The Vertebrate Skeleton

by Sidney H. Reynolds

The anterior end of the basitemporal is pierced near the middle line by a pair of holes, the anterior openings of the Eustachian canals; while just in front of these and a little further removed from the middle line are the anterior openings of the carotid canals. Bristles passed in through the posterior openings of the carotid canals will emerge here (fig. 60, A, 27). In front of the basitemporal the base of the cranium is formed by the rostrum (fig. 60, A, 31), or thickened basal portion of the interorbital septum; this bears two prominent surfaces with which the pterygoids articulate. In some kinds of duck these surfaces are borne by well-marked basi-pterygoid processes.

(d) The side of the cranium. At the base of the posterior end is seen the deep tympanic cavity. The dorsal part of this is divided by a vertical partition into two halves; of these the more anterior is the larger, and forms a deep funnel-shaped cavity, the posterior opening of the Eustachian canal (fig. 60, B, 26). A bristle passed into this opening emerges through the anterior opening of the Eustachian canal. The more posterior of the two is the fenestral recess (fig. 60, B, 28), and is in its turn divided by a slender horizontal bar into a dorsal hole, the fenestra ovalis, and a ventral hole, the fenestra rotunda. During life the fenestra ovalis lodges the proximal end of the columellar chain. Lying at the outer side and slightly dorsal to the tympanic cavity is a deep depression, the lateral tympanic recess, and immediately in front of this is the articular surface for the quadrate. The tympanic cavity is bounded below by the basitemporal, posteriorly by the exoccipital, and above by the squamosal, a membrane bone, which roofs over a good deal of the side of the cranium, and bears ventrally a prominent surface with which the quadrate articulates. Just in front of this is a large round hole, the trigeminal foramen (fig. 60, B, V), behind which the squamosal is drawn out into a short process.

In front of the squamosal there is a prominent forwardly-projecting postfrontal process (fig. 60, 8), which ossifies from a different centre from that forming the squamosal, but in the adult is completely fused with it.

The orbit forms a large more or less hemispherical cavity which lodges the eyeball. It is separated from its fellow of the opposite side by an imperfect partition, the interorbital septum. In the young skull it is seen to be bounded above by the frontal, with which the lachrymal (fig. 60, 7) is fused anteriorly, forming a large backwardly-projecting process; while behind it is bounded by the alisphenoid. The interorbital septum is formed by the ossification and coalescence of the mesethmoid in front, with the orbitosphenoid behind, and the rostrum below. The boundary of the orbit below is very imperfect, the zygomatic arch being incomplete.

The interorbital septum is pierced by the very prominent optic foramen (fig. 60, B, 2), just behind which are the two much smaller foramina for the exit of the oculomotor and pathetic (fig. 60, B, IV) nerves, the more anterior being that for the oculomotor.

Above and slightly in front of the optic foramen is a median opening, the olfactory foramen. This leads into the cranial cavity behind, and in front is continued forwards as a groove between the interorbital septum and the frontal.

(2) The Facial part of the Skull.

This includes the olfactory capsule and associated bones, and the upper jaw.

The bones associated with the olfactory capsules are the nasals and vomer. The nasals (figs. 59 and 60, 5) lie on the dorsal surface immediately in front of the cranium, and are separated from one another by the nasal processes of the premaxillae. Each is completely fused in the adult with the corresponding maxillae and premaxillae, the three bones together forming the boundary of the anterior nares. The vomer (fig. 60, 19) is unpaired and forms a small median vertical plate lying ventral to the anterior continuation of the interorbital septum.

The bones of the upper jaw consist on each side of two slender arcades which in front converge and are attached to the large beak, while behind they diverge but are united by the quadrate.