LECTURE XXI 

 HEREDITY— AIR 



Heredity is the great law of inheritance that "like tends to 

 produce like." This tendency may apply to any peculiarity. 

 Heredity may appear as a factor in the cause of disease when 

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

 local weakness — an ill shaped hoof and sidebone, for example — 

 or general susceptibility appears which is favorable to the de- 

 velopment of disease — for example, a very low resistance to hog 

 cholera. It should be borne in mind that susceptibility and 

 immunity are but varying degrees of the same thing. Statistics 

 are meager, but essential facts are very plain as to the relation 

 of heredity to many diseases. Young pigs from an immune sow 

 are frequently born with considerable immunity; but most of 

 them gradually lose this inherited immunity as they grow 

 older. 



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 the muscle and tendon or 

 bone cells. This theory of lessened resistance applies to such 

 conditions as spavins, ringbones, sidebones, roaring, and internal 

 diseases alike. Under favorable conditions the actual develop- 

 ment of disease may not occur. A stallion with certain defects 

 of the eyes is apt to sire colts with bad eyes. Mares with curby 

 or spavined hocks are equally apt to raise colts with bad hocks. 

 In rare cases the young is born with the actual disease present. 



In-and-in breeding'. — This tends in some cases toward decrease 

 of physical vigor, infertility, tendency to abortion, and various 

 other diseases, especially when long continued and with unwise 

 mating. 



Air 



Air is a very frequent source of disease. Its composition in a 

 general way is: oxygen 1-5, nitrogen, 4-5; more accurately, 



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