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80 ANATOMY.
into the delicate perpendicular grooves and canals of the perpendicular
plate of the bone ; next and external are those for the tilanients distrib-
uted to the roof of the nasal chambers ; and the outer ones pass into
fine canals which subdivide as they penetrate the lateral masses of the
bone. Some anatomists speak of these holes as forming three rows or
lines, but the irregular arrangement of them makes it an ideal rather
than a definite or distinct description.
The Cerebro-naml 8Ut is innnediately posterior to the ethmoidal
wings at the base of the crista galli. It is a narrow or strait opening
uniting the cranial cavity with the nasal chamber. External to this
above may be seen a groove extending posteriorly to the anterior eth-
moidal foramen. The passage, groove, and foramen are for the accom-
modation of the nasal nerve, a branch of the ophthalmic, and also a
branch of the ophthalmic artery.
Development.—The ethmoid bone arises from three points of ossi-
fication—one for each lateral mass, the other for the perpendicular and
cribriform plates. They are deposited in the orbital plates of the late-
ral masses about the fourth or fifth month, and gradually extend into
the turbinated bones. During the first year it commences to ossify in
the perpendicular and cribriform lamellae, the three parts uniting early
in the second year, ossification being completed during the fourth or
fifth year, at which time the ethmoidal cells commence formation.

Vomer.
The Vomer (Fig. 33) is a single bone, situated in the median line
of the nasal chamber, forming the principal portion of the bony sep-
Fio. 33.

"i'^h Sup -MaX'

tum. It usually is more or less deflected to one or the other side, and
is placed in front of and below the sphenoid bone, with the rostrum of
which it articulates, and below th(! ethmoid bone, articulating with the
perpendicular ])late of the latter. It is a thin plate of bone, rhomboidal
in form, having four borders, superior, inferior, anterior, and posterior;
and two surfaces, a right and a left.