324 CONTAGIOUS DISEASES. 



opaque yellowish matter infiltrating the tissues. These vary- 

 ing appearances gave rise to the opinion amongst A^irchow and 

 his followers that they were lesions of different diseases ; and 

 Villemin, who first discovered the inoculability of tuberculosis 

 in 1865, vainly tried to prove their identity by inoculations, 

 but it remained for Koch to prove that they were all due to 

 tubercular bacilli. 



Examined microscopically, the nodules present giant cells 

 with branched processes and small endothelioid cells, which are 

 round or oval in shape, and outside a zone of lymphoid cells. The 

 cells are surrounded by a more or less complete fibrous stroma. 

 These tubercles are sometimes scattered throughout the sub- 

 stance of the organs, having the form of distinctly rounded 

 nodules, rather gelatinous in structure, and a pearly grey trans- 

 parent appearance. In other cases it will be found that beneath 

 the pleurie, both visceral and parietal, there are large yellow 

 nodules, which give off numerous radiating branches. These are 

 called " grapes," from the supposed similarity to that fruit, and 

 between them are seen some red lines in which degeneration has 

 not advanced to any extent. Each yellow patch is seen under a 

 low power to consist of a number of follicles, each being de- 

 generated — caseated — in its centre, embracing the alveoli and 

 their contents. There are no blood-vessels in the centre, but 

 some vessels are seen with difficulty some distance from it, 

 where the alveolar walls are slightly thickened. In order to 

 differentiate the giant cells from those of an endothelial type 

 and from the fibrous capsule, stain with picro-carmine, when the 

 giant cells will present the yellow picric acid colour, and the 

 others will be stained carmine ; for the recognition of the bacilli 

 other methods must be resorted to. 



There is a difficulty in finding the bacilli in caseous tubercles, 

 particularly in cattle, but they can often be demonstrated in 

 material taken from the periphery of the nodule. If cavities 

 containing pus are formed, such as are ]iot uncommon in tuber- 

 culous horses, the bacilli can be detected very readily by the 

 following process : — 



A drop of water is placed on a perfectly clean cover-glass. 

 Eemove a small portion of the pus or caseating matter with 

 the point of a needle, then rub it with a drop of water already 

 placed on the cover glass. The cover-glass is then dried over 



