Hereditary Diseases of Horses. 



113 



several generations. But although corresponding in these 

 respects to many hereditary diseases, it differs essentially 

 from them, inasmuch as it attacks all animals alike when 

 exposed to the same exciting causes, shows no special pre- 

 ference for those bred from a stock in which abortion has been 

 prevalent, does not affect those removed to a distance from 

 the locality in which the disease prevails, and may sometimes 

 be effectually and immediately arrested by a radical change in 

 the system of management. These conditions are quite suffi- 

 cient to disprove the hereditary nature of abortion ; and when 

 such conditions occur in connexion with any other disease, they 

 may be safely accepted as ample evidence of its being produced 

 by external or extraneous circumstances, independently altogether 

 of any hereditary predisposition. 



There are some maladies in which it is comparatively easy 

 to trace the connexion between conformation and disease. 

 In the horse certain sorts of limbs notoriously predispose 

 to certain diseases. Thus, bone spavins are most usually 

 seen where there is a disproportion in the size of the limb 

 above and below the hock ; curbs, where the os calcis is small 

 and the hock straight ; strains of the tendons of the fore-leg, 

 where the limb is round and the tendons and ligaments con- 

 fined at the knee ; and navicular disease, where the chest is 

 narrow and the toes turned out. Amongst horses so formed, 

 these diseases are unusually common, and are developed by 

 causes which Avould be quite inadequate to produce them in 

 animals of more perfect conformation. But it appears to us 

 that internal and constitutional hereditary diseases also depend 

 upon the altered conformation or texture of the parts specially 

 affected, or upon some disturbance of the relation which should 

 subsist between the different constituents of these parts. This 

 abnormal state of the internal parts is seldom within the limits of 

 our means of observation or investigation, but its existence in 

 animals having a hereditary predisposition to disease cannot, we 

 think, be doubted, as we shall now endeavour to show. The 

 ground of our reasoning rests chiefly on the analogy which subsists 

 in all respects between external and internal parts. The same 

 law which regulates the hereditary transmission of form, texture, 

 and relation of external and visible parts, also operates with 

 equal force in regard to the form, texture, and relations betwixt 

 the components of parts internal, and, it may be, inaccessible to 

 ordinary powers of investigation. Then, if, as we have shown, 

 external hereditary diseases, such as lamenesses, are traceable to 

 external hereditary peculiarities of conformation, we do not 

 think it pushing our analogy too far to assert that, in like 

 manner, internal hereditary diseases must, in great part at least, 



VOL. XIV. I 



