_a_, basilar portion; _b_, lateral portions; _c_, squamous portion;
_d_, foramen magnum; _e_, occipital condyles; _f_, jugular processes;
_g_, jugular notch; _h_, lambdoidal ridge; _i_, external occipital
crest; _j_, external occipital protuberance.]
In young kittens four portions may be distinguished in this bone. These
are, a basal portion (the =basioccipital bone=), two lateral portions
(the =exoccipital bones=), and a dorsal portion (the =supraoccipital
bone=). These four bones remain separate through life in many lower
vertebrates. In the adult cat they are completely united into a single
bone, but it is convenient to describe this bone as made up of four
parts: a basal portion (_a_) (=pars basilare=), corresponding to the
basioccipital, two lateral portions (_b_) (=partes laterales=),
corresponding to the exoccipitals, and a squamous portion (_c_) (=squama
occipitalis=) corresponding to the supraoccipital.
The basilar portion (_a_) as seen from the dorsal or ventral surface is
oblong and flattened. It is broadest at the junction of its middle and
last thirds, and tapers toward both ends. It presents a cranial end and
a caudal end, a dorsal, a ventral, and two lateral surfaces. Its caudal
end, which helps to form the ventral boundary of the foramen magnum, is
concave from side to side. The cranial end is transversely elongate,
about five times as broad as high, pointed laterally and roughened for
attachment to the caudal end of the body of the sphenoid, which has a
corresponding form. The joint is a synchondrosis.
The dorsal surface is concave from side to side, forming a longitudinal
groove in which rest the pons and medulla. The concavity is more
pronounced caudad, where the bone is thinner at its middle.
The ventral surface is marked by three parallel longitudinal ridges. One
of them is median and expands caudad into a smooth triangular elevated
area which extends to the foramen magnum. The other two ridges run near
the lateral edges of the bone. Between them and the median ridge the
surface is smooth and is depressed caudad. Laterad of each lateral ridge
is a rough triangular surface overlaid in the natural state by the
medial edge of the tympanic bulla.
The lateral surfaces are smooth and sharp and abut against the petrous
portion of the temporal bone. They pass into the lateral portions of the
occipital caudad.
The =lateral portions= (_b_) of the occipital arise from the caudal
margin of the basilar portion in the transverse plane; a short distance
laterad of the line of junction they turn dorsad at an angle of nearly
ninety degrees. They form the lateral boundaries of the foramen magnum
and pass dorsally into the squamous portion.
The external surface of each presents an elongated elevated spirally
curved surface, the =occipital condyle= (_c_), for articulation with the
atlas. A small part of each condyle is formed from the basal portion.
The two condyles are separated from one another ventrally by a narrow
notch, and each extends (laterad) along the border of the foramen magnum
to a point slightly dorsad of the transverse diameter of the latter.
Laterad of each condyle the bone is elevated into a blunt triangular
projection, the =jugular process= (_f_), which covers the caudal end of
the tympanic bulla. Between the jugular process and the condyle is a
deep depression.
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.