The primitive embryonal skeleton consists of cartilage and fibrous
tissue, in which the bones develop. The process is termed =ossification=
or =osteogenesis=, and is effected essentially by bone-producing cells,
called =osteoblasts=. It is customary, therefore, to designate as
=membrane bones= those which are developed in fibrous tissue, and as
=cartilage bones= those which are preformed in cartilage. The principal
membrane bones are those of the roof and sides of the cranium and most
of the bones of the face. The cartilage bones comprise, therefore, most
of the skeleton. Correspondingly we distinguish =intramembranous= and
=endochondral ossification=.
[Illustration:
FIG. 3.—LEFT FEMUR OF YOUNG PIG, EXTERNAL VIEW, TO SHOW DIVISION OF A
LONG BONE INTO SHAFT (_s_) AND EXTREMITIES.
Proximal extremity consists of two parts, head (_h_) and trochanter
major (_t. m._), which have separate centers of ossification. Distal
extremity consists of trochlea (_t_) and condyles (_c_); _e.l._,
epiphyseal cartilages; _s.f._, supracondyloid fossa.
]
In intramembranous development the process begins at a definite =center
of ossification= where the cells (osteoblasts) surround themselves with
a deposit of bone. The process extends from this center to the periphery
of the future bone, thus producing a network of bony trabeculæ. The
trabeculæ rapidly thicken and coalesce, forming a bony plate which is
separated from the adjacent bones by persistent fibrous tissue. The
superficial part of the original tissue becomes periosteum, and on the
deep face of this successive layers of periosteal bone are formed by
osteoblasts until the bone attains its definitive thickness.
In endochondral ossification the process is fundamentally the same, but
not quite so simple. Osteoblasts emigrate from the deep face of the
perichondrium or primitive periosteum into the cartilage and cause
calcification of the matrix or ground-substance of the latter. Vessels
extend into the calcifying area, the cartilage cells shrink and
disappear, forming primary marrow cavities which are occupied by
processes of the osteogenic tissue. There is thus formed a sort of
scaffolding of calcareous trabeculæ on which the bone is constructed by
the osteoblasts. At the same time perichondral bone is formed by the
osteoblasts of the primitive periosteum. The calcified cartilage is
broken down and absorbed through the agency of large cells called
=osteoclasts=, and is replaced by bone deposited by the osteoblasts. The
osteoclasts also cause absorption of the primitive bone, producing the
marrow cavities; thus in the case of the long bones the primitive
central spongy bone is largely absorbed to form the medullary cavity of
the shaft, and persists chiefly in the extremities. Destruction of the
central part and formation of subperiosteal bone continue until the
shaft of the bone has completed its growth.
A typical long bone is developed from three primary centers of
ossification, one for the diaphysis or shaft and one for each epiphysis
or extremity. Many bones have secondary centers from which processes or
apophyses develop.
The foregoing outline accounts for the growth of bones except in regard
to length. Increase in length may be explained briefly as follows:
Provision for continued ossification at either end of the diaphysis is
made by a layer of actively growing cartilage—the =epiphyseal
cartilage=—which intervenes between the diaphysis and the epiphysis. It
is evident that so long as this cartilage persists and grows, new bone
may continue to be formed at its expense, and increase of length is
possible. When the epiphyseal cartilage ceases to grow, it undergoes
ossification, the bone is consolidated, and no further increase in
length is possible. This fusion takes place at fairly definite periods
in the various bones, and it is of value to know the usual times at
which it occurs in the larger bones of the limbs at least.
After the bones have reached their full size, the periosteum becomes
relatively reduced and inactive so far as its osteogenic layer is
concerned; the bone-forming function may be stimulated by various
causes, as is well seen in the healing of fractures and the occurrence
of bony enlargements.