2. those developed in connection with the special sense organs;
3. those forming the upper jaw and suspensorial apparatus.
Both cartilage and membrane bones take part in the formation of the skull, and a considerable amount of cartilage remains unossified, especially in the ethmoidal and sphenoidal regions.
1. The Cranium proper or Brain-case.
The cartilage and membrane bones of the brain-case when taken together can be seen to be more or less arranged in three rings or segments, called respectively the occipital, parietal, and frontal segments.
The occipital segment is the most posterior of these, and consists of four cartilage bones, the basi-occipital, the two exoccipitals and the supra-occipital; these bound the foramen magnum.
The basi-occipital (figs. 38 and 39, 5) lies ventral to the foramen magnum and only bounds a very small part of it; it forms one-third of the occipital condyle by which the skull articulates with the atlas vertebra. It unites dorsally with the exoccipitals and anteriorly with the basisphenoid.
The exoccipitals are rather small bones, which form the sides and the greater part of the floor of the foramen magnum, and two-thirds of the occipital condyle. Laterally each is united with the pterygoid and opisthotic of its side. At the sides of the occipital condyle each exoccipital is pierced by a pair of foramina, the more dorsal and posterior of which transmits the hypoglossal nerve.
The supra-occipital (fig. 39, 14) is a larger bone than the others of the occipital segment. It forms the upper border of the foramen magnum and is drawn out dorsally into a large crest which extends back far beyond the occipital condyle. In the adult the supra-occipital is completely ankylosed with the epi-otics.
The Parietal segment.