ON THE CAUSES OF DISEASE. 21 



affected, or of 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 observ^ation or investigation ; but its existing in animals having 

 an hereditary predisposition to disease can not, we think, be doubted, 

 as we shall now endeavor to show. The ground of our reasoning 

 rests chiefly on the analogy which subsists, in all respects, between 

 external and internal j)arts. The same law which regulates tlie 

 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 component parts of inter- 

 nal, and, it may be, inaccessible to ordinary powers of investiga- 

 tion. Then if, as we have shown, external hereditary diseases, 

 such as lamenesses, are traceable to external hereditary jieculiari- 

 ties of conformation, we do not tliink it pushing our analogy too 

 far in asserting that, in like manner, internal hereditary dis-easea 

 must, in great jiart at least, depend upon some inherent hereditary 

 ueculiarity of the internal parts affected." 



The common disease known as bone spavin is inherent, or de- 

 pendent on predisposition, as related above; for the author never 

 knew a horse to be the subject of this malady unless he had faulty 

 hocks; or, in other words, in so far as the conformation of the 

 parts were concerned, predisposition was evident. The same n> 

 marks apply to ringbone. Either the pasterns are too upright, 

 thereby causing jar and concussion, or they slant unnaturally, 

 and the consequence is, strain of the ligamentary structures, end- 

 ing often in ringbone and other osseous diseases. 



Ample evidence can be adduced to show that various diseases 

 of the eye are hereditary. The tendency to ophthalmia shows it- 

 self in a peculiar conformation of the eye, and parts in the vicinity 

 of it. Periodical ophthalmia is notoriously hereditary, and usually 

 ends in cataract. 



Colic, also, can be traced to inherent tendencies; for animals 

 subject to flatulent colic are known to have a morbid a])petiie; 

 they have, also, a large, roomy abdomen, which gives them an 

 ungainly appearance. Yet it is true that diseases may oa'ur 

 accidentally, without the intervention of predisposition ; but pre- 

 disposition exists in almost all cases, and it only requires some 

 anatomical and physiological knowledge on the part of husband- 

 men to detect it. 



