oi the head by the nasals anteriorly and laterally, and by the frontals posteriorly,
while inferibrly it is bounded by the palato-Yomerine externally, by the premaxil-
laries anteriorly, and by the approximated inner margins of the palato-vomerine
which cover the point of the parasphenoid. The cartilage which fills up this
opening divides at its anterior extremity and into two short prooesses which are
attached to the posterior nasal prooesses of the premaxillaries, enclosing a small
space.
The palato-vomerines meet anteriorly and then slightly diverge posteriorly,
reaching as far backwards as the hinder border of the pterygoid, resting on each
side of the parasphenoid and having their inner margins covered with teeth.
Bones o f the skull.—The premaxillary (figs. 26 a and b) consists of two
portions, a vertical and horizontal. The former articulates with its fellow and the
nasal, and the latter with the palato-vomerine and maxillary. The vertical portion
of the bone is prolonged backwards as a rather thick plate with a deep pit on its
inner surface and a notch at its posterior border, into which the cartilaginous nasal
septum fits by its divided anterior extremity. When the bones are in position,
a well marked fossa results from the union of the two pits. At the base of the
vertical process on its internal nasal surface a small foramen perforates the bone
and opens externally at the inner angle of the nostril in a well defined groove
which runs round this part of the opening and contains a nutritive vessel.
The nasal (figs. 27 a and 27 b) is a flat triangular bone carrying on its anterior
border the foremost part of the cranial crest. Its anterior margin is arched,
corresponding to the upper border of the bony nostril.
The prefronto- lachrymal (figs. 18 a and 18 b) lies posterior to the nasal, and forms
the outer third of the roof of the nasal passage and the anterior angle of the
orbit. The orbital portion is perforated by a foramen, which opens into the nasal
cavity, probably for the transmission of the nasal branch of the fifth pair of nerves.
The maxillary (figs. 37 and 38) is long and laterally flattened in its anterior
third to form the side .of the face and narrow posteriorly. I t stretches from the
premaxillary to the quadrate. The tip of the pterygoid is in contact with its
orbital margin. The dilated anterior portion consists of a naso-facial and a nasopalatine
plate, the former extending between the anterior angle of the. orbit and
the nostril forms a flat facial surface and the lateral wall of the nasal passage, while
the latter constitutes the outer part of the nasal cavity and a small portion of the
palatine surface anterior to the internal nostrils, the external angles of which are
defined by it. The dentary ridge ceases half-way between the facial plate and
the orbital process, which juts out to meet the pterygoid and defines the posterior
limits of the orbit. The external surface is covered with ridges and minute
grooves. I t articulates with the premaxillary, nasal, palato-vomerine, prefronto-
lachrymal, quadrate, and pterygoid.
The palato-vomerine plate (fig. 34,) is a thin flat] axe-shaped bone, forming the
greater part of the floor of the nostrils and defining their limit posteriorly and
interiorly. I t articulates anteriorly with the premaxillary and maxillary, but the
internal margins of the axe-like heads of the two bones are widely separated from
each other anteriorly for the reception of the cartilage. Behind this, they are
nearly in contact for a short way, but they become widely divergent posteriorly.
The posterior external border of the bone is marked by a deep notch, which
defines the internal border of the inner nostril. The internal margin of each plate
is roughly serrated, the serrations corresponding to the positions of the palatine
teeth. The divergent portions of the plate lie on and between the parasphenoid
and orbito-sphenoid and on the former as far as the pterygoid.
The frontal (figs. 29 a and 29 b) is by far the largest bone of the skull, and
articulates with the nasals, prefronto-lachrymals, temporal, parietals, and orbito-
sphenoids. I t consists of two portions, a flattened body and an appended
orbital process which enters into the middle of the upper wall of the orbit. The
body is slightly convex externally, and the frontal suture carries one-half of the
central cranial crest. The orbital process is covered by a rough osseous surface,
that part of the lateral cranial crest which intervenes between the prefronto-
lachrymal and the squamosal, and it forms the external margin of the temporal
fossa.
The parietal (figs. 30 a and 30 b) is an almost square bone and very much smaller
than the frontal, with its posterior external angle curved upwards to rest on the
temporal and arch of the -exoccipitals. The middle is occupied by the median
cranial crest, which begins near the posterior extremity of the bone, as a prominent
raised ridge.
The exoccipital and auditory capsule (figs. 31a, 315, and 31c) Gonsist of one solid
bone, which forms a large chamber for the organ of hearing and lodges nearly one-half
of the brain. I t presents, however, in its fenestra ovalis, semicircular canals and
condyles, a series of prominent landmarks which enable it to be mapped out.
I t may be described as consisting of two portions, one embracing the exoccipital
condyle and an arched process immediately anterior to that articular surface
formiiig the lateral and upper wall of the brain cavity, posterior to the parietals,
and another, the greater of the two, in which the auditory chamber and its parts
are lodged.
These two portions are defined thus: a ridge, with a groove on its under side,
begins on the superior anterior margin of the fenestra ovalis, and is prolonged
onwards and forwards over the condyle and along the posterior margin of the
arched process of the exoccipital to its tip, from whence it can be traced along
its anterior margin, as a faint line. Another ridge springs from the inferior,
anterior border of the fenestra ovalis and runs forwards and inwards, nearly to the
foramen magnum, and then outwards and forwards on the under surface to"
almost the anterior termination of the cranial floor. I t is true that the latter ridge
defines the surface of the cranial floor covered by the parasphenoid; and as this
area, from its relations to the condyles, may be regarded as exoccipital, may not
all the remaining portion of the capsule external to it and entering into the formation
of the floor of the auditory chamber be considered as the opposite element,