The Vertebrate Skeleton

by Sidney H. Reynolds

(2) the capsules of the auditory and olfactory sense organs are fused;

(3) the skeleton of the jaws and hyoid apparatus. The skull is much flattened and expanded, though not so much as in the frog.

(1) The cranium proper.

The cranium proper or brain case is an unsegmented tube which remains partly cartilaginous, and is partly converted into cartilage bone, partly sheathed by membrane bone. The roof and floor of the cartilaginous cranium are, as is the case also in the frog, pierced by holes or fontanelles, and these are so large that the main part of the roof and floor comes to be formed by membrane bone.

Two pairs of large ossifications take place in the cranial walls. Of these the more posterior on each side represents the exoccipital and all three periotic bones. It bears a small convex patch of cartilage for articulation with the atlas, and with its fellow forms the boundary of the foramen magnum.

Two foramina pierce the exoccipital just in front of the occipital condyle and transmit respectively the glossopharyngeal and pneumogastric (fig. 21, X) nerves. Lying laterally to these nerve openings is seen a patch of cartilage, the stapes, which is homologous with the stapes or proximal element of the columellar chain in the frog. Further forward in front of the stapes is the small opening for the exit of the facial nerve, and seen in a lateral view close to the orbitosphenoid, that for the trigeminal (fig. 21, C, 5).

In front of these large bones the lateral parts of the cranial walls remain cartilaginous for a short distance, and then there follow two elongated bones, the orbitosphenoids (fig. 21, B and C, 11), pierced by the foramina for the exit of the optic nerves. These bones partly correspond to the sphenethmoid of the frog.

The membrane bones connected with the cranium are the parietals, frontals and prefronto-lachrymals on the dorsal surface, and the parasphenoid on the ventral surface.

The parietals (fig. 21, A and C, 6) roof over the posterior part of the great dorsal fontanelle and overlap the exoccipito-periotics. They meet one another along a sinuous suture in the middle line, as do also the frontals which overlap them in front. The frontals and parietals both extend for a short distance down the sides of the cranium and meet the orbitosphenoids. The prefronto-lachrymals (fig. 21, A and C, 7) connect the frontals with the maxillae.