THE SKULL.
The bony structure of the _head_; division into _cranium_
and _face_. Study of the vault, or skull-cap. _Occipital_
bone.--_Parietal_ bone (parietal eminence and temporal
ridge).--_Frontal_ bone (frontal eminences, superciliary ridges:
nasal eminences: supra-orbital arches).--_Temporal_ bone:
mastoid process; zygomatic process.--Sutures of the skull:
sagittal suture; lambdoid suture; sphenoidal sutures.--General
form of skull: long heads; round heads.--Cephalic indices;
dolichocephalic, brachycephalic, and mesaticephalic skulls.
The _skeleton of the head_ is formed of two parts intimately united
to each other: one above and behind, formed for the most part of flat
bones, simple in form, and called the _cranium_, containing the brain:
the other below and in front, consisting of numerous bones, complex in
shape, and constituting the bony structure of the _face_. The facial
bones form the boundaries of the cavities which lodge the principal
organs of sense and the apparatus of mastication.
The _cranium_.--The cranium forms an egg-shaped box with its long axis
directed from before backwards. It may be examined from its _base_,
which we need not study here, or from above or from the side. The
bones which enter into its construction are eight in number, viz. the
_sphenoid_ in the base and side wall, the _occipital_ behind, the
_frontal_ in front, the two _parietal_ above, and the two _temporal_
bones on the sides.
[Illustration: FIG. 56.
THE SKULL (lateral surface).--1, frontal bone;--2, parietal;--3,
occipital;--4, temporal;--5, the great wing of the sphenoid;--6,
coronal suture;--7, lambdoidal suture;--8, 9, parieto-temporal
suture;--10, spheno-parietal suture--11, spheno-temporal
suture;--12, fronto-sphenoidal suture;--13, curved line limiting
the temporal fossa;--14, 15, 16, malar bone;--18, the superior
maxillary bone, with the infra-orbital foramen (19);--20, 21,
22, the bones of the nose;--23, the lachrymal groove;--24, the
nasal eminence;--25, the inferior maxillary bone;--26, the
mental foramen;--27, the angle of the jaw.]
The _occipital_ bone (3, Fig. 56) forms the whole posterior part of the
base and vault of the skull. The bone possesses two distinct parts,
inferior and superior. The _inferior_ part is more or less horizontal,
and is pierced by a large foramen (_foramen magnum_) through which the
cavity of the cranium communicates with that of the vertebral canal.
In front of this foramen is the _basilar process_ of the occipital
bone; upon each side are the _occipital condyles_, by which the skull
articulates with the vertebral column--namely, with the lateral masses
of the atlas (pp. 31, 32). The _superior_ part (3, Fig. 56), called
the _squamous_ or _shell-shaped portion_ of the bone, is more or less
triangular in form, with the apex directed upwards. Its borders are
hollowed out into numerous irregular denticulations, which work in
with similar denticulations on the posterior borders of the parietal
bones (7, Fig. 56); and assist in forming the _lambdoidal suture_.
The external or posterior surface is crossed about its middle by a
_semicircular_ crest (_the superior curved lines_) of which the centre
forms a prominence called the _external occipital protuberance_. This
prominence is placed below the most projecting point of the back of
the skull. The superior curved lines separate the outer or posterior
surface of the occipital bone into an upper smooth part, belonging to
the vault, which is covered by the scalp, and a lower rough surface, to
which the muscles of the neck are attached.
The _parietal_ bones (2 and 13, Fig. 56) are placed on each side of
the vault of the skull in front of the occipital bone. Quadrilateral
in shape, each bone presents four denticulated borders, of which the
superior articulates with the parietal of the opposite side, the
posterior with the occipital (7, Fig. 56), the anterior with the
frontal (6), and the inferior, which is concave (8, Fig. 56), with
the temporal and (slightly) with the sphenoid bone. The parietal bone
presents two points for notice on its external surface--1, near its
centre a projection called the _parietal eminence_, better marked in
young subjects than in the adult, which represents the place where the
ossification of the bone commences; 2, below this prominence a curved
line, the _temporal ridge_, slightly rough (13, Fig. 56), which limits
the temporal fossa and gives attachment to the temporal fascia.
[Illustration: FIG. 57.
FRONTAL BONE (anterior surface).--1, the frontal eminences;--2,
2, superciliary ridges;--3, nasal eminences;--4, 4,
supra-orbital notches;--5, 5, 6, 6, internal and external
angular processes;--7, 7, nasal notches;--8, nasal spine;--9,
9, orbital plates;--11, the superior border;--12, the lateral
borders.]
The _frontal bone_, a single bone (1, Fig. 56), like the occipital,
presents, like it, a vertical and a horizontal part. The latter belongs
to the base of the skull and to the face (9, 9, Fig. 57), and forms
the roof of the cavity of the orbit (see below). The vertical part
forms the anterior and superior wall of the cranium. It has a rounded
superior border (11, Fig. 57) which articulates by its denticulated
edge with the parietal bones and forms the _coronal suture_ (6, Fig.
56). The anterior superficial surface forms the forehead, and upon it
we have to notice the following points:--1, the _frontal eminences_ (1,
1, Fig. 57), better marked in young subjects and in females than in
the adult male; 2, below these, the _superciliary ridges_ (2, 2, Fig,
57), which are directed obliquely downwards and inwards, to end in two
prominent bosses called the _nasal eminences_. These prominences are
best marked in the adult, and owe their prominence to the fact that
the thickness of the bone is hollowed out in this situation into two
cavities called the _frontal sinuses_, which become more developed as
the subject advances in age; 3, at the lower border of this part of the
bone are the _supra-orbital arches_ (4, 4, Fig. 57), at the margin of
the orbit. Curved, with the concavity downwards, each of these arches
forms on the outer side an _external angular process_ (6, 6, Fig. 57),
which articulates with the malar, or cheek-bone (15, Fig. 56), and on
the inner side an _internal angular process_ (5, 5, Fig. 57) which
articulates with the nasal process of the upper jaw. Between the two
internal angular processes is a median rough space called the _nasal
notch_ (7, Fig. 57), into which are received the _nasal_ bones (22,
Fig. 56). Each supra-orbital arch presents, at the junction of its
middle and inner thirds, a small notch called the _supra-orbital notch_
(Fig. 57).
The _temporal bones_, one on each side of the skull (4, 11, and 31,
Fig. 56), are very complex in shape, but we shall study here only one
portion. Each temporal bone is composed of two parts, one belonging
to the base, and another belonging to the lateral wall of the skull;
the part belonging to the base forming a pyramidal mass of very dense
bone, called the _petrous portion_, which contains the delicate organs
of the internal ear; while the part belonging to the lateral wall of
the skull (4, Fig. 56) rises up as an irregular osseous disc from the
base of the petrous portion of the bone. An orifice, the _external
auditory meatus_ which leads into the petrous bone, is found about
its centre (31, Fig. 56). Taking this orifice of the auditory canal
as a centre, we find on the external surface of the temporal bone--1,
behind the auditory canal the _mastoid portion_ of the bone (33, Fig.
56), which articulates with the parietal and occipital bones, and is
prolonged downwards as a conical process in the shape of a nipple
(μαστός, a nipple) called the _mastoid process_; 2, above the auditory
canal, the _squamous portion of the temporal bone_ (4, Fig. 56), a
shell-like bone with a rounded border articulating with the inferior
border of the parietal bone (8, Fig. 56). In front is a process (20,
Fig. 58) which is directed horizontally towards the face and joins the
malar bone (16, Fig. 56). This process, connecting the cranium with the
face, is called the _zygomatic process_ (ζυγός, a yoke), and it forms
with the corresponding part of the malar bone the _zygomatic arch_.
The zygomatic process rises from the temporal bone by two roots (Fig.
57), of which one sweeps backwards above the orifice of the external
auditory canal and helps to form the temporal ridge; the other extends
as a rounded ridge transversely inwards towards the base of the skull,
and as the _eminentia articularis_, forms the anterior limit of the
_glenoid cavity_, into which the condyle of the lower jaw is received.
Finally, below the auditory canal, the temporal bone gives origin to
the _styloid process_ (32, Fig. 56), long and slender, which gives
attachment to certain ligaments and muscles of the neck.
The _sphenoid bone_ is one of the most important in the construction
of the cranium, although as it chiefly occupies the middle of the
base of the skull, it is not very obvious on the surface. It is the
_great wing_ of the sphenoid bone which assists in the formation of the
_temporal fossa_.
The bones of the cranium articulate with one another by denticulated
borders, to which the name of _sutures_ is given. As the artist needs
to study the skull not only with regard to its superficial form,
but also as an _object_ which figures frequently as an accessory in
still-life compositions, and as a true representation of these sutures
contributes to give to the skull its exact physiognomy, we must not
leave the subject without carefully pointing them out. It is necessary
for our purpose to examine the skull on its upper and its lateral
surfaces.
The upper surface of the skull presents a suture in the middle
line, antero-posterior and inter-parietal, called the _sagittal
suture_. Behind, on a level with the superior angle of the occipital
bone (7, Fig. 56), this median suture becomes continuous with the
occipito-parietal suture to which the name of the _lambdoid suture_ is
given-because it resembles the form of a Greek lambda (=Λ=). In front,
the sagittal suture is met by the parieto-frontal suture (6, Fig. 56),
which forms a curved line, to which has been given the name of the
_coronal suture_.
On examining the skull on one of its lateral surfaces, we see that the
sutures form in this situation a more complex design, because of the
articulations of the bones entering into the formation of the _temporal
fossa_ (5, 10, 11, 12, Fig. 56). This fossa is formed by the parietal,
squamous portion of the temporal, frontal, and great wing of the
sphenoid bones. It is limited: below by the zygomatic arch (malar and
temporal), and above by the curved line of the temporal ridge, which
marks the temporal, parietal, and frontal bones. The suture formed
within the fossa, at a point called the _pterion_, is roughly =H= or
=K=-shaped, and is occasioned by the contiguous articulations of the
parietal, sphenoid, frontal, and squamous portion of the temporal bones.
We have said that the general form of the skull is egg-shaped, with its
greater extremity behind. It is, however, a common observation that the
skull may differ remarkably from what may be termed the normal shape.
It may be excessively high, or low; excessively prolonged backwards
or forwards, or the reverse. One of the best-marked and constant
variations is in the ratio of width to length, from which the _cephalic
index_ is derived. This may be an individual peculiarity: for instance,
of two brothers, one may be round-headed, the other long-headed.
Or it may be a racial character: for example, the black races are
long-headed, the yellow races are round-headed, and the white races
have a cephalic index which may be one or the other, or intermediate
between the two extremes.
The cephalic index (index of breadth of the skull) is an important term
in physical anthropology.
As we have previously explained, respecting the proportions of the arm
and forearm (_brachial index_, page 100), we understand by the name of
index the number which indicates the proportion between a short and a
greater length, the latter being considered as equal to 100--that is
to say, the figures obtained by direct measurement being reduced to
a decimal proportion. We find that in certain skulls the transverse
diameter is relatively very short, seeing that it is represented by an
index of 75 or less; and such skulls are known as _dolichocephalic_
(δολιχός, lengthened; κεφαλή, head). Others present a transverse
diameter which approaches the antero-posterior, since it is represented
by an index of 80 or more, the antero-posterior measurement being
100. This cephalic index of 80 or more is called _brachycephalic_
(βραχύς, short). Between these two types are the heads of intermediate
form called _mesaticephalic_, of which the cephalic index is from
75 to 80. The importance of dolichocephalic, mesaticephalic, and
brachycephalic types of skull in relation to racial characters has no
doubt been exaggerated; but the fact remains that certain races are
dolichocephalic, and that certain other races are brachycephalic. A
greater difficulty arises in regard to mesaticephalic skulls, which
are, after all, in a sense only “averages,” and which, occurring as
they do so commonly in the Indo-European race, suggest that either
they are due to individual peculiarity or to the mingling of racial
characters--in either case weakening the value of skull measurements in
the investigation of races.