l hollow
forms, receive the vibratory influence of force externally applied. The
brain receives the concussion of the force applied to its osseous
envelope; and when this latter happens to be fractured, the danger to
life is not in proportion to the extent of the fracture here, any more
than elsewhere in the skeleton fabric, but is solely in proportion to
the amount of shock or injury sustained by the nervous centre.

When it is required to trephine any part of the cranial envelope, the
points which should be avoided, as being in the neighbourhood of
important bloodvessels, are the following--the occipital protuberance,
B, within which the "torcular Herophili" is situated, and from this
point passing through the median line of the vertex forwards to Z the
frontal sinus, the trephine should not be applied, as this line marks
the locality of the superior longitudinal sinus. The great lateral sinus
is marked by the superior occipital ridge passing from the point B
outwards to the mastoid process. The central point B of the side of the
head, Plate 21, marks the locality of the root of the meningeal artery
within the cranium, and from this point the vessel branches forwards and
backwards over the interior of the cranium.

The nasal fossae are situated on either side of the median partition
formed by the vomer and cartilaginous nasal septum. Both nasal fossae
are open anteriorly and posteriorly; but laterally they do not, in the
normal state of these parts, communicate. The two posterior nares
answering to the two nasal fossae open into the upper part of the bag of
the pharynx at 8, Plate 20, which marks the opening of the Eustachian
tube.

The structures observable in both the nasal fossae absolutely
correspond, and the foramina which open into each correspond likewise.
All structures situated on either side of the median line are similar.
And the structure which occupies the median line is itself double, or
duality fused into symmetrical unity. The osseous nasal septum is
composed of two laminae laid side by side. The spongy bones, X W, are
attached to the outer wall of the nasal fossa, and are situated one
above the other. These bones are three in number, the uppermost is the
smallest. The outer wall of each naris is grooved by three fossae,
called meatuses, and these are situated between the spongy bones. Each
meatus receives one or more openings of various canals and cavities of
the facial apparatus. The sphenoidal si