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48 ANAT03IY. ;

this way the central or marrow cavity is formed, and the partitions or
septa of the medullary spaces become absorbed, especially art)iind the
medullary canal, thereby enlarging it. This absorption may be carried
on until the entire embryonal spongy bone is remov^ed.
III. The Intramembraxous Ossification takes place within
membranes of fibrillar connective tissue, independent of any carti-
laginous formation. It is found within the roof of the brain-case, as
in the i^arietal and frontal bones and portions of the occipital and tem-
poral, and within all the facial bones, except the inferior turbinated and
part of the inferior maxilla. (The base of the skull, the two inferior
turbinated, and part of the inferior maxillary bones are developed
within cartilage.) This development of bone is analogous to that of
periosteal formation, which takes place on or around endochondral
bones.
The parietal bone presents a good example of this development. At
first it is composed of a single fibrous membrane ; next it divides near
its centre into two layers, these eventually becoming the external and
internal periosteums. Between these two layers are numerous inter-
lacing bundles of connective-tissue fibres, making an intervening network
between the two membranes and forming irregular medullary spaces
similar to the partially-absorbed cartilage in endochondral bone-forma-
tion. The bundles of connective-tissue fibres forming the walls of the
medullary spaces become infiltrated with calcic salts ; the spaces them-
selves are occupied by blood-vessels and marrow-tissue, and their walls
lined by osteoblasts, M'hich develop bone, as described p. 42.
As the centre thickens the cleavage of tlie membrane extends toward
the circumference, and bony spicula grow outward in radiating lines until
they meet neighboring bones, with which they miite l)y sutures.
While this process is going on, the two osteogenetic membranes deposit
successive layers of bone, causing an increase in thickness, each layer
becoming more dense, thus forming Avhat are known as the external
and internal plates. Between the two tables of a fully-formed bone is
the cancellated structure or dij)loe. These irregularly-formed spaces
are made through absorption of portions of the bony tissue by osteo-
])orosis. The di])loe is a highly vascular tissue, in which the arteries
of the external and internal periosteum anastomose.
Bones arc divided into four classes— viz. long bones, such as those of
the arm and leg; tahidar or JJ((f bones, as those forming the vault of the
cranium ; irref/uldr bones, such as the vertebrae ; and short bones, as
those of the car])us and tarsus.
Many of the bones are arranged in synnnctrical pairs, one on each
side, as illustrated in the ribs, arms, legs, parietal and temporal bones
while the vertebra?, conunencing with the coccyx, and continuing upward
through the skull with the occipital, sphenoid, ethmoid, frontal, and
vomer, arc single l)()ncs, develojK'd from two symmetrical halves. The
inferior maxilla is usually described as a single bone, but in embryonal
life and in some of the lower animals there are two. In man the
bones on either side of the body are seldom of equal size, those of the
right side usually being slightly larger; their markings, such as processes
and foramina, are also dissimilar in size and shape.