Frontal bone – Os frontale
Bones
Originally a paired bone of the cranial vault which develops by intramembranous ossification, the frontal bone is an unpaired pneumatised bone and contains a paranasal sinus: the frontal sinus. The frontal bone consists of a squamous part, a nasal part and two paired orbital parts.
The frontal foramen and the supraorbital foramen (foramina frontalis et supraorbitalis) are variable openings which can replace the frontal and supraorbital notch (incisura frontalis et supraorbitalis).
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• 1 Squamous part (squama frontalis) External surface (facies externa) • 1.1 Frontal tuber (tuber frontale) – paired large elevations containing ossification centers • 1.2 Superciliary arch (arcus superciliaris) – paired arches located above the orbits • 1.3 Glabella – a flattened area above the nasal root and between the superciliary arches • 1.4 Supra-orbital margin (margo supraorbitalis) – the superior border of the orbit • 1.4.1 Frontal notch/foramen (incisura/foramen frontale)
– a notch or foramen above the orbit that is only occasionally present – transmits the medial branch of the supra-orbital nerve 1.4.2 Supra-orbital notch/foramen (incisura/foramen supraorbitalis) – a notch or foramen above the orbit that is always present 1.7 – contains the supra-orbital vessels and the 1.6 1.1 lateral branch of the supra-orbital nerve 1 1.5 Zygomatic process (processus zygomaticus) – connects to the zygomatic bone 1.6 Temporal line (linea temporalis) 1.2 3 – the upper border of the origin 1.5 of the temporalis muscle – continues on the parietal bone 2 1.7 Parietal margin (margo parietalis) – connects with the parietal bone to form the coronal suture Internal surface (facies interna) Lateral view of the skull 1.8 Groove for superior sagittal sinus 1.1 (sulcus sinus sagittalis superioris) 1.7 – contains the superior sagittal sinus 1.2 1 1.9 Frontal crest (crista frontalis) 1.4.2 1.5 – the site of attachment of the falx cerebri 2 2.1 – the anterior continuation of the groove for the superior sagittal sinus 2.2 1.4.1 2 Orbital part (pars orbitalis) 1.3 2.1 Lacrimal fossa (fossa glandulae lacrimalis) 2.4 – a fossa on the lateral part of the roof of the orbit for the lacrimal gland 2.2 Trochlear fovea (fovea trochlearis) Anterior view of the skull – fossa containing the trochlear spine 4 2.5 2.3 Trochlear spine (spina trochlearis) 2 – the attachment of the trochlea 1 of the superior oblique muscle 2.4 Anterior and posterior ethmoidal foramina 2.6 (foramen ethmoidale anterius et posterius) – formed by connection with the ethmoidal bone – transmit the anterior and posterior ethmoidal artery, vein and nerve 2.5 Ethmoidal notch (incisura ethmoidalis) – a notch for the ethmoidal bone 2.6 Sphenoidal margin (margo sphenoidalis) 3 Nasal part (pars nasalis) Internal surface of the cranial base – a small unpaired part of the bone located at the nasal root where the nasal spine and nasal margin are found 1.7 4 Frontal sinus (sinus frontalis) – a paired paranasal sinus 1.8 – develops in children after the first year of life 1.9 – the full volume is reached at the end of the growth period 1 – opens into the middle nasal meatus 2.5 2 2.6 4.1 Septum of the frontal sinuses (septum sinuum frontalium) Posterior view of the frontal bone – separates the left and right frontal sinus 4.2 Opening of the frontal sinus (apertura sinus frontalis) – the entrance to the frontal sinus
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Gyral impressions (impressions gyrarum) are impressions of the cerebral gyri on the inner surface of the frontal bone. The cerebral juga are eminences between these impressions. The spheno-occipital synchondrosis ossifies at the age of eighteen. The transverse occipital suture lies in the upper part of the occipital squama and marks the boundary between the upper part of the occipital bone, which develops by intramembranous ossification, and the lower part of the occipital bone, which develops by endochondral ossification. Failure of fusion the transverse occipital suture (sutura occipitalis transversa) gives rise to the interparietal bones (Inca bones). These bones occur in 30 % of the Native American population but only in 5 % of the European population. The sutura mendosa is formed after incomplete fusion of the squamous part of the occipital bone. It projects medially or vertically from the lambdoid suture and is variable in length. The trochlear spine is a cartilaginous pulley in the trochlear fovea which is ossified in less than 15 % of population. The orbitocranial canal connects the orbit to the anterior cranial fossa. It opens into the anterior ethmoidal foramen. The orbitoethmoidal canal connects the orbit to the ethmoidal air cells. It opens into the posterior ethmoidal foramen. The internal occipital crest (crista occipitalis interna) is a vertical osseous crest. Sometimes it is replaced by the groove for the occipital sinus. The foramen caecum of the frontal bone is a small hole on the cranial surface of the squama of the frontal bone. It represents a remnant of a foetal foramen and contains an emissary vein, which persists after birth in about 1 % of the population.