DISEASES OF THE BONES 381 
same. It takes place along the course of the bloodvessels, 
and is only modified in its attendant phenomena by the 
structure of the parts involved. Swelling, for instance, 
cannot take place in the centre of compact bone tissue. 
Otherwise, other changes occur exactly as in inflammations 
of other structures. 
When the causal irritant has been excessively severe and 
the migration of leucocytes abundant, actual formation of 
pus may occur, the bony tissue being broken down and 
mingled with it, and an abscess cavity 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 inflamma- 
tion 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, how- 
ever, 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 
