242 Manual of Veterinary Microbiology. 



may obtain entrance. The addition to pulverulent 

 virus of bodies of an irregular shape, capable of 

 wounding the respiratory mucous membrane, exerts 

 a similar influence. Joline has demonstrated the 

 great frequency of tuberculosis in cows which respire 

 the fumes of iron foundries; these fumes are always 

 loaded witb fine metallic particles. 



To the number of circumstances favoring the pene- 

 tration of the tubercle bacillus into an organism must, 

 further, be added the simultaneous presence of other 

 microbes which prepare the field for the former. As- 

 sociations of this kind may even start into renewed 

 activity a tubercular focus which had long been latent 

 and regarded as extinct. 



Tlie typical tubercular lesion, the tubercle, is not 

 specific of the disease ; it consists in an inflammatory 

 nodule of peculiar structure which develops under 

 the influence of multiple causes among which may 

 be mentioned microbes (actinomyces, zoogloea of 

 Malassez and Yignal, pseudo-tubercle bacillus of 

 Courmont, etc.), animal and vegetable parasites (do- 

 modex foliculorum, eggs of the strongylus vasorum, 

 aspergillus, etc.), and non-vital agents (powders of can- 

 tharides, pepper, and lycopod, and croton oil). The tu- 

 bercle bacillus must act by a mechanism similar to the 

 preceding causes, that is, through the irritation which 

 it excites in the elements wdiich surround it. These 

 elements are variable ; they may consist of epithelial 

 cells, endothelial cells, or the fixed cells of the con- 

 nective tissue, and along with these should be men- 

 tioned the wandering cells or leucocytes which are 

 every-where present and may play an important part. 

 The cells in contact with the bacillus increase in size, 



