Hereditary Diseases of Horses. 



Ill 



dysentery, to the total exclusion of phthisis, but by and by the 

 dysentery disappears to give place to the phthisis. > 



4. Hereditary diseases occur to a certain extent independently 

 of external circumstances, appearing under all sorts of manage- 

 ment, and being little affected by changes of locality, separation 

 from the diseased stock, or such other causes as modify the pro- 

 duction of non-hereditary diseases. 



5. They are, however, most certainly and speedily developed 

 in circumstances inimical to general good health, and often 

 occur at certain so-called critical periods of life, when unusual 

 demands on the vital powers take place. 



6. They show a striking tendency to modify and absorb into 

 themselves all extraneous diseases. For example, in an animal 

 of a consumptive constitution, pneumonia seldom runs its 

 ordinary course, and, when arrested, often passes into con- 

 sumption. 



7. Hereditary diseases are less effectually treated by ordinary 

 remedies than other diseases. Thus, although an attack of 

 phthisis, rheumatism, or constitutional ophthalmia, may be sub- 

 dued, and the patient put out of pain and danger, the tendency 

 to the disease will still remain, and be greatly aggravated by 

 each attack. 



Hereditary diseases do not necessarily show themselves at 

 birth. In horses and cattle there are only a few which do so. 

 The scrofulous diathesis sometimes presents itself in large col- 

 lections of pus, which occasionally prove fatal within a few 

 days after birth ; and symptoms of hydro-cephalus, rickets, and 

 occasionally rheumatism — all hereditary complaints — are also 

 sometimes found present at that early period. But most here- 

 ditary diseases develop themselves only some considerable 

 period after birth, and the inherent tendency may even remain 

 latent during many years. Thus, in man, gout and gravel do 

 not usually develop themselves until after the meridian of life, 

 and in horses and cattle the tendency to consumption, scrofula, 

 and rheumatism may remain dormant for many years. Nay, 

 more ; diseases of an undoubtedly hereditary nature may remain 

 latent even for a generation or two, and afterwards re-appear 

 with all their wonted severity : " Silente saepe morbo in geni- 

 tore, dum ex cPvo derivatur in nepotem ;" * and such cases are 

 not of infrequent occurrence, and are certainly not at all incom- 

 patible with the hereditary nature of a disease. They may be 

 satisfactorily explained in various ways. The morbid tendency 

 may be so slight as not to interfere with health, or the animal 

 may have been reared in circumstances where the exciting 



* Boeraave, Aphor. de Curandis Morbis, 1075. 



