234 EVOLUTION AND DISEASE. 
as small scattered nodules in various parts of the body. 
In many cases the micro-organism which produces the 
disease, has been satisfactorily isolated. A few tumours 
have been placed in this group because their structure, 
history, and infective properties correspond to those in 
which micro-organisms have been satisfactorily detected. 
The consideration of infective tumours belongs strictly 
to the evolutionist, for the causative agents may be re- 
garded as parasites which have gradually acquired the 
power of flourishing in animal bodies. This group may 
be illustrated by an account of the remarkable disease 
known as actinomycosis. 
Actinomycosis is a disease prevalent among cattle. 
It commences as a rule in connection with the jaws and 
tongue in the form of nodules, which become confluent 
and form large masses. After attaining a certain size 
these tumours suppurate and discharge pus. When the 
jaw is the seat of the disease, the bone becomes eroded 
and expanded on account of the growth invading it. 
Such tumours were formerly described by veterinary 
surgeons as sarcomata, but in 1877 Bollinger detected in 
them microscopic organisms of peculiar radiating struc- 
ture, termed in consequence actinomyces, or the rayed- 
fungus. The matter has received the close attention 
its interest and importance deserves. It appears that the 
spores of the fungus enter the tissues, either through 
ulcers, decayed teeth, or the sockets of teeth, and acting as 
irritants establish inflammation. After lodging in the 
tissues for a time, the spores develop a mycelium (fig. 
119), and the radiated structures thus formed become 
surrounded by leucocytes, round cells, and giant-cells, 
