of Edinburgh, Session 1884-85. 
273 
In all these cases several factors must be taken into account in 
considering the changes which result from the presence, during 
their life history, of micro-organisms in the tissues. They may act 
(a) merely as mechanical irritants, as do particles of coal or stone in 
coal miner’s or stone mason’s lung. ( b ) By the evolution of an 
irritant digestive product or “ptomaine,” which may act generally 
or locally, (c) In the third place, it may be suggested that in certain 
cases there is, in addition to the preceding factors, a subtle physico- 
chemical or molecular reaction between the micro-organisms and the 
cells, which may give rise to changes, local or general, and which 
might be compared to the action of the sperm cell on the germ cell 
or ovum. It is in connection with these points, and with regard to 
the varied degrees in which the infective process occurs in various 
species of parasitic organisms, that we wish specially to speak, for 
recent work of numerous observers upon these questions appears 
capable of throwing a clearer light upon the matter than has pre- 
viously been the case. 
The five following types of tissue reaction may be taken as 
illustrative of the principal pathological processes of micro-organismal 
origin. (1) Septicaemia, in which there is an absorption of septic 
products from an external or localised internal source, but no 
absorption of the organisms which give rise to them. Diffusion of 
products. (2) Specific infective fevers. No localised source after 
period of incubation. General diffusion of both organisms and 
products. (3) Abscess formation. Micro-organisms in area strictly 
localised by “pyogenic membrane.” Rapid death of that area. 
(4) Tubercle. Micro-organisms localised. Slower death and casea- 
tion of area in which they occur. Slight tendency to fibrous tissue 
formation. (5) Actinomycosis. Localisation of micro-organism. 
Little tendency to death of proliferated cells. Greater tendency to 
fibrous tissue formation. 
Without attempting to enter fully into the description of the 
process involved in tissue reaction in each of the classes above 
named, we may briefly summarise the many features, and illustrate 
them by one or two examples. Amongst the chief forms in which 
cell activity exhibits itself during various stages of the life cycle, 
are, according to Mr Geddes, (1) the amoeboid stage, where nutritive 
activity is well marked, but where formative activity is not yet 
