CAUSES OF DISEASE. 219 
which has died of this disease we find the liver and 
intestines presenting numerous rounded nodules of a 
pale yellow colour, varying in size from a pin point to 
that of a filbert. On cutting into the larger nodules the 
centre is found occupied by pus. The smaller ones are 
homogeneous, and on examining them microscopically 
we recognize in the centre small circular cells with larger 
ones—giant-cells—lodged among them, outside these a 
layer of smaller cells with no giant-cells, and lastly a 
layer of fibrous tissue. 
When such specimens are suitably stained, minute 
rod-like bodies — bacilli — are 
seen clustered in the centre 
of the mass and occupying 
the interior of the cells, espe- 
cially the giant-cells. In 
nodules of moderate size the 
centre is occupied by caseous Fic. 115.—Leucocytes ingesting 
material surrounded by a zone pate: 
of cells containing bacilli Adjacent nodules may 
coalesce and thus produce large masses. The blood-vessels 
connected with the nodules frequently present clusters 
of bacilli in their interior. An extensive and prolonged 
study of this disease has convinced me that the bacilli, 
from whatever source arising, are introduced into the 
alimentary canal and find their way into the walls of the 
bowel. Here they are attacked by the leucocytes which 
surround, ingest, and destroy them. The bacilli may be 
too numerous for the leucocytes, and the point where 
the bacilli gain entrance into the tissues become a 
battle-field, large numbers of leucocytes are quickly on 
