The lateral surface looks towards the orbitotemporal fossa. It is notched near the caudal end by the ventral border of the optic foramen (_c_). Caudad of this foramen the surface presents an oblique groove which forms in the natural condition the medial boundary of the orbital fissure. Craniad of the optic foramen the surface is smooth and marked near its ventral border by a longitudinal ridge which forms part of the dorsal boundary of a fossa, the =external pterygoid fossa= (Fig. 40, _p_). The caudal end presents ventrally a quadrangular rough surface for articulation with the body of the sphenoid. The cranial end presents the two sphenoidal sinuses separated by a median partition. The median partition articulates by its free border with the lamina perpendicularis of the ethmoid. At its ventral end is the abruptly truncate end of the median ridge of the ventral surface, which is continuous with the ventral cartilaginous portion of the lamina perpendicularis. The lateral walls of the sphenoidal sinuses are continued craniad of the dorsal and ventral walls and of the median partition, and articulate ventrally with the nasal portion of the palatine bones, and dorsally with the orbital plate of the frontal. Between them is received the caudal ends of the labyrinths of the ethmoid in the middle, while between their dorsal edges is received the caudal end of the cribriform plate, and between their ventral edges the expanded end of the vomer. The =wings= (_b_) arise each from nearly the whole of the dorsolateral angle of the body. They form prominent nearly horizontal triangular projections over the optic foramina. The dorsal and ventral surfaces are smooth and continuous respectively with the dorsal and lateral surfaces of the body. The dorsal surface looks into the cranial cavity, while the ventral looks into the orbitotemporal fossa. Craniad of the apex of the wing its border articulates with the ventral edge of the orbital portion of the frontal bone. Caudad of the apex the border articulates with the cranial border of the wing of the sphenoid. =Temporal Bone.= =Os temporale= (Figs. 22 and 23).--This forms a part of the lateral wall of the cranium, filling the gap between the occipital and the sphenoidal segments. It is made up of three portions which are distinct in kittens but somewhat firmly united in adult cats. In lower vertebrates these portions are distinct bones called the =Squamous=, the =Petrous=, and the =Tympanic= bones. In the cat they may be described as the squamous (_a_), petrous (_b_), and tympanic (_c_) portions of the temporal bone. [Illustration: FIG. 22.--TEMPORAL BONE, EXTERNAL SURFACE. FIG. 23.--TEMPORAL BONE, INTERNAL SURFACE.