INFECTIVE GRANULOMATA. 347 



foodstuff contaminated with the Bacterium tuberculosis have 

 frequentl}^ become aft'ected with primary puhnonary tubercular 

 lesions. (The possibility of inhalation of the infection was care- 

 fully guarded against in these experiments.) It is presumed that 

 the llacterium tuberculosis is incorporated bv leucocytes in the 

 digestive tube and that the leucocytes then pass through the 

 intestinal wall into the lacteals and thence to the thoracic duct 

 to the right heart and on to the lung, the first capillarv system 

 encountered, where they maj^ lodge and estal)lisli tubercular foci. 

 No doubt the resjiiratory tract is the channel of entrance in some 

 cases of tul^erculosis, but the number of animals infected through 

 this channel is ver}^ small. 



An occasional case of tuberculosis mav 1:)C the result of infec- 

 tion through the genitri-urinarv organs. Thus the penis of a 

 bull may become infected by serving a cow afflicted with uterine 

 or vaginal tuberculosis, and this same bull by copulation ma)' 

 infect other cows. Tul^ercular lesions are occasionallv observed 

 in the superficial inguinal glands of steers, and this mav be the 

 result of infection in the castration woimds. 



Conjuncti\-al infection mav occur as a result of forcil:)le dis- 

 charge of infection from tlie respirator)' tube of an aft'ected 

 animal. 



In summarizing, the digestive, respiratory, cutaneous alira- 

 sions, and genito-urinar^r organs are the principal channels of 

 entrance of the Bacterium tulserculosis, the frequenc\- Ijeing in 

 the order mentioned. 



Lesions. — The characteristic lesion of tuberculosis is the 

 tubercle. A tubercle is a nonvascular nodule, composed of leu- 

 cocytes, endothelial, giant and connective tissue cells, with a 

 tendency for the central part of the nodule to undergo necrosis. 

 The lesion maj^ vary in animals of different genera and in diff"er- 

 ent animals of the same genus. Thus tubercular lesions in hogs 

 may differ in some particular from those in cattle because of 

 variations in the resistance of the hog and ox. Variations of the 

 tubercular lesions in dift'erent individuals of the same genus occur 

 because of variation of individual resistance of the infected ani- 

 mal and variation of the virulency of the infecting organisms. 

 Tubercular lesions may be modified or obscured by lesions re- 

 sulting from secondary infections. The initial or primary lesion 

 may occur in any tissue or organ. Lymphoid tissue, however, is 

 more frequently affected than any other. 



The Bacterium tuberculosis and its products are the etiologic 

 factors in the formation of a tubercle. The bacterium having 

 lodged in a tissue favorable for its growth and development, be- 



