THE CRANIUM AND FACE.
The intelligence and all the special senses, except the sense of touch
already spoken of, are gathered together compactly in the head, where
they are carefully protected with bony tissue. Covering the brain is
the skull or cranium, which is made up of eight bones, the frontal, the
occipital, two parietal, two temporal, the sphenoid, and the ethmoid,
while the bones of the face are fourteen in number, two nasal, two
superior maxillary, two lachrymal, two malar, two palate, two inferior
turbinated, the vomer, and the inferior maxillary. For the most part
the bones are arranged in pairs, one on either side.
=The Cranial Bones.=--The cranium or skull is especially adapted for
the protection of the brain and the bones are flat and closely fitted
to its surface. They have two layers of bone, the outer and the inner
tables, of which the outer is the thicker, and between these is a
tissue filled with blood-vessels, the _diploë_. In the infant, whose
brain has not yet attained its full size, opportunity must be left
for growth and the skull therefore consists of a number of bones with
interlocking notched edges, where growth takes place, but in the adult
it forms one solid covering of bone.
The line where the edges of two cranial bones come together is called
a _suture_. The suture between the frontal bone and the forward edges
of the two parietal bones is called the _coronal suture_, that between
the two parietal bones at the vertex of the skull is known as the
_longitudinal_ or _sagittal suture_, and that between the occipital
bone and the back edges of the parietal bones as the _lambdoidal
suture_.
Where the coronal and sagittal sutures meet is a membranous interval
known as the _anterior fontanelle_, while the _posterior fontanelle_
is at the juncture of the sagittal with the lambdoidal suture. These
fontanelles--so called from the pulsations of the brain that can
be seen in them--close after birth either by the extension of the
surrounding bones or by the development in them of small bones known
as _Wormian bones_, the posterior one closing within a few months,
the anterior by the end of the second year. In rickets, however, the
anterior fontanelle remains open a long time, sometimes into the fourth
year.
[Illustration]
[Illustration: FIG. 15.--Cranium at birth, showing sutures and
fontanelles.]
The =frontal bone=, as its name implies, forms the fore part of the
head or forehead. It joins the parietal bones above and the temporal
bones on either side. At the lower edge are the supra-orbital arches,
each with a supra-orbital notch or foramen on its inner margin for
the passage of the supra-orbital vessels and nerve, the nerve most
affected in neuralgia. Just above the arches on either side are the
superciliary ridges, behind which, between the two tables of the skull,
lie the frontal sinuses. On the inner surface the frontal sulcus for
the longitudinal sinus runs along the median line.
The =parietal bones= are the side bones of the skull. They meet each
other in the sagittal suture at the median line above and join the
frontal and occipital bones at either end, while below they touch upon
the temporal bones, the temporal muscles being attached in part along
their lower surface. These muscles are inserted into the coronoid
process of the lower jaw, which they thus help to raise and to retract.
[Illustration: FIG. 16.--Front view of the skull. (After Sobotta.)]
The =occipital bone= is at the base of the skull and at birth consists
of four pieces. In the lower, anterior part is the foramen magnum, an
oval opening through which the spinal cord passes from the skull down
into the spinal canal. Half way between the foramen and the top of the
bone is the external occipital protuberance for the attachment of the
ligamentum nuchæ which holds the head erect. The inner side of the bone
is deeply concave and is divided by a cross-shaped grooved ridge into
four fossæ, the internal occipital protuberance being situated where
the arms of the cross meet. The occipital lobes of the cerebrum lie in
the two upper fossæ and the hemispheres of the cerebellum in the two
lower ones. In the grooves upon the ridge are the sinuses which collect
the blood from the brain.
The occipital and frontal muscles, united by a thin aponeurosis, cover
the whole upper cranium and are known as the _occipito-frontalis
muscle_. At the back this is attached to the occipital bone, while
in front it interlaces with various face muscles. It is a powerful
muscle and raises the brows, wrinkles the forehead, and draws the scalp
forward. Long hair grows on the skin over it as a further protection
against blows upon the skull and sudden variations in temperature.
The =temporal bones=--said to be so named because the hair over them
is the first to turn with age--are situated at the sides and base of
the skull and are in three portions: the squamous or scale-like, the
mastoid or nipple-like, and the petrous or stony portion. The squamous
is the upper portion and has projecting from its lower part the long
arched zygomatic process, which articulates with the malar bone of the
face and from which arises the _masseter muscle_, one of the chief
muscles of mastication, which has its insertion in the ramus and angle
of the lower jaw. Just above the zygomatic process the _temporal
muscle_ has its origin in part, while below is the _glenoid fossa_ for
articulation with the condyle of the lower jaw, the posterior portion
of the fossa being occupied by part of the parotid gland.
The rough mastoid portion of the temporal bone is toward the back and
affords attachment to various muscles, of which the most important are
the occipito-frontalis and the sterno-cleido-mastoid. Within it are the
mastoid cells, which communicate with the inner ear and are lined with
mucous membrane continuous with that of the tympanum. They probably
have something to do with the hearing. In children they often become
the seat of inflammation (mastoid abscess) in infectious diseases and
the mastoid bone has to be cut to let out pus that has collected.
As the lateral sinus is directly behind the mastoid bone, there is
very great danger of going through into the sinus and causing a fatal
hemorrhage.
[Illustration: FIG. 17.--Side view of the skull. (After Sobotta.)]
The petrous portion, which contains the organ of hearing, is between
and somewhat behind the other two portions, at the lower edge of the
temporal bone, wedged between the sphenoid and the occipital bones. On
its outer surface is the _external auditory meatus_, and from below
projects a long sharp spine called the _styloid process_, to which
several minor muscles are attached. In the same angle between the
petrous and squamous portions lies the bony Eustachian tube.
The =sphenoid= or =wedge bone=, so called because in the process of
development it serves as a wedge, lies at the base of the cranium,
forming as it were the anterior part of the floor of the cavity
containing the brain. It is a large, bat-shaped bone and articulates
with all the cranial and many of the facial bones, binding them all
together. It has a body, two large wings, and two lesser wings and,
appears on the outside of the skull between the frontal and the
temporal bones behind the zygomatic process. In the adult the body of
the sphenoid is hollowed out into the sphenoid sinuses, in which pus
sometimes forms.
=The Ethmoid Bone.=--In front of and below the sphenoid and extending
forward to the frontal bone is the ethmoid, the last of the cranial
bones. It consists of a horizontal cribriform or sieve-like plate, from
either side of which depend lateral masses of ethmoid cells. To the
inner side of these masses are attached the thin curved _turbinated
bones_, superior and middle, while between them is a vertical plate
that forms the bony septum of the nose. Rising from the upper surface
of the cribriform plate is another vertical plate, the _crista galli_,
with the olfactory grooves on either side for the reception of the
olfactory bulbs, filaments of the olfactory nerve passing down through
the perforations of the cribriform plate to the nose. For the brain,
which fills almost the entire cavity of the cranium, is supported
by the sphenoid and ethmoid bones internally, as it is protected
externally by the other cranial bones.
=Ossification of Sutures.=--If premature ossification of all the
sutures occurs, _idiocy_ results, while in _cephalocele_ there is a gap
in the ossifying of the bones so that the membranes or brain protrude.
In _rickets_ the forehead is high and square and the face bones
poorly developed, so that the head looks larger than it really is. In
_Paget’s disease_ the bones enlarge and soften. This affects the head
but not the face and often the first thing noticed is that the hat is
too small. _Craniotabes_ is thinning of the bone in places, the bone
becoming like parchment and being easily bent. It is generally caused
by pressure of the pillow or the nurse’s arm.
=Bones of the Face.=--The facial bones serve to form the various
features of the face, which after all are merely organs of special
sense. Many delicate muscles control the facial expression which,
consciously or unconsciously, reflects the character of their owner.
Surgically the most important of the facial bones are the two _superior
maxillary bones_, because of the number of diseases to which they
are liable. They meet in front, together forming the upper jaw, and
with the malar bone help form the lower part of the orbit of the eye.
They are cuboid in shape and are hollowed out into a pyramidal cavity
called the _antrum of Highmore_, which opens by a small orifice into
the middle nasal meatus and which sometimes becomes infected and has
to be tapped. The nasal process for articulation with the frontal and
nasal bones has, at its lower edge, a crest for the inferior turbinated
bone, and close beside this on the inside, extending down from the
upper edge, is a deep groove which, with the lachrymal and inferior
turbinated bones, helps to form the lachrymal canal for the nasal tear
duct. The bones give attachment to many small muscles, connected for
the most part with the nose and mouth, of which the masseter is the
only important one.
The two _malar_ or _cheek bones_ are small quadrangular bones, which
form the prominences of the cheeks and help form the orbits of the
eyes. Projecting backward from each is a zygomatic process for
articulation with the zygomatic process of the temporal bone, while a
maxillary process extends downward for articulation with the superior
maxillary. Here again the most important muscle attached is the
masseter. If the malar bone is crushed great deformity results.
The _lachrymal bones_ are two small bones, about the size and shape of
a finger-nail, situated at the front of the inner wall of the orbit. At
the external edge is a groove which lodges the lachrymal sac above and
forms part of the lachrymal canal below.
The two _palate bones_ are at the back of the nasal fossæ and help to
form the floor of the nose, the roof of the mouth, and the orbit. Each
has a vertical and a horizontal plate, and it is these last that by
their juncture form the _hard palate_. Oftentimes in cases of hare-lip
_cleft palate_ also occurs, the result of incomplete development. To
remedy the consequent opening in the roof of the mouth, which makes
articulation difficult, operation is generally resorted to, though
sometimes a plate is fitted over the opening by a dentist.
The _nasal bones_ are two small oblong bones which articulate with the
frontal and superior maxillary bones and with each other. They form the
bridge of the nose, the rest of the nose being wholly of cartilage,
except for the _vomer_, a bone shaped like a plough-share, which forms
part of the nasal septum, articulating along its anterior edge with the
ethmoid and the triangular cartilage.
The two _inferior turbinated bones_ lie along the outer walls of the
nasal fossæ. They are thin scroll-like bones covered with mucous
membrane and serve to heat the air as it passes in. Sometimes when one
has a cold, the membrane and the bone too swell up and close the nares.
Loss of the sense of smell in a bad cold may be due to such swelling
and the consequent impeding of the entrance of odoriferous particles--a
condition that would likewise interfere with the sense of taste. Part
of the bone is sometimes removed, to enlarge the passage, enough being
left to warm the air.
Lastly, there is the _inferior maxillary bone_ or _lower jaw_. This
has a horseshoe-shaped body and two _rami_, one at either end. Each
ramus has a pointed process in front called the _coronoid process_,
into which is inserted the temporal muscle. At the back, and separated
from the coronoid process by the sigmoid notch, is the _condyle_, which
articulates with the glenoid fossa on the temporal bone. The rami also
give attachment to the masseter muscle at its point of insertion. In
adult age the ramus is almost vertical but in old age the portion of
the jaw hollowed out into alveoli for the teeth becomes absorbed and
the angle of the jaw becomes very obtuse. On the inner side of the jaw
near the middle on either side is the fossa for the _sublingual gland_,
while the _submaxillary gland_ lies in a fossa farther back on either
side.
Sometimes the lower jaw is dislocated and when once this has occurred
it is liable to occur again, the ligaments becoming stretched.