Part 14

Four foramina leading into the cranial cavity are visible in a lateral view of the skull, craniad of the tympanic bulla. The one nearest the bulla is the =foramen ovale= (u) for the third division of the fifth nerve; next craniad of this is the =foramen rotundum= (v) for the second division of the fifth nerve. These two foramina pierce the alisphenoid: just craniad of them, between the alisphenoid and the orbitosphenoid, is the large =orbital fissure= (r) (foramen lacerum anterius), which transmits the third, fourth, and sixth cranial nerves and the first division of the fifth. Dorsocraniad of the orbital fissure is the =optic foramen= (w), for the =optic nerve=.

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.

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).

a, foramen magnum; b, caudal end of hypoglossal canal; c, jugular foramen; d, internal auditory meatus; e, tentorium, forming the cranial boundary of the cerebellar fossa; f, dorsum sellæ; g, sella turcica; h, anterior clinoid processes; i, foramen ovale; j, foramen rotundum; k, orbital fissure; l, optic foramen; m, chiasmatic groove; n, presphenoid bone; o, cribriform plate; p, lamina perpendicularis of ethmoid; q, labyrinths of ethmoid; r, nares; s, foramina incisiva or anterior palatine foramina; t, infraorbital foramen; u, opening of the lachrymal canal; v, caudal opening of posterior palatine canal; w, sphenopalatine foramen; x, frontal process of the malar; y, zygomatic process of the temporal; z, appendicular fossa, in the petrous bone.]

=Cavities of the Skull= (Figs. 42 and 43).--The bones of the cranial portion of the skull enclose the =cranial cavity= for the brain; the facial bones enclose the =nasal cavity=, for the olfactory organ.

The =cranial= cavity is divisible into three principal fossæ: the =cerebellar= fossa (Fig. 43, I) caudad, for the cerebellum; the =cerebral= fossa (II) in the middle, for the cerebrum; the small =olfactory= fossa (III) at the cranial end for the olfactory bulb of the brain.

The =cerebellar= fossa (I) is bounded caudally by the occipital bone enclosing the foramen magnum (Fig. 42, a). Its ventral surface is formed by the basilar portion of the occipital and the petrous portions of the temporals; its lateral surface by the mastoid portions of the temporals and parts of the parietals and occipital. Its roof is formed by the parietals and interparietal. Craniad the cerebellar fossa is partly separated from the cerebral fossa by the tentorium (Fig. 42, e; Fig. 43, f) formed by the two parietals: this encloses a quadrangular opening by which the two fossæ communicate. The caudal, dorsal, and lateral walls of the cerebellar fossa are deeply marked by fossæ for the lobes of the cerebellum; the small =appendicular= fossa (Fig. 43, e), forming a deep indentation in the petrous bone near its dorsocaudal end, is particularly noticeable.

The following openings are found in the walls of the cerebellar fossa. Caudad is the large foramen magnum (Fig. 42, a) by which the brain-cavity communicates with the vertebral canal. Near the caudal margin of the foramen magnum, on its lateral side, just mediad of the dorsal end of the occipital condyle, is the caudal opening of the condyloid canal (Fig. 43, a) which passes craniad through the substance of the occipital bone to open just caudad of the petrous: it transmits a vein. The condyloid canal varies greatly in size in different specimens. A few millimeters craniad of the edge of the foramen magnum on the floor of the fossa is the small opening of the hypoglossal canal (Figs. 42 and 43, b), for the twelfth nerve. Just craniad of this, at the caudomedial border of the petrous, is the large jugular foramen (c). On the petrous itself, near the middle, is the internal, auditory meatus (d) divided into the dorsal =facial canal= for the seventh nerve, and a ventral passage for the eighth nerve. At the cranial end of the cerebellar fossa is the large opening bounded by the free edges of the tentorium.

The =cerebral= fossa forms much the largest part of the cranial cavity. It is bounded by the parietals (Fig. 43, 3′), squamous portions of the temporals (4), frontals (8), the sphenoid (5), and presphenoid (6). A slight rounded ridge on its lateral wall at about the position of the suture between the frontals and parietals separates a smaller cranial portion sometimes called the =anterior= fossa, from a larger caudal portion sometimes known as the =middle= fossa of the cranial cavity. The walls of the cerebral cavity are marked with numerous ridges and shallow furrows for the cerebral convolutions.

The floor of the cerebral cavity is bounded caudad by the prominent dorsum sellæ (Fig. 42, f; Fig. 43, g), just craniad of which is the rounded depression known as the sella turcica (Fig. 42, g; Fig. 43, h), for lodgment of the hypophysis. A number of foramina pierce the floor of the cavity in this region. Just ventrad of the cranial tip of the petrous portion of the temporal is the small =foramen lacerum= (medius). Craniad and laterad of this is a row of four foramina: the caudal one is the foramen ovale (Fig. 42, i); then come in order the foramen rotundum (j), the orbital fissure (k), and the optic foramen (Fig. 42, l; Fig. 43, k). The two optic foramina are connected by the shallow transverse =chiasmatic groove= (Fig. 42, m), for the optic chiasma. Another small foramen continues caudad from a groove on the floor of the orbital fissure; this opens on the ventral surface of the sphenoid, between the wing and the body of the bone. The groove and foramen constitute the =pterygoid canal=, which transmits a nerve,--the =nerve of the pterygoid canal=, or Vidian nerve.

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.

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=.

V. BONES OF THE THORACIC EXTREMITIES.

=Scapula= (Figs. 44 and 45).--The scapula may be described as a flat triangular bone with one angle rounded. It lies beneath the muscles on the lateral face of the thorax near its cranial end. From its lateral surface there projects a flat ridge (Fig. 44, g), the =spine= of the scapula. The ventral end of the ridge is free as a curved process, the =acromion= process (Fig. 44, j).

FIG. 45.--SCAPULA, MEDIAL SURFACE.

I, fossa supraspinata; II, fossa infraspinata; III, fossa subscapularis; IV, fossa for teres major. a, vertebral border; b, coracoid border; c, glenoid border; d, glenoid angle and fossa; e, coracovertebral angle; f, glenovertebral angle; g, spine; h, tuberosity of the spine; i, metacromion; j, acromion; k, supraglenoidal tubercle; l, incisura scapulæ; m, coracoid process; n, groove indicating portion of spine; o, o′, ridges for attachment of muscle-fibres.]