The frontal bone (os frontale) an unpaired, membrane bone, contributes to the formation of the vault of the skull and develops in connective tissue. It is, moreover, associated with the organs of sense (smell and vision). In accordance with this double function, the frontal bone is made up of two parts: a vertical part, squama (squama frontalis) and a horizontal part. Ac­cording to its relation to the organs of vision and smell, the paired orbital part (pars orbitalis) and an unpaired nasal part (pars nasalis) are distinguished in the horizontal part. As a result, the following four parts are distinguished in the frontal bone. 1. The frontal squama (squama frontalis) as any membrane bone, has the shape of a plate, externally convex and internally concave. It ossifies from two ossification points, which are apparent even in an adult as two frontal tubers (tuberas frontalia) on the external surface (jacies externa). They are pronounced only in man due to the development of the brain. They are absent not only in anthropoid apes but also in extinct forms of man. The inferior border of the squama is called the supraorbital border (margo su­praorbital). Approximately at the junction of the medial and middle third of this border is the supraorbital notch (incisura supraorbitalis) (which trans­forms sometimes into a foramen supraorbitale), transmitting the supraorbital arteries and nerve. Eminences, the superciliary arches (arcus superciliares) varying greatly in size and length, are seen immediately above the supraorbit­al border; they are continuous medially on the midline with a more or less prominent area, the glabella, the superior part of the bridge of the nose. The glabella is an important feature in distinguishing the skull of modern man from a fossil skull. The lateral end of the supraorbital border stretches out to form the zygomatic process (processus zygomaticus), which arti­culates with the zygomatic bone. A clearly detectable temporal line (li-nea temporalis) extends upward from the process; this line delimits the temporal surface (jacies temporalis) of the squama. A small groove, sagittal groove (sulcus sinus sagittalis superioris) runs on the midline of the internal surface (jacies interna) from the posterior border and is continuous at the lower end with the frontal crest (crista frontalis). These structures provide attachment for the dura mater. Depressions for the pacchionian granulations (arachnoid villi) are seen near the midline. 2 and 3. The orbital parts (partes orbitales) are two horizontal plates whose inferior concave surface faces the orbit. The superior surface faces the cranial cavity, and the posterior border articulates with the sphenoid bone. The superior cerebral surface bears marks of the brain, namely cerebral ridges of cranium (juga cerebralis) (BNA) (L juga yoke), and digitate impres­sions (impressiones digitatae). The inferior surface (jacies orbitalis) forms the superior orbital wall and bears marks of adjacent accessories of the eye: the lacrimal fossa (fossa glandulae lacrimalis) near the zygomatic process; trochlear fossa (fovea trochlearis) near the supraorbital notch; and trochlear spine (spina trochlearis) where the trochlea for the tendon of one of the muscles of the eye is attached. The orbital parts are separated by the ethmoid notch (incisura ethmoidalis), which in an intact skull is filled by the ethmoid bone (Fig. 54). 4. The nasal part (pars nasalis) occupies the anterior part of the ethmo­id notch on the midline. A projection ending as a sharp process, the nasal spine (spina nasalis), is found here; it helps to make up the nasal septum. On either side of the spine are depressions, which serve as the superior wall for the sinuses of the ethmoid bone. To the front of them is an opening leading into the frontal sinus (sinus frontalis) located in the thickness of the bone to the back of the superciliary arches; the sinus varies greatly in size. The frontal sinus contains air and is separated by the septum of the frontal inus (septum sinuum frontalium). Accessory frontal sinuses are sometimes encountered to the back of or between the main sinuses. Among all the skull bones, the frontal bone is most typical of man. It changed most in the process of evolution. In the earliest hominids (as in the anthropoid apes), it was sloped sharply backward, forming a forehead sloping to the back. Beyond the orbital narrowing it was sharply divided into the squama and the orbital parts. A continuous thick elevation stretched on the edge of the orbit from one zy­gomatic process to the contralateral one. The elevation diminished consider­ably in modern man, and only the superciliary arches remained. In accordance with the development of the brain, the squama straightened out to a ver­tical position. The frontal tubers developed at the same time; as the result the shape of the forehead changed from sloping to convex, lending the skull its characteristic appearance.