284 HEREDITY AND DISEASE 



it is said, in 16 per cent.), the young calf is very rarely tubercular. 

 Leclerc found only five cases out of 400,000. As to man, only about 

 a dozen instances of congenital tuberculosis were admitted by an 

 expert as securely established in 1905, and Dr. R. Schl (iter's ex- 

 ceedingly careful scrutiny (1905) of alleged cases of congenital 

 tubercle in human infants led him to the conclusion that for prac- 

 tical purposes the possibility of ante-natal infection might be in 

 this case disregarded. Thus, Prof. D. J. Hamilton writes : " With 

 extremely few exceptions — so few that they may almost be 

 neglected — children are not born tubercular even of tubercular 

 mothers, nor are the young of animals born tubercular under like 

 conditions " (1900, p. 293). Even if the mother have genital 

 tuberculosis, specific contamination of the unborn child seems rare, 

 and there is no proof that genital tuberculosis in the father has any 

 specific effect on his offspring. 



(4) In all ordinary cases, then, the infection with tubercle bacillus 

 occurs after birth, and in many cases long after. 



The fact that tubercular disease may be a shadow over a family 

 history for generations is doubtless mainly due to an inheritance of 

 what began as a truly germinal or blastogenic variation, which is 

 only a biological way of expressing what the physician means by 

 " a particular predisposition," " a tubercular temperament," " a 

 diathesis," and so on. To discuss what the particular weakness 

 precisely is does not fall within our province ; Prof. Hamilton 

 says, " Most likely the particular vulnerability resides in the epi- 

 thelial protective coverings of the body being too little resistant, too 

 easily stimulated by external agencies, too readily penetrated by the 

 parasite of the disease " (1900, p. 294). " In support of this 

 assertion are to be taken into account certain epithelial manifesta- 

 tions which accompany the tubercular habit — namely, the very 

 dark or very light degree of colour of the hair, the overgrowth of 

 hair in the bushy eyebrows and long eyelashes, and, lastly, the 

 occurrence of a lanugo-like overgrowth in tubercular children 

 along the spine and over the legs. To my mind, these aU point 

 to an anomaly of the epithelial type which is peculiar to the tuber- 

 cular habit of body " (Hamilton, 1900, p. 295). 



If it be the case that the tubercle bacillus usually gains access, 

 even to the lungs, mainly by the digestive tract, and almost entirely 

 through the intestine, and may penetrate into the large lymph 

 channels without any apparent lesion, we have still perhaps to do 



