Part 4

The internal surface (Fig. 18) is concave dorsoventrally, following the outline of the foramen magnum. It is convex from side to side. It is smooth except at its outer margin, which is rough for articulation with the mastoid portion of the temporal bone. The cranial face of the jugular process (_f_) shows a rough concavity for the reception of the bulla tympani. Mediad of the jugular process is a notch (=jugular notch=) (_g_) which when the bones are articulated forms part of the boundary of the jugular foramen. Mediad of this notch is a foramen which forms one end of the =hypoglossal canal=. It passes dorsocaudad into the cranial cavity and transmits the hypoglossal nerve. Dorsad of the hypoglossal canal is the cranial opening of the =condyloid canal=, which passes caudad and opens just craniad of the dorsal end of the condyle. It transmits a vein. The outer border of this portion is rough for articulation with the petrous and mastoid portions of the temporal.

The =squamous portion= (_c_) has the form of a sector of a circle whose arc is a little more than ninety degrees. The central angle of the sector is truncated and bounds the foramen magnum dorsally. The arc of the sector forms the dorsal margin of the bone, while along the radii it passes into the lateral portions. Its dorsal portion is thick and porous; its ventral portion near the foramen magnum is thin and compact.

The external surface (Fig. 17) is marked by a prominent ridge, the =lambdoidal ridge= (_h_), parallel with the dorsal border and near to it. The narrow portion of the outer surface that lies dorsad of the ridge forms an angle of about ninety degrees with the remainder of the surface.

A median crest (_i_) extends ventrad from the middle of the lambdoidal ridge toward the foramen magnum; this is the =external occipital crest=. At its junction with the lambdoidal ridge it is elevated into a tubercle, the =external occipital protuberance= (_j_).

The inner surface (Fig. 18) presents depressions for the convolutions of the cerebellum.

The dorsal border is thick and rough for articulation with the parietals and interparietal. The ventral border abuts on the foramen magnum and is thin and smooth.

=Interparietal Bone.= =Os interparietale= (Fig. 19).--This is a small triangular bone lying between the parietals, with its apex directed craniad, and its base in contact with the squamous portion of the occipital.

Its dorsal surface is arrow-shaped and has its posterior border notched. It is marked by a median crest (part of the =sagittal crest=) which is continued craniad from the middle of the lambdoidal crest.

The ventral surface is irregularly triangular, smooth, and concave. The three borders are rough for articulation with the parietals and occipital.

[Illustration: FIG. 19.--INTERPARIETAL BONE, OUTER SURFACE.

FIG. 20.--SPHENOID BONE, DORSAL OR INTERNAL SURFACE.

Fig. 19, showing the sagittal crest running craniocaudad across its middle.

Fig. 20.--_a_, body; _b_, wings; _c_, pterygoid process; _d_, tuberculum sellæ; _e_, dorsum sellæ; _f_, sella turcica; _g_, notch which aids in forming the foramen lacerum; _h_, longitudinal groove of alisphenoid; _i_, notch which aids in forming the orbital fissure; _j_, foramen rotundum; _k_, foramen ovale.]

=Sphenoid Bone.= =Os sphenoidale= (Fig. 20).--The sphenoid bone of man is represented in the cat by two entirely distinct bones,--one cranial, the other caudal. The cranial portion may be designated as the =presphenoid= (Fig. 21); the caudal part will be described as the =sphenoid= (proper) (Fig. 20).

The sphenoid bone in the kitten is in three parts: a central portion, the =basisphenoid=, and two lateral portions, the =alisphenoids=. In many lower vertebrates these three bones are permanently distinct, but in the adult cat they are united to form the sphenoid bone. To these there is added a fourth element, separate in many vertebrates as the =pterygoid= bone. The sphenoid may thus be described as composed of a central portion, the body (_a_) (basisphenoid), and of two thin expanded wings (_b_) (alisphenoids, alæ magnæ of the human sphenoid); each of which has arising from it a thin curved process, the =pterygoid process= (_c_), directed craniad and largely made up of the pterygoid bone.

The body of the sphenoid (_a_) lies in the middle line of the base of the skull. It is wedge-shaped, with the converging sides of the wedge directed laterad and its apex pointed craniad.

It has six surfaces, of which the dorsal and a part of the laterals look into the cranial cavity. The cranial end articulates with the body of the presphenoid, and the caudal with the body of the occipital.

The dorsal surface is triangular, with one apex of the triangle truncated, elevated, and directed craniad. This elevation is the =tuberculum sellæ= (_d_). Just caudad of the middle the surface presents a rectangular elevation with rounded angles, the =dorsum sellæ= (_e_). The cranial end of the dorsum sellæ presents at each dorsolateral angle a very small smooth tubercle which represents one of the posterior clinoid processes of man. Between this elevation and the elevated cranial end of this surface there is a deep excavation, the =sella turcica= (_f_), in which in the natural state is lodged the hypophysis. Near the cranial end of the sella is a small foramen, probably nutrient. At the caudal end of the body a slight notch (_g_) separates it from the wing: this notch forms a part of the =foramen lacerum=. Against this notch fits the apex of the petrous bone, and from it a groove (=carotid groove=) is continued mediocraniad to the sella turcica.

The ventral surface (Fig. 41, 3) is triangular, smooth, and nearly flat; it is marked by a median ridge which is the continuation craniad of the ridge on the ventral face of the basilar portion of the occipital.

Its caudal angles are separated from the rest of the bone by sharp triangular elevations, laterad of which are rough triangular areas, overlaid when the bones are articulated by a triangular spine from the tympanic bulla.

Its lateral surfaces are mostly covered by the wings. They appear at the sides of the elevated cranial end of the dorsal surface as triangular areas.

The caudal end is concave, rough, and has the form of the cranial end of the basilar part of the occipital.

The cranial end is nearly square and rough for articulation with the body of the presphenoid.

=The Wing= (=alisphenoid=; ala magna of the human sphenoid) (Fig. 20, _b_).--This is a thin quadrilateral plate of bone attached by its medial border to nearly the whole of the lateral surface of the body. Its middle portion lies nearly in the same plane as the body, but its ends are curved dorsad so that its internal surface is concave and its external surface is convex. The curvature is most pronounced near the long lateral border, so that this border forms nearly a semicircle.

The internal surface supports the occipital lobe of the cerebrum. It is marked by a rounded groove (=h=) which is parallel with the lateral surface of the body. The dorsal margin of the groove projects mediad in the form of a sharp ridge which is broadest caudad, where it often reaches nearly to the posterior clinoid process. The groove passes craniad into three foramina. The first (cranial) of these, the =orbital fissure= (_i_), is large and lies between the wing, the body, and the pterygoid process. It is incomplete, but is completed by the presphenoid. The second foramen is small and rounded; it is the =foramen rotundum= (_j_). The third, =foramen ovale= (_k_), is larger and oval and penetrates the wing through about the middle of its longitudinal axis. Another minute foramen penetrates the sphenoid between the wing and the body of the bone, just laterad of the tuberculum sellæ. This foramen is continuous craniad with a groove on the dorsal surface of the pterygoid process; the groove and foramen constitute the =pterygoid canal=. It transmits a nerve.

The external surface shows the orbital fissure, the foramen rotundum and the foramen ovale, bounded ventrally by a sharp ridge, which is continued onto the pterygoid process. Between this ridge and the body the surface is longitudinally grooved for the tuba auditiva or Eustachian tube.

The semicircular margin of the bone articulates with the squamous portion of the temporal. At the junction of its caudal and middle third there is sometimes a toothlike projection which underlies the root of the zygoma.

The whole of the cranial margin, except the lateral end, articulates with the wing of the presphenoid. At this end the angle formed by the junction of lateral and cranial borders is produced into a flat process, which passes dorsocaudad between the squamous portion of the temporal and the frontal, and articulates by the roughened internal surface of its free end with a similar process from the parietal.

The caudal margin laterad of the groove is bevelled and roughened at the expense of the dorsal surface and is overlaid by the ventral end of the tentorium. Mediad of the groove it projects caudad as a slender point, the =lingula= of the sphenoid. This is received into a narrow cleft between the apex of the petrous bone and the bulla tympani.

The pterygoid process (_c_) is a nearly square, thin plate of bone. The medial surface is smooth and concave, the lateral face is convex and marked by two parallel ridges. The medial one of these is continued craniad from the bony septum which separates the orbital fissure from the foramen rotundum, and the lateral one from the septum which separates the foramen rotundum from the foramen ovale. A sharp triangular spine projects laterad from near the caudal end of the lateral ridge.

The two ridges and that part of the lateral surface of the bone included between them form a part of the sphenoid bone known as the pterygoid process of the sphenoid bone, in those cases where the pterygoid is a separate bone.

The remainder of the process is equivalent to the pterygoid bone of other vertebrates.

Between the caudal margin of this bone and the lateral of the two ridges, i.e., between the pterygoid bone and the pterygoid process of the sphenoid, is a long deep fossa, the =internal pterygoid fossa= (Fig. 40, _s_). The laterocaudal margin of the pterygoid process projects caudad, as a curved triangular spine, the =hamulus= or =hamular process= (Fig. 40, _t_; Fig. 43, _i_).

The =Presphenoid Bone=. =Os presphenoidale= (Fig. 21).--In a young cat this bone is in three pieces, a basal portion (=presphenoid=) and two wings (=orbitosphenoid bones=). These bones remain distinct throughout life in many lower vertebrates, but in the adult cat they fuse to form a single bone. We may nevertheless conveniently describe this bone as made up of a body (_a_) (the basisphenoid), and two wings (_b_), the orbitosphenoids (the alæ parvæ of the human sphenoid).

[Illustration: FIG. 21.--PRESPHENOID BONE, VENTRAL VIEW.

_a_, body; _b_, wings; _c_, optic foramina.]

The body (_a_) lies in the base of the skull in the median line, craniad of the basisphenoid. It has the form of a rectangular prism about twice as long as broad. It is hollow, and the cavity is divided by a median longitudinal partition into two cavities (=sphenoidal sinuses=, Fig. 43, _l_). The sphenoidal sinuses are continued craniad into the cavities of the ethmoid. The body has six surfaces:

The dorsal or internal surface (Fig. 42, _n_) looks into the cranial cavity and is continuous with the dorsal surface of the wings. The caudal end of the body is depressed, and when united to the basisphenoid aids in forming the cranial wall of the sella turcica. At each caudolateral angle is a short spine, the =anterior clinoid process=. At about one-third the length of the bone from the caudal end is a transverse groove (=chiasmatic groove=, Fig. 42, _m_) for the optic chiasma. Its ends lead into two round foramina (the =optic foramina=, Fig. 42, _l_; Fig. 21, _c_) which pass craniolaterad between the body and the wings of the presphenoid and transmit the optic nerve and the ophthalmic artery.

The ventral surface (Fig. 21) is hour-glass-shaped and marked by a smooth median ridge, continuous with the ridge on the basisphenoid and overlaid at its cranial end by the vomer. The caudal end presents a rough triangular area on each side, for articulation with the pterygoid process of the sphenoid bone, while the cranial end has similar areas overlaid by the nasal portion of the palatine bones.

The lateral surface looks towards the orbitotemporal fossa. It is notched near the caudal end by the ventral border of the optic foramen (_c_). Caudad of this foramen the surface presents an oblique groove which forms in the natural condition the medial boundary of the orbital fissure.

Craniad of the optic foramen the surface is smooth and marked near its ventral border by a longitudinal ridge which forms part of the dorsal boundary of a fossa, the =external pterygoid fossa= (Fig. 40, _p_).

The caudal end presents ventrally a quadrangular rough surface for articulation with the body of the sphenoid. The cranial end presents the two sphenoidal sinuses separated by a median partition.

The median partition articulates by its free border with the lamina perpendicularis of the ethmoid. At its ventral end is the abruptly truncate end of the median ridge of the ventral surface, which is continuous with the ventral cartilaginous portion of the lamina perpendicularis. The lateral walls of the sphenoidal sinuses are continued craniad of the dorsal and ventral walls and of the median

## partition, and articulate ventrally with the nasal portion of the

palatine bones, and dorsally with the orbital plate of the frontal. Between them is received the caudal ends of the labyrinths of the ethmoid in the middle, while between their dorsal edges is received the caudal end of the cribriform plate, and between their ventral edges the expanded end of the vomer.

The =wings= (_b_) arise each from nearly the whole of the dorsolateral angle of the body. They form prominent nearly horizontal triangular projections over the optic foramina.

The dorsal and ventral surfaces are smooth and continuous respectively with the dorsal and lateral surfaces of the body. The dorsal surface looks into the cranial cavity, while the ventral looks into the orbitotemporal fossa.

Craniad of the apex of the wing its border articulates with the ventral edge of the orbital portion of the frontal bone. Caudad of the apex the border articulates with the cranial border of the wing of the sphenoid.

=Temporal Bone.= =Os temporale= (Figs. 22 and 23).--This forms a part of the lateral wall of the cranium, filling the gap between the occipital and the sphenoidal segments. It is made up of three portions which are distinct in kittens but somewhat firmly united in adult cats. In lower vertebrates these portions are distinct bones called the =Squamous=, the =Petrous=, and the =Tympanic= bones. In the cat they may be described as the squamous (_a_), petrous (_b_), and tympanic (_c_) portions of the temporal bone.

[Illustration: FIG. 22.--TEMPORAL BONE, EXTERNAL SURFACE.

FIG. 23.--TEMPORAL BONE, INTERNAL SURFACE.

_a_, squamous portion; _b_, petrous portion; _c_, _c′_, tympanic portion (_c_, entotympanic; _c′_, ectotympanic); _d_, zygomatic process; _e_, mastoid portion of the petrous; _f_, mandibular fossa; _g_, postmandibular process; _h_, tuberculum articulare; _i_, external auditory meatus; _j_, stylomastoid foramen; _k_, pit for tympanohyal bone; _l_, mastoid process; _m_, grooves bounding the jugular foramen; _n_, internal auditory meatus; _o_, appendicular fossa; _p_, hiatus facialis; _q_, styliform process of tympanic bulla.]

The =squamous portion= (_a_) (=squama temporalis=) is thin and oval or has the form of an equilateral triangle with rounded angles, with a curved process, the =zygomatic process= (_d_), arising from its ventral border. Its outer surface (Fig. 22) is convex and smooth and gives origin to part of the temporal muscle. Its inner surface (Fig. 23) is concave and smooth except near the margins, where it is bevelled and rough. The roughened border is broader dorsad and caudad. The ventral margin of the bone is turned mediad at its cranial end so that the lateral face of the inverted portion looks ventrad. By this portion of its lateral face the squamous rests upon the tympanic bulla, and its edge articulates with the tentorium and the wing of the sphenoid. The remaining (caudal) portion of the ventral border overlies the mastoid portion (_e_) of the petrous. By the remainder of its border the bone articulates with the parietal dorsad and with the wing of the sphenoid craniad. The roughened portion of its inner surface overlies the margins of both these bones.

The zygomatic process (_d_) is formed by the confluence of two roots. One of these starts from the ventral end of the lambdoidal ridge and passes along the ventral margin of the squama dorsad of the external auditory meatus. The other arises abruptly from the cranioventral angle of the bone. The process thus formed is at first broad and passes horizontally laterad and slightly craniad. It soon grows more slender and turns gradually craniad, while at the same time it twists so that the surface which is dorsal at the base becomes medial at the tip; the posterior root which is continuous with the caudal border at the base is continuous with the dorsal border at the apex. On the ventral surface of the base is a transversely elongated concave articular surface, the =mandibular fossa= (_f_), for the condyloid process of the lower jaw. Caudad of this is a sharp transverse ridge, the =postmandibular= or =postglenoid process= (_g_), and craniad of the lateral end of the fossa a slight tubercle, the =tuberculum articulare= (_h_). Near its apex the zygomatic process is more slender and its ventral border is bevelled for articulation with the malar or zygomatic bone.

The =tympanic= (Figs. 22 and 23, _c_; Fig. 24) is expanded into a large hollow olive-shaped bone which is known as the =auditory bulla= and encloses the tympanic cavity. Its substance is very compact. Unlike the tympanic of most other mammals it is developed from two bones, known as the =ectotympanic= (Fig. 22, _c′_) and =entotympanic= (_c_). These are strongly marked in young kittens, and can usually be easily distinguished in adult cats. The entotympanic (Figs. 22 and 23, _c_) forms the larger part of the bulla, constituting its ventral and medial surfaces; it is thin, smooth, and transparent. The ectotympanic (Fig. 22, _c′_) surrounds the external auditory meatus: it is thicker and more opaque than the entotympanic. The bulla lies ventrad of the squamous, and in an external view conceals a large part of the petrous.

On its lateral surface it presents near the dorsal border an irregular oval opening, that of the =external auditory meatus= (Fig. 22, _i_), which leads into the tympanic cavity. Caudad of the external auditory meatus is a nearly dorsoventral groove, which, when the bone is articulated, forms a part of the boundary of the stylomastoid foramen (Fig. 22, _j_); just ventrad of this groove is a pit (_k_) which lodges the tip of the tympanohyal bone.

Craniad the bone is produced into a short spine, the =styliform= process (_q_), which lies in a horizontal groove in the ventral surface of the basisphenoid. Laterad of this spine is a groove for the tuba auditiva or Eustachian tube.

[Illustration: FIG. 24.--TYMPANIC BULLA, ISOLATED, MEDIAL SURFACE.

_a_, inner end of auditory meatus; _b_, partition dividing tympanic cavity; _c_, styliform process.]

The medial surface (Fig. 24) presents in the middle near its ventral margin a short triangular spine which lies in the natural state against the ventral surface of the basilar portion of the occipital.

Caudad of this spine the surface is marked by two or three vertical parallel grooves (Fig. 23, _m_). They indicate the portion of the bone which bounds the jugular foramen, and are possibly impressions of the ninth, tenth, and eleventh nerves.

The dorsal two-thirds of the medial surface is lacking in the disarticulated bulla (Fig. 24), so that the cavity of the bone is exposed. This opening is in the natural state closed by the petrous bone. The caudal end is rough where it is overlaid by the jugular process.

On the inner surface of the tympanic bulla is seen the thickened margin of the inner end of the auditory meatus (Fig. 24, _a_). To it is attached the membrana tympana. In the median dorsal line this margin is notched for the reception of the incus and head of the malleus. From the lateral wall of the cavity at the line of junction of the ectotympanic and entotympanic a thin bony partition (_b_) rises. It runs almost directly mediad; is concave dorsally and divides the tympanic cavity into two chambers.

=The Petrous Portion= (Fig. 23, _b_, and Fig. 25).--This consists of two parts, a very dense part (the petrous portion proper, Fig. 25), which has the form of a triangular pyramid and encloses the auditory labyrinth, and a less dense part, the =mastoid= portion (Figs. 22 and 23, _e_), which is flattened and triangular and is attached by its base to the base of the pyramid.

The petrous portion may be described as having a base and three sides, lateral, dorsal, and medial. It completes the medial wall of the tympanic bulla, so that it is not possible to see it from the exterior of a skull except through the auditory meatus (Fig. 22, _i_). When the bones of the skull are articulated its dorsal surface is covered by the tentorium and alisphenoid. Its lateral face looks into the tympanic cavity, while the medial face looks into the cranial cavity.

[Illustration: FIG. 25.--PETROUS BONE OF RIGHT SIDE, LATERAL SURFACE, ENLARGED.

_a_, fenestra cochleæ; _b_, promontory; _c_, fenestra vestibuli; _d_, fossa for the tensor tympani muscle; _e_, fossa for incus and malleus; _f_, fossa continuous with stylomastoid foramen; _g_, foramen leading to facial canal.]

Its lateral face (Fig. 25) (medial wall of the tympanic cavity) presents just ventrad of the middle of its base a large circular foramen, the =fenestra cochleæ= (_a_) (or fenestra rotunda), which looks caudolaterad; it leads into the cochlea. The fenestra cochleæ lies at the summit of a nipple-like elevation, the =promontory= (_b_), which is continued toward the apex of the bone as a gradually diminishing semicylindrical ridge, due to the presence within it of the bony cochlea. Dorsad of the fenestra cochleæ is the much smaller =fenestra vestibuli= (_c_) (or ovalis) which leads into the vestibule. It is occupied in the natural condition by the base of the stapes.

Dorsocraniad of the fenestra vestibuli is a large fossa (_d_) which contains the tensor tympani muscle. Dorsocaudad of this, partly bounded by the squamous portion of the temporal, is another large fossa (_e_), the cranial end of which is occupied by the incus, while its caudal end is occupied by the head of the malleus.

Nearly caudad of this fossa and separated from it by an oblique bony septum is a third fossa (_f_) which is narrow and curved. It is continuous with a notch in the mastoid portion of the bone. When the tympanic is articulated the notch is converted into a foramen (=stylomastoid foramen=, Fig. 22, _j_) for the exit of the seventh nerve. The fossa gives passage to the seventh nerve and also lodges the stapedius muscle. A groove may be traced from the stylomastoid foramen to the caudal border of the fossa for the tensor tympani muscle, where it passes into a canal (_g_). The groove and canal are parts of the =canalis facialis= or =facial canal= (aqueductus Fallopii) for the passage of the seventh nerve through the petrous bone.