Its base is formed by the =presphenoid= (fig. 75, 12), a very deep unpaired bone, narrow and compressed ventrally, and with an irregular dorsal surface. The presphenoid is continuous with a second pair of wing-like bones, the =orbitosphenoids=. Each orbitosphenoid meets the alisphenoid behind, but the relations of the parts in this region are somewhat obscured by a number of large foramina piercing the bones, and also by an irregular vacuity, the =foramen lacerum anterius= or =sphenoidal fissure=, which lies between the orbitosphenoid and alisphenoid, separating the lateral parts of the parietal and frontal segments, in the same way as the space occupied by the auditory bones separates the lateral parts of the occipital and parietal segments. The orbitosphenoids pass obliquely forwards and upwards, and are united above with a second pair of large membrane bones, the _frontals_ (fig. 73, 3). The outer side of each frontal is drawn out into a rather prominent rounded =postorbital process= (fig. 73, 10), from which a ridge converges backwards to meet the sagittal crest. The anterior part of the frontal is produced to form the long nasal process, which is wedged in between the nasal and maxillae. [Illustration FIG. 73. DORSAL VIEW OF THE CRANIUM OF A DOG (_Canis familiaris_) × 2/3. 1. supra-occipital. 2. parietal. 3. frontal. 4. nasal. 5. maxillae (facial portion). 6. premaxillae. 7. squamosal. 8. jugal. 10. postorbital process of frontal. 11. infra-orbital foramen. 12. anterior palatine foramen. 13. lachrymal foramen. _i_ 1. first incisor. _c._ canine. _pm_ 4. fourth premolar.] The cranial cavity is continuous in front with the =nasal= or =olfactory cavities=, but the passage is partially closed by a screen of bone, the =cribriform plate= (fig. 72, 5), which is placed obliquely across the anterior end of the cranial cavity, and is perforated by a number of holes through which the olfactory nerves pass. The plane of the cribriform plate is called the =ethmoidal plane=, and as was the case also with the occipital plane, the angle that it makes with the basicranial axis varies much in different mammals, and is of importance. The =olfactory fossa= in which lie the olfactory lobes of the brain, is partially separated from the =cerebral fossa=, or cavity occupied by the cerebral hemispheres, by ridges on the orbitosphenoids and frontals. The presphenoid is connected in front with a vertical plate formed partly of bone, partly of unossified cartilage; this plate, the =mesethmoid= (fig. 72, 7), separates the two olfactory cavities which lodge the olfactory organs. Its anterior end always remains unossified, and forms the septal cartilage of the nose. The brain case may then, to use the words of Sir W.H. Flower, be described as a tube dilated in the middle and composed of three bony rings or segments, with an aperture at each end, and a fissure or space at the sides between each of them.