The cranial cavity narrows at its cranial end to form the small olfactory fossa ([Fig. 43], III) which lodges the olfactory bulbs. This is bounded by the frontals and the lamina cribrosa ([Fig. 42], o) of the ethmoid; caudad it opens directly into the cerebral fossa. Numerous openings through the lamina cribrosa for the olfactory fibres connect the olfactory fossa with the nasal cavity. The roof of the fossa is marked by a prominent median crest from the united edges of the frontals.

The nasal cavity is almost completely filled by the ethmoid and vomer and the conchæ nasales. Its roof is formed by the nasal bones and portions of the frontals; its sides by the frontals, lachrymals, maxillaries, premaxillaries, and palatine bones; its floor by the horizontal plates of the palatines, maxillaries, and premaxillaries.

[Fig. 43].—Skull, Median Longitudinal Section, showing the Cavities.

I, cerebellar fossa; II, cerebral fossa; III, olfactory fossa. 1, occipital bone; 2, interparietal; 3, 3′, parietal; 4, temporal (4, squamous portion; 4′, petrous portion; 4″, tympanic portion); 5, sphenoid; 6, presphenoid; 7, palatine; 8, frontal; 9, maxillary; 10, premaxillary; 11, ethmoid; 12, nasal; 13, incisor teeth; 14, canine; 15, 16, 17, premolars; 18, molar. a, condyloid canal; b, hypoglossal canal; c, jugular foramen; d, internal auditory meatus; e, appendicular fossa; f, tentorium; g, dorsum sellæ; h, sella turcica; i, hamular process; j, pterygoid process of sphenoid; k, optic foramen; l, presphenoid sinus; m, m′, frontal sinus; n, lamina perpendicularis of the ethmoid (broken at cranial edge).

The nasal cavity opens craniad by the large nares ([Fig. 39], j; [Fig. 42], r), which are bounded by the premaxillary and nasal bones. In the natural condition this opening is divided by a median cartilage which is continuous with the lamina perpendicularis ([Fig. 43], n) of the ethmoid, thus forming a partition which divides the nasal cavity into two separate halves. From the floor of the cranial part of the cavity rises a ridge formed of the nasal crests of the maxillaries and premaxillaries, and the cranial portion of the vomer. Farther caudad the vomer spreads out in a horizontal plane and separates from the floor of the cavity, so that the nasal cavity is thereby divided by a horizontal partition into dorsal and ventral portions. The ventral portion is small, forming the inferior meatus of the nose; it ends caudally at the choanæ (posterior nares, [Fig. 41], o) which lead into the nasopharynx. That portion of the nasal cavity lying dorsad of the vomer is almost completely filled by the ethmoid and the conchæ nasales, superior and inferior. It is bounded caudally by the lamina cribrosa of the ethmoid ([Fig. 42], o). The nasal cavity communicates with the cranial cavity by the foramina for the olfactory fibres in the lamina cribrosa; with the nasopharynx by the choanæ; with the exterior of the body by the nares; with the mouth-cavity by the foramina incisiva or anterior palatine foramina ([Fig. 42], s); with the orbit by the sphenopalatine foramen and the nasolachrymal canal. It communicates directly also with the frontal sinuses ([Fig. 43], m, m′), the sphenoidal sinuses ([Fig. 43], l), and with the cells of the labyrinths of the ethmoid.

JOINTS AND LIGAMENTS OF THE SKULL.

Sutures of the Skull.—The bones of the skull join each other by means of immovable articulations known as sutures. These sutures are designated by combining the names of the bones between which they are situated: as, sphenofrontal suture (sutura sphenofrontalis), between the sphenoid and frontal; nasomaxillary suture (sutura nasomaxillaris), between the nasal and maxillary bones. When a suture joins the two corresponding bones of opposite sides the prefix inter is used, as the intermaxillary suture (sutura intermaxillaris) between the maxillaries. The sutures bounding the parietals have, however, received special names not derived in this manner. The suture caudad of the parietals, separating them from the occipital and interparietal, is known as the lambdoidal suture; that between the two parietals is the sagittal suture; that separating the parietals and squamous portions of the temporals is the squamous suture; that between the parietals and frontals is the coronal suture. The suture separating the two frontals also is known as the frontal suture, in place of interfrontal.

Articulations of the Mandible.—In man the two halves of the mandible are united craniad, so as to form a single bone. In the cat the two halves are separate, but articulate closely at the symphysis menti by a thin interarticular cartilage.

The articulation of the mandible at the mandibular fossa of the temporal is covered with a close articular capsule. The mandibular fossa is lined with cartilage. A slender ligament passes from the angular process of the mandible caudad to the external auditory meatus, being attached to the latter about 8 millimeters from its medial end. This is the stylomandibular ligament.