ANATOMY AND PHYSIOLOGY OF THE HORSE'S FOOT. 185 
as bones of this class usually are. So that there is nothing 
at all remarkable in the arrangement of the bony tissue of this 
portion of the foot. 
In the hind foot the navicular bone is shorter and less 
bulky. 
The posterior face of the navicular bone is that which is so 
often the seat of disease. Standing at an angle perhaps a 
little more acute than that of the front surface, it exceeds it 
in length and width, and, like it, has a prominent transverse 
ridge nearly in its centre, with a slight and irregularly 
undulating depression on each side; that portion towards 
the outside of the ridge, and consequently corresponding to 
the outside of the foot, being the largest side. Over the 
whole of this surface the wide expansion of the flexor tendon 
of the foot glides vertically, to a limited extent, during the 
various movements of the lower half of the limb. This face 
of the bone is covered by a particular membrane, whose 
structure and special characteristics it is necessary to recog- 
nise, if for no other than pathological reasons. And these 
reasons are all the more urgent, as it would appear that in dis- 
cussing the pathology of navicularthritis, the nature of the 
tissue covering the face of the bone has been sometimes 
most unaccountably overlooked or mistaken. 
For instance, it has been stated that this tissue is articular 
cartilage , and the pathological changes incidental to that 
substance, with the various hypotheses as to the causes which 
induce these changes, have been brought to bear in explaining 
the several phases of disease on the lower face of this bone. 
Theories have been promulgated, in the belief that the 
disease commenced in the spongy texture of the bone, and 
if not successfully combated when localised there, that it 
extended to the compact tissue, and ultimately attacked the 
contiguous face of the supposed articular cartilage. 
Now, if the other two faces of the bone — those articulating 
with the second and third phalanges — were always or ever 
involved in this often-inflammatory and not unfrequently 
ulcerative disease, the supposition would, in all probability, 
be correct, as they are both covered by a layer of that peculiar 
substance which is interposed between the surfaces of bones 
forming a diarthrodial joint, and which is appropriately 
designated articular cartilage. 
But this is not the case. I have never heard of or wit- 
nessed an instance in which these surfaces were affected by 
this particular form of disease, though they could scarcely 
fail to be implicated did the malady commence in the in- 
terior of the bone, as conjectured by some; indeed destruc- 
