The =basilar part= or =process= (Pars basilaris) is a strong, somewhat prismatic bar which extends forward from the ventral margin of the foramen magnum. It is much narrower in front than behind. The ventral surface is rounded. The cranial surface is concave and smooth; its posterior part, supports the medulla, and its anterior part has a shallow cavity on which the pons rests. The lateral borders are thin and sharp, and form the inner margin of the =foramen lacerum= (Foramen lacerum et jugulare). The anterior end has, in the young subject, a semicircular, flat, pitted surface which is attached to the body of the sphenoid bone by a layer of cartilage; in the adult there is complete fusion. On the ventral aspect of the junction are tubercles for the attachment of the ventral straight muscles of the head. [Illustration: FIG. 28.—VENTRAL SURFACE OF SKULL OF HORSE, POSTERIOR HALF WITHOUT MANDIBLE. The skull is inclined slightly. (Notation on key Fig. 27.) ] The =squamous part= (Squama occipitalis) is the somewhat quadrilateral mass situated above the lateral portions, from which it remains distinct till the second year. The =outer surface= is crossed by a very prominent ridge, the =occipital crest=; the middle part of this is thick, transverse in direction, and forms the highest point of the skull when the head is in the ordinary position; laterally it becomes thinner and runs downward and forward to join the temporal crest.[7] The crest divides the surface into two very unequal parts; the small anterior area (Planum parietale) presents a median ridge which is the posterior part of the external sagittal crest; the large area below the crest (Planum nuchale) also has a central eminence, the =external occipital protuberance=, on the sides of which the funicular part of the ligamentum nuchæ is attached. The =internal surface= is concave and presents a deep central depression and two shallower lateral ones which adapt it to the surface of the cerebellum. The occipital bone is connected by suture with the interparietal, two parietals, and two temporals, and by synchondrosis with the sphenoid; the condyles articulate with the atlas. =Development.=—The occipital ossifies in cartilage from =four centers=, and consists at birth of four pieces as described above.[8] The lateral parts unite with the basilar at three to four months, and with the squama in the second year, when the bone is consolidated. The parieto-occipital suture and the spheno-occipital synchondrosis are obliterated about the fifth year usually. The temporo-occipital suture partially ossifies in old subjects. THE SPHENOID BONE The =sphenoid bone= (Os sphenoidale) is situated in the base of the cranium, its central part lying in front of the basilar part of the occipital. It consists of a central part, the body, two pairs of wings, and two pterygoid processes.