1. Changes in the Bursa.—­Upon the internal surface of the bursal membrane is first noticed a slight inflammatory hyperaemia, accompanied by more or less swelling and tumefaction, owing to its infiltration with inflammatory exudate.  The portion covering the hyaline cartilage of the navicular bone has lost its peculiar pearl-blue shimmer, and become a dirty yellow.

Remembering that the bursal membrane is a synovia-secreting one, and bearing in mind what happens in ordinary synovitis and arthritis (with which, of course, this may be very closely compared), we shall first expect changes in the bursal contents.  It is highly probable, though difficult of proof, that in the very early stages the chronic inflammatory stimulus has the effect of increasing the flow of synovia.  In every case, however, where it can with any certainty be said that navicular disease exists, it is too late to meet with this condition.  The disease has then progressed until destruction of the secreting layer of the bursal membrane has been seriously interfered with, and in this case we find a distinct deficiency in the quantity of synovia in the bursa.  In advanced cases it is even found that the bursa is absolutely dry.

2. Changes in the Cartilage.—­Directly that portion of the bursal membrane covering the cartilage is the subject of inflammatory change, the cartilage itself, by reason of its low vitality, soon suffers.

Under a process, which we may term ‘dry ulcerative,’ the cartilage covering the ridge on the lower surface of the bone commences to become eroded, and in appearance has been likened, both by English and Continental writers, to a piece of wood that has been worm-eaten (see Fig. 161).

[Illustration:  FIG. 161.—­NAVICULAR BONE (POSTERO-INFERIOR SURFACE) SHOWING THE ‘WORM-EATEN’ APPEARANCE CAUSED BY EROSION OF THE HYALINE CARTILAGE, AND COMMENCING RAREFACTIVE ARTHRITIS.]

’At this stage, or much earlier’—­we are quoting Colonel Smith, A.V.D.—­’may be found calcareous deposits in the fibro-cartilage and the bone.  They are scattered like fine sand here and there, generally across the inferior half of the face of the bone; they are sometimes numerous, frequently scanty, occasionally entirely absent.  The amount of calcareous degeneration depends upon the lesions present.  If much destruction of bone exists, there will be but few calcareous deposits; whilst if there are many calcareous deposits, there may be but slight ulceration of bone tissue, and perhaps none at all.  In fact, I have held the opinion, and see no reason to modify it, that calcareous deposits are safeguards against caries.’[A]

[Footnote A:  Journal of Comparative Pathology and Therapeutics, vol. vi., p. 195.]