DISEASES OF THE JOINTS 433 



articulation often participate in the inflammatory changes, 

 and in many cases become completely ossified. The true 

 articulatory surface of the bone, that articulating with the 

 OS pedis and with the os coronae, is never affected. 



Causes. — In enumerating the causes of navicular disease, 

 we shall follow the example of Colonel Smith and classify 

 them under certain headings — namely, (1) Hereditary Pre- 

 disposition ; (2) Compression ; (3) Concussion ; (4) A Weak 

 Navicular Bone ; (5) A Defective or Irregidar Blood-supply 

 to the Bone ; and (6) Senile Decay. 



Fig. 163. — The Navicular Bone from a Case of Long-standing 

 Navicular Disease. 



The erosion of the cartilage on its central ridge is most marked, and 

 the porous appearance of the bone thus uncovered points to the 

 existence within it of a rarefactive ostitis. Along its edges large 

 osteophytic outgrowths speak of the effects of an osteoplastic 

 periostitis. 



1. Hereditary Predisposition. — That navicular disease is 

 hereditary is a fact that has for a long time been insisted 

 on, and has come to be so generally admitted that we do 

 not intend to dwell on it here. As we have said before, it 

 is found in the lighter breeds of horses (and, according to 

 Zundel, especially in the English breeds), and is there seen 

 to be frequently transmitted from parent to offspring. 



2. Compression. — By this is meant the compression of 

 the navicular bone between the os pedis and the os coronae 

 in front, and the perforans tendon behind. 



In order to appreciate this explanation of the causation 

 of navicular disease at its true value, it will be well to con- 

 sider briefly the physiology of the parts in question. 



28 



