It contains the medulla, pons, and cerebellum. In front is a median depression (Fossa pontis) for the pons. The surface behind this is concave transversely and slopes gently downward to the foramen magnum; it supports the medulla. On either side are the =foramen lacerum basis cranii= and the =hypoglossal foramen=. The =anterior= or =nasal wall= is formed by the cribriform plate of the ethmoid, which separates the cranium from the nasal cavity. It is perforated by numerous foramina for the passage of the olfactory nerve-bundles. [Illustration: FIG. 36.—MEDIAN SECTION OF SKULL OF HORSE WITHOUT THE MANDIBLE. The septum nasi is removed, but the mucous membrane on the turbinal bones is retained. _a_, _a′_, Superior turbinal bone, dotted line indicating limit between anterior coiled part and posterior uncoiled part; _b_, _b′_, superior turbinal folds, inclosing bars of cartilage; _c_, fold of mucous membrane formed by union of _b_ and _b′_; _d_, _d′_, anterior coiled and posterior uncoiled part of inferior turbinal, dotted line indicating septum between them; _e_, _f_, inferior turbinal folds, former (alar fold) inclosing bar of cartilage; _g_, _h_, _i_, superior, middle, inferior meatus; _o_, _o′_, frontal sinus; partial septum between _o_ and _o′_; _q_, nasal part of frontal sinus (nasal sinus); _r_, lateral mass of ethmoid bone; _s_, sphenoidal sinus; _t_, cranial cavity; _u_, opening made in superior turbinal bone at point where drainage of frontal sinus may be obtained. (After Ellenberger, in Leisering’s Atlas.) ] THE NASAL CAVITY The =nasal cavity= (Cavum nasi) is a longitudinal passage which extends through the upper part of the face. It is divided into right and left halves by a median =septum nasi=. The =lateral walls= are formed by the maxilla, premaxilla, and the perpendicular part of the palate bones. Attached to them are the turbinal bones, which subdivide each nasal fossa into three =meatuses= (Meatus nasi). This wall is crossed obliquely by the =canal= and =groove= for the =naso-lacrimal duct=, and its posterior part is perforated by the =sphenopalatine foramen=. The =superior wall= or =roof= is formed by the frontal and nasal bones. It is concave from side to side, and nearly straight longitudinally, except in the posterior part, where it curves downward. It presents a median elevation, the =nasal crest=. The =inferior wall= or =floor= is formed by the palatine processes of the premaxillæ and maxillæ, and the horizontal parts of the palate bones. It is wider but considerably shorter than the roof. It is concave transversely, and nearly horizontal from before backward, except in the posterior third, where there is a slight declivity. The anterior part presents a median groove for the cartilaginous septum, and a furrow for the organ of Jacobson on either side. On either side of the palatine processes of the premaxillæ is the =palatine cleft=.