THE INFECTIOUS DISEASES. 251 



ful whether the tubercle bacillus furnishes a direct formative stimulus, or whether such 

 is furnished by damaged cells, or whether the cell proliferation may not be an expression 

 of reparative activity in the presence of damaged tissue made possible by disturbed 

 organic control, see page 302. In any event the new tissue which forms under the in- 

 fluence of the tubercle bacillus apparently owes its morphological as well as biological 

 characteristics to impulses toward tissue formation which are exerted in the presence of 

 agencies doubtless poisons restraining within narrow bounds the new connective- 

 tissue growth, no matter how extensive or persistent this may be, and tending con- 

 stantly to its destruction. 



It is interesting in this connection to note that when lesions in many respects simi- 

 lar to those of the ordinary tuberculosis are induced experimentally in animals with 

 dead tubercle bacilli (see above), the poisonous substances leading to necrosis are not 

 produced continually and for indefinite periods, as is the case in infection with living 

 bacilli, but are soon exhausted, so that after a certain amount of initial necrosis the new 

 tissues go on to develop in the usual reparative way, blood-vessels are formed, and heal- 

 ing by a cicatrix under favorable conditions regularly takes place. It is probable that 

 effective healing in tuberculosis in man takes place only after the local production of 

 destructive poison ceases through the death or diminished virulence of the tubercle 

 bacilli present. 



THE NUMBER OF TUBERCLE BACILLI IN LESIONS. 



The number of bacilli which are present in the lesions of tuberculosis 

 is subject to great variation. They are usually abundant in the walls 

 and contents of phthisical cavities, and in tubercle tissue which is under- 

 going cheesy degeneration and disintegration. In these situations they 

 may be found in myriads, forming sometimes a large part of the disin- 

 tegrated mass. They are found in cells and scattered among them. 

 Sometimes they are present in considerable numbers in the giant cells of 

 miliary tubercles. In the acute general tuberculosis of children they 

 are often present in large numbers, particularly in the lungs (Fig. 120). 

 They may be found in tuberculous inflammation in any part of the body, 

 and have been seen in the blood. The bacilli are almost constantly dis- 

 charged in the sputa of patients suffering from pulmonary tuberculosis, 

 often in enormous numbers from one to four billion in twenty-four hours, 

 according to Nuttall's estimate and their presence sometimes affords 

 valuable diagnostic aid in early stages of obscure forms of the disease. 



Under a variety of conditions, especially in the older tuberculous 

 lesions, the bacilli may not be demonstrable. This apparent occasional 

 absence of the bacilli is probably due either to their disappearance as 

 the process grows older, or to some unknown changes which interfere 

 with the ordinary staining procedures. 



FREQUENCY OF TUBERCULOSIS IN MAN AND THE LOWER ANIMALS. 



Tuberculosis is a very common disease not only of man 1 but also of 

 many of the lower animals, 2 especially of cattle, and inasmuch as the 



1 Carefully prepared statistics show that tuberculous lesions are present in the body 

 in more than ninety per cent of the cases examined at autopsies. See for a thorough 

 and suggestive analysis of five hundred autopsies Naegeli, Virchow's Archiv, Bd. clx., 

 p. 426, 1900. 



2 For bibliography of animal tuberculosis see Eber, Lubarsch and Ostertag's "Ergeb- 

 nisse," Jahrg. iv. for 1897, p. 859 



