Modes of Infection. 529 



Of course any organ of the body may occasionally be found to be 

 particularly predisposed to infection and it seems that primary affec- 

 tion of the bones and articulations is thus explained (traumatic influ- 

 ences). The species of animal is also of importance. Thus subcutane- 

 ous infection of guinea pigs usually produces disease of the lungs, 

 almost never of _ the spleen, while in rabbits, next to the lungs, the 

 kidneys are particularly prone to disease, and in chickens the lungs are 

 practically immune to infection with avian tuberculosis (Weber) . Simi- 

 lar differences are observed among the domesticated animals, e. g., in the 

 pig the spleen is particularly susceptible in contrast to other animals, 

 in cattle the serous membranes, and in dogs the pericardium. Finally, 

 the variety of tubercle bacilli is to be considered. Thus, in man, the 

 bovine type usually causes disease only in the digestive tract and in its 

 lymphatic glands. 



From what lias been stated it is evident that in tuberculosis 

 the infection of any internal organ whatever may result through 

 the respiratory apparatus (aerogenic infection) or through the 

 digestive apparatus (alimentary infection). As to which of 

 these modes of infection is the more common, views differ at 

 the present time. It is not probable either that the solution 

 to the problem will be an easy one. Bacilli carried with the 

 inhaled air to the uaso-pharynx may readily be absorbed through 

 the alimentary mucous membrane. But, whichever view may 

 be the correct one, there can be no doubt as to the danger of 

 inhaling air laden with bacilli. The manner of infection de- 

 pends no doubt, at least, in a great measure, upon the conditions 

 already discussed. Patients with open pulmonary tuberculosis, 

 by virtue of the exhaled infected droplets constitute a source 

 of either mode of infection ; an open tuberculosis of other organs 

 of the body constitutes a danger for alimentary infection almost 

 exclusively. 



The localization and character of the tuberculous lesion 

 is determined to some extent also by the intensity of the in- 

 fection. After infection with a Jarge mass of virulent bacilli 

 the morbid process usually, especially in older animals, begins 

 its development at the point where the largest number of bacilli 

 became lodged on the mucous membrane. At these points 

 tubercles and ulcers are formed, such as are observed in the 

 tonsils (especially in swine) and in the ileum. In the latter 

 they usually begin in the lymph follicles of the mucous mem- 

 brane. If the infection happens to be slight the mucous mem- 

 brane remains intact and the lymph g-lands only are affected. 

 This is because most of the bacilli remain lodged in these glands 

 and the few that happen to pass through are either destroyed 

 (phagocytosis) or, on account of their small numbers, produce 

 lesions that are not observed until a later time. Thus it may 

 occasionally be observed in cattle that the retropharyngeal 

 glands contain large tuberculous foci, the upper, middle^ and 

 lower cervicals and finally the mediastinal glands successively 

 smaller and more recent foci (Breuer). By this means we may 

 often be enabled, in the beginning of the disease, at least, to 



Vol. 1—34 



