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OSSIFICATION. 585

III. tSub-periosteal Bone-fonnation.—As the name implies, this form
of development takes place underneath the periosteum, and is, of neces-
sity, a later product tlian interstitial development. We have seen that
repair of bone after caries is generally due to this mode of growth.
The vascular supply for the nourishment of bones is largely located
in the periosteum. Small branches of the larger trunks of vessels,
which are found in tlie periosteum, penetrate the cortical portion and
form anastomosing loops with other branches which are found in the
marrow-cavity. It is thus that Haversian canals are formed at right
angles to the surface of the bone, and are seen to radiate toM'ard the
Fig. 325.

Section of Phalangeal Bone of Human Fcetus, 5 months (magnified about 7o diameters) : o-, the cartilage-
cells iu the centre, midway between the epiphyses, are enlarfjed and separated from one another
by a dark-looking calcified matrix ; im, layer of bone deposited underneath the periosteum ; «,
layer of osteoblasts by which the layer has been formed. Some ol the osteoblasts are already
embedded in the new bone as bone-cells in the lacuna. The cartilage-cells are becoming enlarged
and flattened, and are arranged in rows above and below the calcified centre. At the ends of the
cartilage the cells are small, and the groups are irregularly arranged ; the cartilaginous heads are
surrounded by perichondrium.
centre like the .spokes of a wheel, following no regular cour.se, but
winding here and there, sometimes crossing and recrossing one anotiier.
In the shaft of a lono: bone the Haversian svstems follow the line of