The occipital segment is the most posterior of the three, and consists of four cartilage bones, which in the adult are commonly completely fused together. They surround the greatforamen magnum (fig. 75, 2) through which the brain and spinal cord communicate. Forming the lower margin of the foramen magnum is a large flat unpaired bone, the basi-occipital (fig. 75, 5). Above this on each side are the exoccipitals, whose sides are drawn out into a pair of downwardly-directed paroccipital processes, which are applied to the tympanic bullae[141]. The inner side of each exoccipital is converted into the large rounded occipital condyle (fig. 72, 13) by which the skull articulates with the atlas vertebra. The dorsal boundary of the foramen magnum is formed by a large unpaired flat bone, the supra-occipital (figs. 72 and 75, 1), which is continuous with a small bone, the interparietal, prolonged forwards between the parietal bones of the next segment.

In old animals the interparietal forms the hind part of a prominent ridge running along the mid-dorsal surface of the skull and called the sagittal crest, while the junction line of the occipital and parietal segments forms a prominent occipital crest.

The plane in which the bones of the occipital segment lie is called the occipital plane; the angle that it makes with the basicranial axis varies much in different mammals.

The parietal segment consists of both cartilage and membrane bones. It is formed of five bones, which are in contact with those of the occipital segment on the dorsal and ventral surfaces, while laterally they are separated by the interposition of the auditory bones, and to some extent of the squamosal. The basisphenoid (fig. 75, 6), an unpaired bone forming the ventral member of this segment, is the direct continuation of the basi-occipital. It tapers anteriorly, but is rather deep vertically, its upper or dorsal surface bearing a depression, the sella turcica, which lodges the pituitary body of the brain. From the sides of the basisphenoid arise the alisphenoids (fig. 75, 11) a pair of bones of irregular shape generally described as wing-like; each gives off from its lower surface a pterygoid plate, which is united in front with the palatine, and below with the pterygoid. The alisphenoids are united above with a pair of large nearly square bones, the parietals (fig. 73, 2), which meet one another in the mid-dorsal line. The line of junction is frequently drawn out into a strong ridge, which forms the anterior part of the sagittal crest.

Fig. 72. Vertical longitudinal section taken a little to the left of the middle line through the skull of a Dog (Canis familiaris) × 3/5. (Camb. Mus.)

1. supra-occipital.18. palatine.
2. interparietal.19. alisphenoid.
3. parietal.20. internal auditory meatus.
4. frontal.21. tentorium.
5. cribriform plate.22. foramen lacerum posterius.
6. nasal.23. floccular fossa.
7. mesethmoid.24. coronoid process.
8. maxillae.25. condyle.
9. vomer.26. angle.
10. ethmo-turbinal.27. mandibular symphysis.
11. maxillo-turbinal.28. inferior dental foramen.
12. premaxillae.29. stylo-hyal.
13. occipital condyle.30. epi-hyal.
14. basi-occipital.31. cerato-hyal.
15. tympanic bulla.32. basi-hyal.
16. basisphenoid.33. thyro-hyal.
17. pterygoid.XII. condylar foramen.

The frontal segment, which surrounds the anterior part of the brain, is closely connected along almost its whole posterior border with the parietal segment.

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.