucous membrane retained. Each mass consists of six turbinals which extend almost to the perpendicular plate and are termed =endoturbinals=. These diminish in size from above downward; the largest is attached to the nasal bone, and is hence usually called the naso-turbinal or superior turbinal; the second is much smaller, and is very commonly termed the great ethmoid cell. Between the endoturbinals are twenty-one small =ecto=turbinals, and all are beset with secondary and tertiary coiled lamellæ.

The =perpendicular plate= or =mesethmoid= (Lamina perpendicularis) is median, and forms the posterior part of the septum nasi. Its =lateral surfaces= are nearly plane, but are marked below by some grooves and ridges; they are covered by the nasal mucous membrane. The =anterior border= is irregular and is continuous with the septal cartilage. The =posterior border= projects into the cranial cavity as a ridge, the =crista galli.= The =dorsal border= joins the frontal bones at their line of junction—the frontal suture. The =ventral border= is received into the groove of the vomer.

=Development.=—The ethmoid develops in cartilage from five centers, two for each lateral mass, and one for the perpendicular plate; from the latter ossification extends into the cribriform plate. At birth the perpendicular and cribriform plates are entirely cartilaginous. By the time ossification is complete the ethmoid has united with surrounding bones to such an extent that it cannot be separated intact for study.

FIG. 30.—SKULL OF HORSE, DORSAL VIEW.

12, Supraorbital process; 14, parietal bone; 14′, external sagittal crest; 15, frontal bone; 15′, frontal crest; 21, lacrimal bone; 22, nasal bone; 26, maxilla; 24, incisor teeth. (After Ellenberger-Baum, Anat. für Künstler.) ]

THE INTERPARIETAL BONE

This bone (Os interparietale) is centrally placed between the squamous part of the occipital and the parietal bones. It is usually described as a single bone, although it ossifies from two chief lateral centers, and is sometimes distinctly paired in skulls of young foals.

The =external surface= (Facies externa) is quadrilateral and is flat and smooth in the very young foal; later it presents the median external sagittal crest.

The =internal surface= (Facies cerebralis) presents the =internal occipital protuberance=, a three-sided process which projects downward and forward into the cranial cavity between the cerebral hemispheres and the cerebellum; it has three concave surfaces and three sharp borders which form part of the tentorium osseum.

The =posterior border= is thick; it joins the squamous part of the occipital bone. The =lateral= and =anterior borders= are united by suture with the parietal bones.

=Development.=—The interparietal ossifies in membrane from two chief lateral centers. It fuses first with the parietals, somewhat later with the occipital, but the period at which this union takes place is quite variable.

THE PARIETAL BONES

The two =parietal bones= (Ossa parietalia) form the greater part of the roof of the cranium; they unite in the median line, forming the =sagittal suture=. Each is quadrilateral in outline and has two surfaces and four borders.

The =external surface= (Facies parietalis) is convex, and is marked by a more or less prominent curved line, the =external sagittal crest=; this is median in its posterior part, and is continuous with the crest of like name on the occipital bone; in front it curves outward and is continuous with the frontal crest. The surface external to the crest (Planum temporale) enters into the formation of the temporal fossa, and is roughened for the attachment of the temporal muscle.

The =internal= or =cerebral surface= (Facies cerebralis) is concave. It presents numerous =digital impressions= (Impressiones digitatæ) which correspond to the gyri of the cerebrum. There are also furrows (Sulci vasculosi) for the meningeal arteries. Along the inner border there is a =sagittal groove= (Sulcus sagittalis) for the superior longitudinal sinus.

The =anterior border= joins the frontal bone at the =parieto-frontal suture= (Sutura coronalis).

The =posterior border= meets the occipital bone at the =parieto-occipital suture= (Sutura lambdoidea). Below this junction it curves inward and concurs with the temporal bone in the formation of the =parieto-temporal canal= (Meatus temporalis). A =transverse groove= (Sulcus transversus) connects this canal with the sagittal sulcus.

The =internal border= is thick and serrated. It joins its fellow at the =sagittal suture=, and (in the young subject) meets the interparietal at the =interparietal suture=. The line of junction is marked internally by the =internal sagittal crest= (Crista sagittalis interna).

The =external border= is beveled and is overlapped by the squamous temporal bone, forming the parieto-temporal suture (Sutura parieto-squamosa). The angle of junction of the external and posterior borders articulates with the posterior angle of the temporal wing of the sphenoid.

=Development.=—Each parietal bone ossifies in membrane from a single center. In the young foal the central part of the bone is much more convex than in the adult and forms a prominence similar to the pronounced tuber parietale of the young child; the external sagittal crest is not present, and the external surface is smooth.

The sagittal suture is usually closed at four years, the parieto-occipital at five years, and the parieto-temporal at twelve to fifteen years.