288 STRCTURE AND DISEASE OF THE NAVICULAR BONE. 
covered by a synovial membrane; the surface of other joints 
are not covered by synovial membrane ; this does not 
extend beyond the edges of the joint. The cartilage cover- 
ing the inferior surface of the navicular bone is thickest at 
the posterior edge of the bone, and gradually becomes 
thinner as it approaches the anterior edge which adjoins the 
coffin-bone. In aged animals, articular cartilage has a 
tendency to become fibrous. In proof of stratified fibrous 
cartilage being subject to the same process of disease as non- 
fibrous articular cartilage, I have only to refer to the case 
of the mare, in Mr. Fleming's note — which, by the bye, he 
showed me at Chatham in February, 1869, as being then 
about to be condemned. 
Mr. Fleming cites this case in support of his theory that 
the disease commenced on the long flexor muscle of the 
forearm and extended to the joint, producing ulceration of 
the stratified fibrous cartilage. In my opinion, the disease 
of the mare's shoulder did not commence in the muscle, but 
in the head of the humerus, and subsequently extended to the 
soft tissues of the joint. If I mistake not, there was a con- 
siderable bony enlargement on the humerus, and I also 
believe that Mr. Fleming told me that the exact seat and 
nature of the lameness was not at first very clear. The disease 
constituting spavin, navicular disease, and ulcerated joints 
(“ injuries to the capsule excepted"), commences in the bone, 
and extends to the soft tissues of the joint, and does not, as 
conjectured by some persons, commence in the soft tissues 
and extend to the bone. I believe such cases, however, to 
be rare. 
Mr. Fleming states that the navicular disease in some cases 
may be confined for a long time to the synovial, or rather 
the fibrous membrane covering this face of the bone without 
the slightest change being perceptible in the spongy texture ; 
indeed, so dense is the compact tissue that, even in advanced 
stages of the malady, it yields but slowly to the disinte- 
grating process, and thus protects the more vascular, but 
less resisting, cancellated structure, until it is itself de- 
stroyed. I shall feel exceedingly obliged to Mr. Fleming 
if he will show me specimens to support this statement. I 
have some hundreds, but not one that would prove it. 
I have stated in my paper that the disease usually com- 
mences in. the centre of the bone, where the nutrient action 
is supposed to be lowest. I believe that cases do occur 
occasionally where the cartilage is the first to become affected 
by the defective nutrition of the bone, but the disease never 
becomes extensive without involving the spongy structure 
