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.