where they bend forward to meet the squamous and parietals. The precondyloid
foramen is large and well-developed, with the cartilaginous stylohyal rod firmly
applied to it sideways and bending round the posterior extremity of the, tympanic,
from behind forwards, and between it and the paroccipital process to the periotic.
The facial nerve is seen coming out through a foramen formed by a notch in the
anterior margin of the exoccipital, immediately external to the cartilage aforementioned
and the paroccipital process.
The exoccipitals ( PI. XL, figs. 3 and 4), in a young skull in which they are
quite distinct, do not form the upper margin of the foramen magnum; hut the
condyles are almost entirely formed by them, only a fifth share being contributed
by the basi-occipital. The anterior exoccipital face is- concave internal to the
transverse ridge and has the precondyloid notch immediately internal to the paroccipital
process. The notch in the skull is converted into a foramen. The transverse
ridge terminates above the inner border of the paroccipital process on a line with
the anterior basi-occipital angle of the bone; the whole of the paroccipital process
below that level is outside and deeply concave, especially at its lower margin,
where a distinct pit is formed. A small but deep fossa occurs external to the
lower end of the transverse ridge, separated by a fine ridge from the paroccipital
concavity, into which the posterior superior external angle of the periotic abuts.
The lower concavity faces the posterior surface of the periotic and pars mastoidea
and the hinder aspect of the tympanic cavity, being separated from these bones
by the stylohyal cartilage passing in behind the tympanic bone and posterior to the
facial nerve, which is protected anteriorly by the paroccipital process.
In a skull measuring 12 inches long, in which the supra-occipita! has not been
amalgamated with the exoccipitals, the latter bones are completely united to the
basi-occipital; .and in this skull there is an imperfection of ossification representing
the posterior condyloid foramina, which are [almost always present either on one or
both sides associated with a thinning in this region where they do occur.
The paroccipital process becomes much thickened with age, and in an adult
male its external margin below is grooved for the passage of the facial nerve, while
its anterior aspect is much roughened; the concavity all but disappearing, the lower
pit being occasionally converted into a short canal by a broad spiculum of bone
arching over it.
The internal surface of the paroccipitals in the adult skull has the transverse
ridges greatly developed and grooved for the lateral sinuses. Immediately external
to the occipital protuberance, they are indented on their under surfaces by a rather
deep pit, which is overhung above by their sharp edges. This pit is continuous with
a broad and deep furrow that lodges the posterior occipital sinus. There is no trace
of an internal occipital crest nor of a falx-cerebelli.
Basi-occipital (PI. XL, fig. 5).—The supra-occipital would appear to unite
with the exoccipitals about the same time that the basi-occipital amalgamates with
the basisphenoid. In the very young skull it is perfectly flat on its under surface
between the well-developed tympanic borders, with the exception of a thickened
portion on its anterior fourth. This thickening becomes much intensified with age
and deeply concave on its posterior aspect. The tympanic borders also become
strongly developed as the animal increases in years, and it appears evident that, as
the basisphenoid and the anterior thickened portion of the basi-occipital constitute
the greater part of the upper wall of the posterior sinus, the increased thickness
of this portion of the basicranial axis of the skull is for the protection of the
brain against the pressure which it would sustain in the respiratory effort were its
structure in this portion of a less solid nature.
The posterior three-fourths of the bone is very much thinner than in its anterior
fourth. In the very young example captured in August, the bone is rather
deeply concave in the mesial line for the reception of the medulla oblongata, with
a concave depression on its sides for the reception of the lobes of the cerebellum.
The concavity for the medulla is marked by two posteriorly divergent grooves,
probably arterial. On the anterior extremity of this surface there is a small,
well-marked depression, apparently for the reception of the pituitary body. In
the foetal skull the pituitary body is true to its position on the hinder end
of the basisphenoid, as is demonstrated by the fact that a pin passed through
the middle of the cartilage separating the two bones appears on the inner surface
of the base of the skull posterior to the pituitary fossa. The remarkable change
of position which overtakes this fossa is, however, inaugurated at a very early
period. Passing outwards and backwards from the sides of the pituitary depression
there is a faint ridge which terminates in a strong process of bone, which is
transverse to the basicranial axis and defines on either side the anterior limit of the
cerebellar lobe. This transverse process is separated by a transverse incision from
another but less prominent process lying immediately below it. These two processes
contribute to define the anterior border of the foramen laeervm posterius.
With increasing age the uppermost process is prolonged inwards to near the
median line of the skull, constituting two false, posterior clinoid processes,
defining the hinder border of the pituitary fossa. In some skulls I have observed
only one on the right side, which is the process usually most strongly marked.
The anterior surface of the base of this process to the free margin of the basi-occipital
external to the pituitary fossa, presents a rough triangular surface on which
lies the posterior angle of the alisphenoid. With age these two united portions of
the two basicranial bones become enormously thickened, projecting outwards and
slightly backwards.
At the anterior angle of the exoccipital surface, there are two processes, a
superior and inferior, the former contributing to form the anterior surface of the
base of the process that defines the front border of the anterior condyloid foramen,
the latter being the hinder termination of the downwardly and outwardly projecting
plate against which the tympanic abuts. This plate terminates anteriorly
in a process for the support of the basisphenoid, being applied to the inner
surface of the base of the corresponding plate of that bone. The anterior surface
presents a transversely elongated concave rough surface, which becomes united to