The Frontal Bone (os frontale) is a flat bone, located at the anterior and superior part of the skull, consisting of two principal parts, the orbital and frontal, or the vertical and horizontal portions.
The frontal bone presents for study the following—on the external surface; the interfrontal suture—a seam or ridge formed by the
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fusion, in early life, of the two symmetrical halves of the bone; the frontal eminences, on either side of the centers of the horizontal or upper portion, and thot to be the center of ossification; superciliary arches, two curved broad ridges on which are mounted the eyebrows and between the superciliary ridges is an elevation called the glabella or nasal eminence; supraorbital arches, two curved prominent arches which form the upper boundary of the orbit and separate the vertical from the horizontal portion of the bone; the external and internal angular processes (called also the zygomatic processes), two sharp processes of bone at the outer and the inner angle of each arch, the external being the more prominent, and the supraorbital notch, an indentation, on the inner margin of each arch, which is converted into a foramen, the supra-orbital foramen, by a small band of tissue—the
` transverse ligament derived from the muscles covering the anterior portion of the head.
On the orbital portion of the bone are found the orbital cavities, two concave depressions whose walls protect the eyeball, on the outer margin of each cavity is a hollow depression which lodges the lacrimal gland, and, on the inner surface, a small depression for the attachment of the trochlear muscle, the nasal spine is a sharp pointed process which extends outward, from the frontal bone, and articulates with the nasal bones; and the frontal sinuses, two in number, are openings formed where the frontal articulates with the nasal bone, forming the air sinuses which open into the nose.
On the internal surface are the following: a crest, a ridge which gives attachment to the falx cerebri; a sagittal groove, which lodges
the longitudinal sinus, a large vein of the skull; depressions and elevations for convolutions of the brain and for the Pacchionian bodies; also the foramen caecum, a blind opening, found between the frontal bone and the ethmoid, where the crest of the frontal terminates.
This foramen, when present, transmits a small vein from the mucous membrane of the nose to the superior longitudinal sinus.
The frontal bone articulates with the two malar, two parietal, sphenoid, ethmoid, two nasal, two superior maxillary, and the two lacrimal bones. Muscles attached to the frontal bone are: the temporal, corrugator supercilii, the orbicularis oculi, and the occipito-frontalis.
The frontal bone, aside from forming the forehead, helps in the formation of the orbits. Each orbit consists of 7 bones, and the two orbits of 11 bones, namely: the frontal, sphenoid, ethmoid, two lacri-mal, two superior maxillary; two palatal, and two malar.
THE TEMPORAL BONE (Illustrations Nos. 10, 11 and 12.)
The Temporal Bones—(os temporale) two in number, are located at the sides and lower part of the skull. They are called
“temporal” because it was thot that the hair turned gray first at that place. Each bone presents for study three descriptive parts—squam-ous, scale-like, mastoid, nipple-like, and the petrparts—squam-ous, stony part. The squamous is the thinner part, scale-like in form, and made up of an external and an internal layer, having little connective tissue within.
The mastoid part is nipple-shaped, usually rough, and is sometimes perforated with many openings on its outer surface. The petrous por-tion is the hard, dense, ivory-like part containing the greater part of the middle and inner ear.
Other points for study on the bone are the following: a temporal ridge on the squamous portion, for the temporal artery, also for the temporal muscle; the zygoma, a broad arch-shaped piece of bone, which projects anteriorly to articulate with the zygomatic process of the malar: the glenoid fissure, a groove underneath the zygoma;
glenoid fossa, a depression also beneath the zygoma, for the articu-lation of a process known as the condyle of the mandible; the styloid and vaginal process, sharp and pen-like in form, each usually about 1/2 to 1 inch in length and giving attachment to muscles and ligaments; the external auditory meatus, a canal seen on the external surface, partly in the squamous and partly in the mastoid portions of the bone, leading to the middle ear; digastric groove, a deep groove on the posterior surface of the mastoid process, for the attachment of the posterior belly of the digastric muscle; occipital groove, below the digastric groove for the occipital artery; tympanic or mastoid antrum,
a recess in the mastoid process, communicating with the tympanum;
mastoid cells, air cells, which extend from the antrum into the mastoid process in a downward and backward direction, they also extend forward over the roof of the external auditory meatus and inward for a certain distance into the petrous portion; and the supra-mental spine is a sharp scale of bone which gives attachment to a part of the cartilage of the external ear.
On the petrous portion is found the opening of the carotid canal, for the internal carotid artery and plexus, the internal auditory meatus, which transmits the 7th and 8th pairs of cranial nerves and the auditory artery; the jugular fossa, a depression for the sinus of the
internal jugular vein, and which, when joined with the occipital bone, forms the jugular foramen; subarcuate fossa, a fossa in the posterior inner surface beneath the protrusion of the semicircular canal; an arcuate eminence, an eminence on the upper surface marking the location of the superior semicircular canal; and the facial canal or aqueduct of Fallopius, a canal for the facial nerve in the petrous portion. Numerous other openings, foramina, depressions, and eminences are found on this bone. The temporal bone articulates with the occipital, the parietal, the sphenoid, the malar and the mandible.
Muscles attached to the temporal bone are the following: the temporal, masseter, epicranius, sterno-mastoid, splenius capitis, trachelo-mastoid, digastric, retrahens aurem, tensor tympani, levator palati, stapedius, stylo-glossus stylo-hyoid and the stylo-pharyngeus.
THE SPHENOID BONE (Illustrations Nos. 13, 14 and 15.)
The Sphenoid Bone (os sphenoidale) is an irregular bone, wedge-shaped, and resembling a bat with wings extended. It is
located at the base of the skull, anterior to the occipital, posterior to the frontal and the ethmoid, with the temporal on either side. It enters into formation of the cranial, orbital and nasal cavities. For descriptive parts it has a body, two greater wings, two lesser wings and two pterygoid processes or plates.
The body is placed in the median line, cuboid in shape, hollowed out in the interior and containing cavities called the spheroidal sin-uses, or spheroidal cells. It presents on its upper surface an ethmoidal spine articulating with the ethmoid; a vaginal process, a process of bone on the inferior surface of the body running inward from the base of the pterygoid plate, it articulates with the ala of the vomer, and on
the inferior surface of this process is a groove, which is converted by a process of bone on the palate bone, into a canal, called the palatine canal for the passage of the pharyngeal nerve and pterygo-palatine artery, the olive, a smooth oval-shaped eminence; an optic groove for the optic nerve, and the sella turcica (turkish saddle), a depression which lodges a gland known as the pituitary body. The posterior portion of the sella turcica, called the dorsum sellae, is somewhat curved and forms a complete septum. Four sharp processes, the clinoid processes, two anterior and two posterior, are found
at the front and back part, or wall, of the sella turcica; the clivus (a slope) is a bony surface, which slopes down from the sella turcica; this is sometimes called the basilar process of the sphenoid, it unites
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with the basilar process of the occipital bone, and lodges a band of fibers of the brain, called the pons Varolii.
On either side of the body are the following openings:—optic foramen, transmits the optic nerve, and the ophthalmic artery;
spheroidal or superior orbital fissure, or foramen lacerum anterius, an elongated opening, transmits the 3rd and 4th branches of the 5th and 6th cranial nerves, also branches of arteries, foramen rotundum, transmits the maxillary division of the 5th cranial nerve; foramen ovale, transmits the mandibular division of the 5th nerve and a small artery; foramen Versalii, transmits a small vein; the foramen spinosum transmits an artery, a vein, and numerous filaments of nerves; and the pterygoid or vidian canal for the vidian artery and nerve.
The sphenoid bone has two greater wings, also called the temporal wings. These are two broad, wing-like processes of bone, extending from the sides of the body, curving upward, outward and backward. A portion of each wing receives some of the convolutions of the cerebrum. They are rough on either side. The two lesser or orbital wings are thin, triangular processes of bone extending from
the upper and lateral portions of the body. They project transversely outward, and their two roots encircle the optic foramen. The two pterygoid processes or least wings are small pieces of bone, which extend downward from the lateral portions of the body of the sphe-noid. They terminate in two hook-like processes known as the hamu-lar processes.
On the anterior portion are two processes called the lingulae, two ridges of bone, found in between the body and the greater wings, to which are attached small tendons and ligaments. There are two grooved pieces of bone which contain air sinuses, lined with mucous membrane, and these communicate with the nasal fossae. A ridge, or crest, known as the ethmoidal crest, or spheroidal crest, articulates
with the cribriform plate of the ethmoid bone, and assists in forming the nasal septum. The sphenoid bone articulates with all of the cranial bones and with five of the facial bones, the two malar, two palatal and the vomer. To the sphenoid bone are attached the six pairs of the orbital muscles, the temporal, external and internal pterygoid, the muscles of mastication, the superior constrictor of the pharynx, the tensor palati, and the laxator tympani.
THE ETHMOID BONE (Illustrations Nos. 16. 17 and 18.)
The Ethmoid Bone (os ethmoidale) is so named from its form and shape, ethmoid meaning “sieve-like.” It is an irregular bone lying anterior and superior to the sphenoid, between the orbits, and it helps in the bone formation of the orbits, and of the nasal fossae. The parts of which the bone is composed are the two lateral masses, or labyrinths, the perpendicular plates and the horizontal plates.
The two lateral masses are formed of curved, hollowed-out thin bone, and their cavities contain the ethmoidal cells. Their orbital plates help to form the inner wall of the orbits, and they are notched
above to form, with the frontal bone, the two ethmoidal foramina. The superior and middle turbinated processes, large curved pieces of bone, bound the superior meatus of the nose, and are considered by some as detached pieces of the turbinated bones. On the outer side of each lateral mass is a thin, smooth, oblong plate of bone, called the os plenum or orbital plate, it forms the principal part of the inner wall of the orbit. Immediately beneath this plate of bone, is an irregular hook-like layer of bone called the unciform or uncinate process, which closes the upper part of the orifice of the antrum.
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The perpendicular plate is a vertical plate, running from before backward, and forms part of the septum of the nose. It is usually inclined to one side, and projects into the cranial cavity as the crista galli. The perpendicular plate is grooved for filaments of the olfactory
nerves, and has attached to it the cartilage of the nose. By the articu-lation of the crista galli with the frontal bone, it gives formation to the foramen caecum, a sinus communicating with the nasal fossae.
The crista galli is so named from its fancied resemblance to a cock’s comb. It gives attachment to a process of the dura mater, called
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the falx cerebri, on a level between the orbits. The cribriform plate lies horizontally at the sides of the crista galli. It is concave for the olfactory bulbs, and is perforated for the olfactory nerves and some small vessels. The olfactory nerves are fifteen or twenty in number on each side, and small perforations of the ethmoid bone form the openings thru which they pass from the mucous membrane of the nose.
The ethmoid articulates with thirteen bones of the skull, this number includes two cranial bones, the sphenoid and frontal, and eleven bones of the face; the two nasal, two palatal, two superior maxillary, two lacrimal, two turbinated and the vomer. There are no muscles attached to the ethmoid.