272 
Proceedings of the Royal Society 
external conditions or moulding forces may be so altered that the 
reaction of the tissue to these forces manifests itself in a form which 
varies somewhat from that observed under normal conditions. 
The constant presence of micro-organisms upon the free surfaces, 
and within the cavities of the body, gives rise to the question, Why 
are they not normally present in the tissues 1 Here we may adopt 
Lister’s theory of standard vitality, based on the absence of micro- 
organisms from the healthy urethra and from a wound healing by 
first intention. Unless this standard vitality of cell elements be 
in some way impaired, all the conditions are unfavourable to the 
growth of micro-organisms. The circumstances in which we have 
depression of tissue vitality include, in addition to general organic 
depressants, such structural features as the formation of cavities, 
pouches, and wounds, in which there may be accumulation of 
secretions or other effete products, the structure, function, and 
arrangement of the epithelium covering a free surface, the position 
of the surface and its relations, first, to the external world, and 
second, to the deeper tissues, especially to the lymphatics, the 
number, size, and relation of these lymphatics, the tissues in which 
they occur, and the relations of the lymphatics to the small veins. 
Several instances might be quoted from the paper in illustration of 
the bearing of these several factors on the invasion of the tissues by 
micro-organisms. According to certain differences in the character 
and distribution of these invasions, it is possible to form a general 
classification of the action of the organisms which give rise to them. 
(1) This action may be purely local, and accurately confined to one 
area, beyond which neither the micro-organisms nor their products 
have any effect. (2) There may be a purely local action in the first 
instance, but this localisation is only temporary. The invasion goes 
on in stages, point after point being attacked. The spores 
developed at one point are transported to others, and so the process 
goes on intermittently, as in actinomycosis. (3) The organism may 
be limited to a definite tract, but its products may be diffused 
throughout the whole of the invaded organisation, as in certain cases 
of septicaemia. (4) There is no limitation to the sphere of activity 
of the micro-organism, which, with its irritant products may pene- 
trate to all parts of the tissues, and so give rise to a general disease, 
as in the case of the organisms which give rise to the specific fevers. 
