114 



Hereditary Diseases of Horses. 



depend upon some inherent hereditary peculiarity of the internal 

 parts affected. The following remarks will, we think, tend to 

 support our hypothesis. Particular conditions of the blood often 

 become hereditary, and, if an excess of the red globules of the 

 blood be hereditary, the disease of plethora to which that excess 

 gives rise will also become hereditary. We have a striking 

 example of this in many of the improved breeds of cattle, in 

 which is conjoined a remarkable excess of the red globules of the 

 blood with a highly plethoric habit of body. If the eye be 

 predisposed to deep-seated ophthalmia, a slight exposure to cold, 

 or even an error in diet, will be sufficient to induce the disease. 

 But before an acute attack there is seldom noticeable in the eye 

 any alteration of texture or of function indicating the existence 

 of such a tendency. That such a tendency does, however, exist 

 there can be no doubt, and we think that it must consist in an 

 altered condition of some of the deeply-seated parts of the eye. 

 Our conclusion is, therefore, that every hereditary disease de- 

 pends upon some hereditary abnormal condition predisposing to 

 that disease. This abnormal condition may be either local or 

 general. It may affect the form, structure, texture, quantitative 

 or qualitative composition either of solids or fluids. It may 

 constitute so powerful a predisponent to disease as speedily to 

 cause impairment of health, or it may be so slight, that without 

 the co-operation of exciting causes it will fail to produce any 

 apparent disturbance of the general health. But animals with 

 such inherent defects are always predisposed to disease. In- 

 fluences which are harmless in others often produce in them 

 serious and irremediable disease. Thus, ordinary work causes 

 spavins or curbs in horses with badly-formed hocks ; a slight 

 exposure to cold brings on phthisis in a cow of consumptive 

 diathesis ; simple engorgement of the stomach causes an attack 

 of ophthalmia in a subject predisposed to it. Hence, an animal 

 having a hereditary tendency to disease labours under many dis- 

 advantages, and his health, and even his life, are in constant 

 jeopardy. He is always liable to suffer from slight and tempo- 

 rary errors in diet and regimen, and bears about with him from 

 birth an ill-fated inheritance which affords a congenial soil for 

 the reception and development of disease, and is transmitted to 

 his posterity unimpaired in power, and undiminished in extent. 



II. The Hereditary Diseases of Horses. 



In regard to the hereditary diseases of horses we shall consider, 

 first, those of a local nature, afterwards proceeding to those which 

 are more general in their character, and which affect the system as 

 a whole. 



Local hereditary diseases are usually simple in their nature, 



