376 



INFECTIVE DISEASES. 



cells, in which it is possible, though often with extreme difficulty, 

 to demonstrate the tubercle bacillus. The cells originate in the 

 proliferation of endothelial connective tissue and white blood cells. 

 Later on, large oval or circular multi- nucleated cells, or giant cells, 

 make their appearance. The tubercle bacilli are only occasionally 

 found in the interior of human giant cells, whereas in the lower 

 animals, in equine and bovine tuberculosis more especially, the bacilli 

 are often present in great numbers, and very commonly in the form 

 of conspicuous rings, visible under a low power of the microscope. 



FIG. 161. TUBERCLE OF THE LUNG IN A VERY EARLY STAGE, x 400 : a, An alveolar 

 wall ; b, blood-corpuscles in capillaries of the same ; c, blood-corpuscles 

 extra vasated into the alveolar cavities; d, alveolar capillaries filled with 

 blood-corpuscles carried forward by the tubercle which is growing into the 

 alveolar cavity ; e, large endothelium-like cells, of which the tubercle in this 

 stage is mainly composed ; /, portion of a branch of the pulmonary artery 

 injected (HAMILTON). 



Whether the absence of blood-vessels or the action of the bacillus is 

 the main factor in producing caseation, is an open question. When 

 suppuration follows caseation, as commonly happens in tuberculosis 

 of the lungs in man, and in experimental tuberculosis in animals, an 

 abscess forms. In cattle there is a remarkable tendency to the 

 formation of calcareous deposit in the caseous masses. 



The tubercle may not degenerate and die, but live and develop. 



