tes, between which there is, in most places a
cancellated or cellular tissue. The external plate is fibrous, the
internal, compact and vitreous. The skull is nearly oval in form, convex
externally, the bone being much thicker at the base than elsewhere, and
it is, in every respect admirably adapted to resist any injury to which
it may be exposed, thus affording ample protection to the brain
substance which it envelops. The internal surface of the cranium
presents eminences and depressions for lodging the convolutions of the
brain, and numerous furrows for the ramifications of the blood-vessels.
The bones of the cranium are united to one another by ragged edges
called _sutures_, which are quite distinct in the child but which in old
age are nearly effaced. Some authorities suppose that by this
arrangement the cranium is less liable to be fractured by blows; others
think that the sutures allow the growth of these bones, which takes
place by a gradual osseous enlargement at the margins. The bones of the
_Face_ are joined at the lower part and in front of the cranium, and
serve for the attachment of powerful muscles which assist in the process
of mastication. Although the soft parts of the face cover the bony
structure, yet they do not conceal its principal features, or materially
change its proportions. The form of the head and face presents some
remarkable dissimilarities in different races.

[Illustration: Fig. 11.
_1_. The first bone of the sternum (breast-bone).
_2_. The second bone of the sternum.
_3_. The cartilage of the sternum. _4_. The
first dorsal vertebra (a bone of the spinal
column). _5_. The last dorsal vertebra. _6_.
The first rib. _7_. Its head. _8_. Its neck. _9_.
Its tubercle. _10_. The seventh or last true
rib. _11_. The cartilage of the third rib. _12._
The floating ribs.]

[Illustration: Fig. 12.
A vertebra of the neck. _1_. The
body of the vertebra. _2_. The spinal
canal. _4_. The spinous process
cleft at its extremity. _5_. The
transverse process. _7_. The interior
articular process. _8_. The
superior articular process.]

THE TRUNK has fifty-four bones, which are as follows: The _Os Hyoides_,
the _Sternum_, twenty-four Ribs, twenty-four _vertebrae_ or bones of the
Spinal Column, the _Sacrum_, the _Coccyx_, and two _Ossa Innominata_.
The _Os Hyoides_, situated at the base of the tongue, is the most
isolated bone of the skeleton, and serves for the attachment of muscles.
The _Sternum_, or br