The =occipital bone= has an extensive squamous part or supraoccipital,
which forms a very broad and prominent crest. The latter is concave, and
is thick and rough above, where it forms the highest part of the skull;
laterally it becomes thinner, turns downward, and is continuous with the
temporal crest. Two divergent ridges pass upward from the foramen
magnum, and the surface between them is concave and smooth. The greater
part of the inner (or anterior) surface of the supraoccipital is united
with the parietal bones, but a lower concave area faces into the cranial
cavity. The foramen magnum is almost triangular, and is narrow above,
where it is flanked by two small tuberosities. The paramastoid processes
are extremely long and project almost straight downward. The hypoglossal
foramen is at the inner side of the root of the process. The basilar
part is short and wide; its lower surface bears a thin median ridge and
two lateral imprints or tubercles which converge at the junction with
the sphenoid bone.
[Illustration:
FIG. 110.—SKULL OF PIG, DORSAL VIEW.
]
The =interparietal= fuses before birth with the occipital. The tentorium
osseum is absent.
The =parietal= is overlapped by the occipital bone behind and concurs in
the formation of the occipital crest. Its external surface is divided by
the parietal crest into two parts. The inner part (Planum parietale)
faces upward and forward, and is flattened and smooth. Its inner border
is short and straight and unites early with the opposite bone. Its
anterior border is concave and joins the frontal bone. The outer part
(Planum temporale) faces outward and is more extensive; it is concave,
forms a large part of the temporal fossa, and is overlapped below by the
squamous temporal. The parietal crest extends in a curve from the
occipital crest forward and outward to the supraorbital process. The
internal surface is concave and is marked by digital impressions. The
lower border projects into the cranial cavity and forms a crest which
separates the cerebral and cerebellar compartments laterally. The
interior forms part of the frontal sinus in the adult. There is no
parieto-temporal canal.
The =frontal bone= is long. The frontal surface slopes downward and
forward, the inclination varying in different subjects. The anterior
part is concave and is marked by the supraorbital foramen and the groove
leading forward from the foramen to the nasal bone. The supraorbital
canal opens into the orbit at the upper part of the inner wall of the
latter. The supraorbital process is short and blunt-pointed, and is not
connected with the zygomatic arch. The gap in the orbital margin is
closed by the orbital ligament in the fresh state. The orbital part is
extensive and forms the greater part of the inner wall of the orbit. Its
upper part is perforated by the orbital orifice of the supraorbital
canal, in front of which is the distinct fovea trochlearis. The
ethmoidal or internal orbital foramen is situated in the lower part near
the junction with the orbital wing of the sphenoid. The temporal part is
very narrow and is separated from the orbital plate by a ridge which
joins the pterygoid crest below. The interior of the bone is excavated
by the frontal sinus in practically its entire extent in the adult. In
the young subject the cavity is confined to the anterior part and the
rest of the bone is thick.
The =temporal bone= has a general resemblance to that of the ox. The
zygomatic process is short and stout and is bent at a right angle. The
upper border of the process is thin; traced from before backward it
curves sharply upward and forms a high prominence in front of the
external auditory meatus; beyond this it drops rather abruptly and is
then continued upward to the occipital crest. The anterior part of the
lower border joins the zygomatic process of the malar, which is deeply
notched. The condyle is concave in the transverse direction. The
postglenoid process is absent, but the articular surface is bounded
behind and internally by a crest. There is no parieto-temporal canal.
The external auditory canal is very long and is directed upward and
outward. The bulla ossea is large, compressed laterally, and bears a
pointed muscular process in front. A narrow space intervenes between the
bulla and the basilar part of the occipital bone, so that the foramen
lacerum resembles that of the horse. The small hyoid process is situated
in a deep depression in front of the root of the paramastoid process,
and the stylo-mastoid foramen is immediately external to it. The petrous
part presents no important differential features. The squamous part
(including the root of the zygomatic process) contains an air-cavity,
which is continuous with the sphenoidal sinus.
[Illustration: