d of the thigh-bone
sawn across, showing its central
cavity.]

The bony plates form the boundaries of the _Haversian_, or nutritive
canals of the bones. In the _second stage of ossification_, the
cartilage corpuscles are converted into bone. Becoming flattened against
the osseous lamellae already formed, they crowd upon one another so as to
entirely obliterate the lines that distinguish them; and, simultaneously
with these changes, a calcareous deposit takes place upon their
interior. Bones grow by additions to their ends and surfaces. In the
child, their extremities are separated from the body of the bone by
layer of cartilage, and the cancellated, or cellular structure, which
remains for a time in the interior, represents the early condition of
the ossifying substances.

The bones contain more earthy matter in their composition than any other
part of the human body, being firm, hard, and of a lime color. They
compose the skeleton or frame work, and, when united by natural
ligaments, form what is known as the _natural_ skeleton; when they are
wired together, they are called an _artificial_ skeleton. The number of
bones in the human body is variously estimated; for those regarded as
single by some anatomists are considered by others to consist of several
distinct pieces. There are two hundred distinct bones in the human
skeleton besides the teeth. These may be divided into those of the Head,
Trunk, Upper Extremities, and Lower Extremities.

[Illustration: Fig. 10.
The bones of the skull separated. _1_. Frontal,
only half is seen. _2_. Parietal. _3_. Occipital, only
half is seen. _4_. Temporal. _5_. Nasal. _6_. Malar.
_7_. Superior maxillary (upper jaw). _8_. Lachrymal.
_9_. Inferior maxillary (lower jaw). Between
_4_ and _6_ a part of the sphenoid or wedge-shaped
bone, is seen. Another bone assisting to form
the skull, but not here seen, is called the _ethmoid_
(sieve-like, from being full of holes), and is situated
between the sockets of the eyes, forming the
roof of the nose.]

THE BONES OF THE HEAD are classed as follows: eight belonging to the
Cranium, and fourteen to the Face. The bones of the Cranium are the
_occipital_, two _parietal_, two _temporal, frontal, sphenoid_, and
_ethmoid_. Those composing the face are, the two _nasal_, two _superior
maxillary,_ two _lachrymal_, two _malar_ two _palate_, two _inferior
turbinated, vomer_, and _inferior maxillary_. The cranial bones are
composed of two dense pla