and the region superior and anterior to the ex-occipital as the pro-otic? This view
would seem to he supported by an examination of the relation of the constituent
parts of the: capsule. The exoccipital lies wedged in between the two elements.
I t is just possible that the sides of this, part of the cranial cavity may he formed
by the pro-otic, but the apparent overlapping of the exoccipital by the pro-otic
appears to negative this view of their relations. The position of the fenestra ovalis,
and the rudiments of the stapes and of the semi-circular canals and other important
structures, would appear to establish the foregoing opinion regarding the nature of
this anditory capsule, and which will now he described in detail.
Fenestra ovalis.—The general direction of this structure is backwards and outwards,
and it is placed somewhat external to the exoccipital condyle, from which it
is separated by the foramen of the 8th pair of nerves. I t is a short cylinder with
a distinctly rounded orifice, closed by a membrane containing a small bony nodule,
evidently the stapes. The inner surface of the lower wall of the tube is on a level
with the floor of the auditory cavity, and the internal opening is placed a little
behind the centre and immediately below the inner opening of the semi-circular
canals which lie in the roof of the chamber.
Semi-circular canals.—These structures can be detected on the external surface
of the roof of the chamber as two rounded ridges, and their presence can be at once
demonstrated by a vertical section of the capsule, or by cutting the roof away
gradually (fig. 31c). They are on the same* level and close to the roof of the
chamber. Their construction is very simple. A flat process arches in from either
side of the chamber to the roof, and both meet in the centre. A canal is thus
formed between the upper surface of each process and the roof. The floor of the
anterior canal is not so broad as the posterior one, which is slightly curved outwards
and downwards and forwards, while the former is simply forwards and outwards.
Their external openings are on a line with the external border of the fenestra ovalis,
and their whole length occupies about the middle half of the total transverse, length
of the chamber. Their.extemal openings overlook a crypt-like depression or sacculus ,
in the outer extremity of the chamber, which probably represents the cochlea. On *
a vertical section of the dry capsule, its cavity is seen to have been filled with a
pure snow-white friable substance resembling finely-ground r.tmlTr invested by a
delicate membrane. In a specimen which had been preserved in alcohol, it is a
shrivelled and contracted mass suspended in the centre of the chamber from the
internal openings of the semi-circular canals. The sac is doubtless the equivalent
of the utricle, and it is probable that the white contents are calcareous.
On the under surface of the capsule, external to the parasphenoid ridge, there
are three, or sometimes more, small foramina on the anterior and inner side of thé
inferior pterygoid process. Two of them communicate and open externally, but the
other passes through the substance of the floor of the capsule and opens into the
cranial cavity close to a foramen, which appears to transmit the acoustic branch of
the 7th nerve. If this is the course of the facial nerve, we have a very clear indication
of the homologies of this capsule. The anterior extremity of the pro-otic
element is a pointed process with a foramen for the 5th nerve near its extremity.
There are two other foramina immediately above it, but they are frequently united
into one. They appear to be for the passage of blood-vessels as the roof of the
cranial cavity within, and is covered with a dense plexus.
The exoccipital forms the wall of the cranium internal to the auditory capsule,
and a small part of the floor on each side of the parasphenoid. A narrow plate of
bone rises immediately behind the condyle, bending upwards, forwards and inwards
over the cavity, forming the superior margin of the foramen magnum. The base of
this arched process springs, as already stated, from the outer wall of the fenestra
ovalis, so that only the narrow line along the, posterior margin of the ridge is exoccipital,
while all anterior to it superiorly is pro-otic.
Each exoccipital, as a rule, has two articulating surfaces (fig. 25) for the 1st
vertebra, an external one for the lateral facet and an internal one for articulation
with the so-called odontoid process. The latter surface is a true articulation,
and it is confluent sometimes with the former^ producing an elongated facet.
In cases where the odontoid process is not divided, each exoccipital carries a facet
and a half. The lateral facet is directed downwards, backwards and inwards, the
concavity being from without inwards, and the odontoid surface upwards and backwards
with a prominent lower margin.
The condyle is connected to the, fenestra ovalis by a ridge, which, along with the
superior one, marking the limits of the pro-otic, encloses the foramen of the 8th pair
of nerves. There is a prominent notch in front of the odontoid articulation, which
receives a rounded flattened process on the encephalic aspect of the parasphenoid.
Its posterior margin sometimes separates the odontoid articulations from each other.
The parasphenoid intervenes between the exoccipitals and forms the greater part of
the cranial floor.
The two anterior of the three foramina on the wall of the cavity above, and
slightly before the parasphenoid notch of the exoccipital, lie in a depression common
to both; but whilst the posterior of these two foramina passes directly into the
auditory chamber, the other pierces the substance of the opisthotic and opens below
at the base of the pterygoid. I t appears to enclose the facial branch of the 7th
nerve, while the other transmits a nerve which also probably enters this simple
ear labrynth by the posterior foramen.
The parasphenoid (fig. 33) is a long, rather flat, thin bone, stretching from, the
posterior margin of the palatine canal to the inferior margin of the exoccipital
foramen, into which it enters. I t has a slight constriction in its middle, corresponding
to the middle of the cranial cavity. The posterior portion is broader than the
anterior, and forms, as usual in this group of animals, the greater part of the floor of
the cavity posterior to the frontals. I t is feebly convex below and concave above, with
a well-defined margin for the articulation of the orbito-sphenoid, and a broader one
posterior to this on which the exoccipital rests. Close to. its posterior border, there is
a central rounded flattened nodule which fits into the precondyloid notch of the exoccipital,
and sometimes sends a process between the odontoid facets of the two bones.
m 5