Ventrad of the cranial portion of the orbit is the large sphenopalatine foramen (q), for the nerves and arteries of the same name. Just craniad of this is the small caudal opening of the posterior palatine canal, which passes through the substance of the palatine bone and opens on its ventral surface near its cranial margin. Just dorsad of the cranial root of the zygomatic arch is the opening of the lachrymal canal (x), while the root of the arch is pierced by the large infraorbital foramen (y), which transmits the infraorbital nerves and artery from the orbit.

The teeth (13-18), implanted along the alveolar border of the maxillary and premaxillary, form a prominent feature in a lateral view: they are described in the account of the [alimentary canal].

The ventral surface of the skull ([Fig. 41]) is very complex. It is separated by the orbits into a caudal and a cranial portion, united by a narrow median trough-like part. Laterad of this trough-like part are visible parts of the orbit and the zygomatic arches, which do not properly belong to the ventral surface and have already been [described].

[Fig. 41].—Skull, Ventral View.

1, occipital bone; 2, temporal; 3, sphenoid; 4, presphenoid; 5, frontal; 6, malar; 7, vomer; 8, palatine; 9, maxillary; 10, premaxillary. a, foramen magnum; b, occipital condyles; c, jugular process; d, tympanic bulla; e, mastoid process; f, stylomastoid foramen; g, external auditory meatus; h, jugular foramen; i, styliform process; j, groove for Eustachian tube; k, foramen ovale; l, foramen rotundum; m, pterygoid process of sphenoid; n, perpendicular plate of palatine; o, choanæ or posterior nares; p, zygomatic arch; q, cranial end of posterior palatine canal; r, palatine grooves; s, foramina incisiva or anterior palatine foramina; t, opening of pterygoid canal.

Caudally there appear in the ventral view the foramen magnum (a), occipital condyles (b), and jugular processes (c). In front of the jugular processes the two tympanic bullæ (d) form prominent features, with the mastoid process (e), the stylomastoid foramen (f), and the external auditory meatus (g) on their lateral surfaces. All these structures have been described. The tympanic bullæ (d) are placed with long axes directed craniomediad, so that they converge toward their cranial ends. At the caudomedial angle of each bulla is the large jugular foramen (h), for the ninth, tenth, and eleventh nerves. Opening into the mediocaudal margin of the jugular foramen is the smaller hypoglossal foramen, for the twelfth nerve.

The craniomedial end of the tympanic bulla projects craniad as the styliform process (i). Just laterad of this process is the opening (j) into the tympanic bulla by which the tuba auditiva or Eustachian tube passes into the middle ear. A faint groove for the tube passes craniomediad from this opening, on the surface of the sphenoid. Craniolaterad of the opening for the tuba auditiva is the foramen ovale (k); craniad of this the foramen rotundum (l) is faintly indicated. On the surface of the sphenoid just craniad of the styliform process of the bulla tympani is the minute opening of the pterygoid canal (t). The orbital fissure and optic foramen are not seen in the ventral view.

The middle region of the ventral surface is narrow: it is formed by a trough-like fossa which is bounded laterally by the pterygoid processes (m) of the sphenoid and the perpendicular plates of the palatines (n). Ventrad of this lies, in the natural condition, the soft palate, converting the fossa into the nasal portion of the pharynx or nasopharynx. Craniad this fossa is bounded by the free caudal edges of the palatines; beneath which the fossa communicates with the nasal cavity by the two choanæ (o). Laterad of this median fossa are visible in the ventral view parts of the temporal and orbital fossæ, bounded laterally by the zygomatic arches (p).

The cranial part of the ventral surface is a somewhat triangular plane area formed by the palatal portions of the palatines (8), maxillaries (9), and premaxillaries (10), which together constitute the hard palate (palatum durum). Laterad and craniad this area is bounded by the alveolar borders of the maxillaries and premaxillaries bearing the teeth. The hard palate is marked near the cranial border of the palatine bones with two or more foramina which form the cranial termination of the posterior palatine canal (q). Two faint grooves pass from these foramina a short distance craniad, gradually converging: these are known as the palatine grooves (r) (sulci palatini). Near the cranial end of the hard palate are two large openings close together near the middle line: these are the foramina incisiva (or anterior palatine foramina) (s).