In this article we will discuss about the meaning, parts and surfaces of frontal bone which belongs to human skull.
Meaning and Parts of Frontal Bone:
This is a membrane bone, forms forehead and lodges frontal air sinus, i.e., pneumatic bone.
Three parts are:
1. Squamous part
2. Orbital plates are on either side
3. Nasal part.
I. Squamous Part:
Squamous part has following surfaces – external, internal and right and left temporal surfaces.
Surfaces of Squamous Part of Frontal Bone:
(a) External Surface:
It is smooth and convex. Inferiorly it is limited by nasal notch and supra orbital borders, superiorly limited by coronal suture. Laterally superior temporal lines bound this surface.
i. Frontal Tuberosities:
Situated on either sides of the midline – site of maximum convexity.
ii. Super Ciliary Arches:
Lies above the supra orbital margin.
iii. Supra Orbital Margin:
It separates the squamous part from the orbital plate. It is rounded medially and sharp laterally. It has – supra orbital notch lies between lateral 2/3 and medial 1/3 of the supra orbital border. Through this notch supra orbital nerve and vessels are passing. Sometimes this notch in converted into a foramen – called supra orbital foraman.
It is a smooth elevation in the midline in between two super ciliary arches.
(b) Internal Surface:
It is concave, lodge’s frontal lobe of cerebrum.
i. Median frontal crest is a bony elevation in midline gives attachment to falx cerebri.
ii. Foramen caecum – Lies in between frontal crest and crista galli of ethmoid bone. An emissary vein passes through it from nose to superior sagittal sinus in early childhood, later on it obliterates.
iii. Median sagittal sulcus lodging superior sagittal sinus.
iv. Impression caused by frontal gyri, sulci and meningeal vessels.
v. Pits on either sides of the median sagittal sulcus to lodge the arachnoid granulations and villi of the superior sagittal sinus.
(c) Right and Left Temporal Surfaces:
It forms the anterior part of the temporal fossa. Superior temporal lines separate this surface from the external surface. Temporal fascia and temporalis muscle is attached to the lines.
Posterior- Frontal bone articulates with parietal bones at coronal suture. Junction between two parietal and frontal bone is called Bregma.
Nasion is the junction of two nasal bones with the frontal bone.
Frontal air sinuses- These are situated within the frontal bone separated by a thin septum. Lies deep to glabella and super ciliary arches making the bone pneumatic bone.
2. Orbital Plates:
Lower surface forms the roof of the orbit, while upper surface forms the floor of the anterior cranial fossa.
Medial aspect of the lower surface has a spine called trochlear spine, a fibro cartilaginous ring is attached that forms a pulley for the superior oblique muscle.
The lateral aspect of the orbital surface has a depression called lacrimal fossa – lodges lacrimal gland.
It is a “U” shaped gap present in between the two orbital plates. There are anterior and posterior ethmoidal notches – which are converted into foramina by the ethmoid bone. Through these foramina anterior and posterior ethmoidal nerves and vessels are passing.
Cranial surface is marked by impressions of sulci and gyri of frontal lobe of cerebrum.
Posterior border articulates with the lesser wing of sphenoid bone.
3. Nasal Part:
It is situated below to glabella. It shows a triangular nasal notch, which articulates with the nasal bone, frontal process of maxilla and lacrimal bone.
Nasal spine is a midline projection from the nasal notch. Behind the nasal spine nasal grooves are present, forms the nasal cavity.
It is a midline suture present in the foetal frontal bone. It represents the line of fusion between two halves. Rarely, it may be present in the adult crossing the midline of squamous part of the bone.
Two primary centres appear by 8th week of intrauterine life near the super ciliary arch. They unite at the age of 2-8 years. The secondary centres rarely develop at the nasal part of the bone by 10th year.