Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

J<> ANATOM Y.
Special with a longitudinal interval on the mesial plane, a trans-
^Anatomy^ verse one on the sit;e of the corona] suture) an(j a trian.
The fron- gVlar cha.s™ at the ang]e between the two. This chasm,
tal hone. witb a similar one between the frontal bones, forms a
quadrilateral lozenge-shaped space, at which the motion
of the brain is distinctly felt both at birth and for months
after, and which is therefore named the fontanelle, or the
'dwter'xov fontanelle (fans pulsatilis ; bregma), or the open of
the head. As the ossific process aclvances, the lateral
margins of the bone extend, and the mesial margins ex¬
tending mutually, at length coalesce, first at the nasal
protuberance and along the forehead, afterwards above, un¬
til the fontanelle is progressively diminished and at length
obliterated. This junction is effected by the formation
of serrated processes, which are mutually dove-tailed into
each other; and for some years after birth the frontal
bone consists of two similar halves articulated by a middle
suture. In some few instances, especially in the female,
this continues for many years; and the individual is
found after death to have the frontal bone in two halves,
with a middle suture. More frequently, however, the
suture is obliterated by the consolidation of its serrated
margins, and the frontal bone consists of one piece. The
points of ossification remain long distinct in the form of
the frontal protuberances. But eventually, from the uni¬
form elevation of the margins of the bone, they become
less conspicuous ; and in old age they disappear more or
less completely, leaving a surface uniformly uneven.
The eth- The ethmoid or sieve-like bone (ps cribriforme), which
mold bone. js symmetrical, and occupies the quadrilateral notch
of the frontal bone, consists of several bony plates ar¬
ranged at right angles, and parallel with each other, so
as to give the whole a cubical shape. It consists of four
parts, a horizontal plate, occupying both sides of the
mesial plane; a vertical plate at right angles to it, and
corresponding with the mesial plane; and a lateral plate
on each side, also vertical and parallel to the middle plate.
In the bone thus formed the following circumstances de¬
serve attention. (Fig. 5.)
The superior surface, cerebral, covered by dura mater,
is formed by the horizontal plate, perforated by numerous
holes {lamina cribrosa), through which pass the fibrils of
the olfacient or first pair of nerves depressed longitu¬
dinally on each side, but surmounted in the middle to¬
wards its anterior half by a strong process of a triangular
shape, named the cock’s comb (crista galli), and to which
is attached the anterior inferior extremity of the falx, or
dichotomous membrane. The anterior margin of this
process is generally marked by a groove which, with that
of the frontal bone, forms a passage for the nasal vein into
the longitudinal sinus. The posterior margin of the per¬
forated plate is marked by an angular notch between two
horns, for articulation with a salient angle of the sphenoid
bone. The crista galli may be regarded as the upper di¬
vision of the vertical plate, which occupies the mesial
plane, and which is thick and sometimes bifid before, but
thin and rough behind, where it acts as a partition to the
lateral halves of the ethmoidal cavities. The sides of this
middle vertical plate are furrowed by minute canals (ca-
nalicult), traced from the foramina, above, in which the
nervous fibres are lodged. This plate, and indeed the
lower surface of the perforated plate, are covered by a
fibro-mucous membrane, which has been named the pitui¬
tary or the Schneiderian. The vertical ethmoid plate is
articulated at its lower margin with the vomer and the tri¬
angular cartilage of the nose, before with the nasal spine
of the frontal bone, and behind with the median crest
(processus azygos) of the sphenoid bone.
The lateral portions of the ethmoid bone consist exter¬
nally of a smooth flat bone (os planum of the ancients), Special
which forms the inner or nasal wall of the orbit, internal- Anatomy,
ly of two bones convoluted on themselves, and which are
distinguished as the superior and middle turbinated bones Th® e(th-
(conclue, ossa turbinata superiora et media, ossa spongi-ni0Ul bime'
osa). These bones are seen most distinctly behind; but
to form a correct notion of their figure, it is requisite to
detach the lateral portions and examine them separately,
when the following peculiarities may be recognised. The
ossa plana on each side terminate in concave oblong
quadrilateral plates, diverging outwards from the vertical
plane. At the internal edge of these quadrilateral plates
is seen above a convex bone, with numerous minute per¬
forations, the orifices of short canals. This is the su¬
perior turbinated or spongy bone. Below, and a little to
the external side, is a small groove, separated by a thin
plate from a larger cavity, which is the superior meatus,
leading into the posterior ethmoid cells. Below this,
again, is the osseous plate, with perforated edges turned
on itself from within outwards, so that its convex side is
towards its fellow, and its concavity is below and laterally,
and separated by another thin plate from the lower mar¬
gin of the ossa plana. This is the middle turbinated
bone, the longitudinal cavity of which communicates with
the lower or nasal surface of the bone, and is bounded on
its outer margin by the lower margins of the ossa plana,
where they are articulated with the inner margin of the
orbitar plate of the maxillary bone. The anterior defi¬
ciency of the ossa plana is occupied by the lacrymal
bones.
The internal surface of these bones generally is cover¬
ed by a thin fibro-mucous membrane, partaking of the
characters of periosteum at its attached, and of mucous
membrane at its free surface.
The ethmoid is articulated with the frontal bone, the
sphenoid, the superior maxillary bones, the nasal, the
lacrymal, the palate bones, the inferior turbinated bones,
and the vomer, at the parts already indicated.
In structure the component plates are compact, unless
at the crista galli, which contains some cancellated tissue,
and the middle and superior turbinated bones, which seem
less dense than the horizontal plate.
The ethmoid bone consists in the foetus of loose, soft,
brown-coloured substance, contained in a thick vascular
membrane, and disposed in the cubical shape, but with¬
out the complicated arrangement of convoluted plates by
which it is afterwards distinguished. Into this the ner¬
vous fibrils penetrate, and are observed to be ramified.
This continues at least four or five months after birth,
when these fibrils become surrounded with compact bone
deposited in their interstices, and in this manner the per¬
forated plate is formed by deposition round the nerves.
About the same time, on the mesial plane is observed a
vertical plate, which gradually becomes condensed into
solid bone, in the shape of the crista galli and middle par¬
tition. Soon after, as the bone increases in size, excava¬
tions are formed, and the soft uniform substance is remov¬
ed, while plates of thin but solid bone interposed between
thick vascular membranes are observed to be formed.
The plates, which are slightly convoluted, become thinner
and more solid, and are at length moulded into the supe¬
rior and middle turbinated bones. The ethmoid is gene¬
rally complete about eighteen months after birth; and
about the second year its different component parts may be
recognised. The holes of the perforated plate are, how¬
ever, larger and more numerous at this age than after¬
wards. The minute grooves (canaliculi,) described by Scar¬
pa, in the lateral portions, are also more distinct and larger
than subsequently. It ossifies therefore in four points;