234 E VOL UTION 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 Bellinger 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, 



