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 



