BACILLUS TUBERCULOSIS. 273 



Infected with tuberculosis; in rare cases the tubercle 

 bacilli may also penetrate into the body through wounds 

 of the skin. At times tubercular animals are the source 

 of the infection ; but the disease seems only to be carried 

 by the milk of tubercular cows when the mammary 

 glands are themselves affected with the disease. 



In apparent contrast to the influence of the local 

 spread of the tubercle bacilli, we have the experience 

 that as the result of living in the neighbourhood of 

 tubercular individuals by no means all the healthy 

 persons are infected; indeed, the percentage of infections 

 is only slightly greater than where there is, as far as we 

 can judge, much less opportunity for infection. The Important 



,. ... ., rl , J .j . part played bv 



predisposition evidently plays a very important role in the individual 

 the occurrence of tubercular infection ; it practically j^ lsposl " 

 controls the mode of spread of the tuberculosis, and is 

 more especially important because it has the greatest 

 influence on the treatment of the disease. What this 

 individual predisposition consists of is a question which 

 has been as yet very little investigated as regards its 

 relation to the infective agents which have as yet been 

 recognised. We know that there are certain protective 

 arrangements in the body which, along with the very 

 limited conditions of life of the tubercle bacilli, render it 

 difficult for the latter to obtain a foothold ; in this respect 

 we may point out the moist and sticky lining membrane 

 at the entrance to the respiratory tract, the mode in 

 which elements which have passed further are expelled 

 foy the ciliated epithelium and by coughing, and the 

 resisting power of the normal cells, more especially of the 

 epithelial cells (Veraguth), towards the individual para- 

 sites. It is only when, for example, as the result of 

 chronic catarrh, there are lesions of the epithelial covering, 

 or when there is stagnation of the secretion in certain 

 parts of the lung, or when the nutrition and the energy 

 of the epithelial cells is defective, that the bacilli which 

 have entered can develop and multiply undisturbed ; and 

 these conditions again chiefly occur only in individuals 

 in whom the thorax is abnormal or where the nutrition 

 is bad, or in those who do not breathe sufficiently deeply, 



18 



