LECTURE XX. 



CAUSES AND PREVENTION OF DISEASE. 



HEREDITY. 



Definition. — The great law of inheritance that "Uke tends to 

 produce Hke." This tendency may apply to any peculiarity. Her- 

 edity may appear as a factor in the cause of disease when actual 

 disease germs are transmitted, which is probably rare, or when a 

 local weakness, general susceptibility, appears, which is favorable 

 to the development of disease. 



Theory. — The modern idea of heredity in relation to disease 

 is that the thing actually inherited is usually only a tendency or 

 a lessened resistance. This may refer to the white corpuscles and 

 serum of the blood as well as to muscle and tendon or bone cells. 



This theory of lessened resistance applies to spavins, bad eyes 

 and certain internal diseases alike. Under favorable conditions 

 the actual development of disease may not occur. 



A stallion with defective eyes is apt to sire colts with bad 

 eyes. Mares with curby or spavined hocks are apt to raise colts 

 with bad hocks. In rare cases the young is born with the actual 

 disease preseent. 



The suggestion is obvious. Breed from naturally sound ani- 

 mals, and it should be borne in mind that a fat animal is not neces- 

 sarily a sound or healthy animal. Domestic animals in show con- 

 dition may be diseased either internally or externally. 



In and in breeding. — This frequently tends toward physical 

 decav. infertility, tendency to abortion and various diseases of 

 lungs and eyes. In human family it tends toward idiocy, freaks, 

 consumption and a variety of diseases. Statistics are meager 

 but essential facts are very plain as to the relation of heredity to 

 animal and human diseases. 



