formed. In
milder cases, affected and necrotic tissue is removed by a process of
phagocytosis, and new tissue (this time osseous) formed in its place.

In the periosteum we may take it roughly that inflammation runs a course
similar to that occurring in soft tissues elsewhere. There is but one
exception, and that, as we shall mention shortly, is connected with its
deeper layer.

As we know, the periosteum consists of two layers, an outer fibrous and
an inner yellow elastic, and is extremely vascular. Numerous bloodvessels
ramify in it, and, with their attendant nerves, break up to enter the
numberless canals of the Haversian system. This extreme vascularity, of
course, favours abundant exudation. The exudate, however, is, as it were,
shut in by the dense fibrous layer of the membrane, and the result is that
in periostitis it collects between the membrane and the bone, causing
swelling and raising of the membrane, and giving rise to excruciating pain
from pressure upon the nerves.

Should the periostitis be complicated by the formation of pus, then the
vessels entering and supplying the bone are, in the suppurative area,
destroyed. With their destruction it may happen that we get also death of
a portion of the osseous tissue. This, however, when the suppuration is
abundant, cannot commonly occur, as the bloodvessels within the bone--those
of the medulla--commence to supply blood to the affected part. In cases of
trouble with the bones of the foot, these last few remarks have a special
significance. Here we have three bones whose medullary cavity is extremely
small--almost nil, in fact--which explains in some measure how easy it is
when suppuration exists to get necrosis and exfoliation of, say, portions
of the os pedis. Necrosis and sloughing of the periosteum itself may also
happen, but as the extreme vascularity of the membrane is a fairly strong
safeguard against that it is of only rare occurrence.

In connection with the deep layer of the periosteum, and forming part of
it, are found numerous bone-forming cells (_osteoblasts_). These, under
ordinary conditions, are relatively quiescent. Under the slightest
irritation or stimulation, however, their bone-forming functions are
stirred into abnormal activity, thus explaining how easy it is (especially
with bones so open to receive slight injuries as are those of the foot) to
get ossific deposits, the starting-point of which we are quite unable to
account for.

With