The =orbital part= (Pars orbitalis) forms the major part of the inner wall of the orbital cavity. It is separated from the frontal part by a prominent ridge which is part of the orbital margin. Its =external= or =orbital surface= is concave and smooth, and presents superiorly a small depression (Fovea trochlearis), which is bridged by a small bar of cartilage, around which the superior oblique muscle of the eye is reflected. The lower border concurs with the orbital wing of the sphenoid in the formation of the ethmoidal or internal orbital foramen. The =internal surface= faces into the frontal sinus and gives attachment to the lateral mass of the ethmoid. The =temporal part= is separated from the orbital part by the deep =sphenoidal notch= (Incisura sphenoidalis), which is occupied by the orbital wing of the sphenoid. Its =external surface= forms part of the inner wall of the temporal fossa. The =internal surface= is largely covered by the orbital wing of the sphenoid in the young subject, but later forms part of the wall of the frontal sinus. The principal connections of the frontal bone are as follows: (1) The inner border joins its fellow at the =frontal suture=. (2) The anterior border meets the nasal and lacrimal at the =naso-frontal= and =fronto-lacrimal sutures=. (3) Laterally it forms the =spheno-frontal= suture with the orbital wing of the sphenoid, and also joins the palate bone and maxilla. (4) Posteriorly it meets the parietal at the =parieto-frontal= (or coronal) =suture=, and articulates below this with the squamous temporal. (5) The extremity of the supraorbital process unites with the zygomatic process of the temporal bone. =Development.=—Each ossifies in membrane from one center which appears in the root of the supraorbital process. In the new-born foal there is a slit between the cranial plate and the orbital and temporal plates which receives the cartilaginous margin of the orbital wing of the sphenoid. THE TEMPORAL BONES The =temporal bone= (Os temporale) forms the greater part of the lateral wall of the cranium. It is situated between the occipital behind, the parietal above, the frontal in front, and the sphenoid below. It consists of two distinct parts, =squamous= and =petrous=. 1. The =squamous temporal= (Squama temporalis) is a shell-like plate which has two surfaces and four borders. The =internal surface= (Facies cerebralis) is largely overlapped by the surrounding bones, but its central part is free and presents digital impressions and vascular grooves.