186 ANATOMY AND PHYSIOLOGY OF THE HORSe’s FOOT. 
tion of the articular cartilage on these surfaces would be 
merely the sequel to disease in the spongy tissue. A casual 
glance at the three surfaces of the bone will at once show 
that the inferior face — that which is in apposition to the 
inner surface of the deep flexor tendon differs from the others 
in not being covered with articular cartilage, but with a 
tissue which is somewhat widely different from that sub- 
stance. The overlooking this fact has, doubtless, led to the 
theory that the affection which we term “ navicularthritis 33 
commenced its ravages in the spongy tissue, extending them 
only and always to one side of the compact tissue and the 
covering external to it. 
The inferior face of the navicular bone is covered by a 
membrane ,* which, so far as I am capable of judging, differs 
but little, if in anything, from those membranes which are 
usually classed by the name of stratified fibrous cartilages , 
and cover bony prominences or grooves, over or through 
which tendons glide. We have an excellent example imme- 
diately above the navicular bone, on the posterior face of the 
second, phalanx, and better still, perhaps, on the posterior 
surfaces of the sesamoid bones of the fetlock. In these 
fibro-cartilaginous membranes the diversely arranged fasci- 
culi of white fibrous tissue can be distinctly perceived by 
the naked eye beneath the synovial membrane covering 
them ; and their appearance is widely different from that 
presented by articular cartilage, which is a homogeneous, 
semi-transparent and elastic substance, having no synovial 
membrane on its face. Examined microscopically, they are 
found to be still more dissimilar; in the first there is found 
to be but little, if any, hyaline matrix. 
The covering on the navicular bone is chiefly remarkable from 
its constituent fasciculi being derived entirely from the liga- 
ments surrounding it, and which attach it to the other bones. 
The glistening white fibres from these are seen to pass inwards 
in bundles, ramifying over the face of the bone in a somewhat 
irregular manner, and becoming fewer in number towards 
the centre where the friction is greatest — another feature 
which distinguishes this membrane from articular cartilage, 
which is generally thickest where most pressure has to be 
sustained. 
Add to this that a synovial capsule lines the face of the 
# Professor Bouley does not describe this membrane or its structure. He 
merely remarks that this face of the bone “est revetu d’une epaisse mem- 
brane fibreuse, analogue, par sa disposition, aux cartilages diarthrodiaux, 
mais differente par sa texture, son organisation et ses proprietes.” — ‘ Traite 
du Pied du Cheval/ p. 37. 
