226 EVOLUTION AND DISEASE. 
it survives the injury, a series of events, varying accord- 
ing to the extent of tissue damaged, ensues. The 
simplest case is when the tissues are severed with a 
‘clean, sharp knife. In this case, unless a large artery is 
wounded, if the two surfaces of the wound be brought 
together and maintained in strict apposition, the bleeding 
will cease, and in the course of a few hours the whole of 
the damaged surface will be invaded by leucocytes, these 
gradually elongate and become transformed into tissue 
and form a uniting medium between the two surfaces 
of the wound. . 
When a wound is thus inflicted, and the surfaces not 
brought into contact, the result is different. Exposure 
to the air gradually arrests the bleeding, unless large 
vessels are cut; as in the preceding case the damaged 
tissue is invaded by leucocytes, and many cells lying 
exposed on the surface, being too far removed from the 
living tissue, die. The surface of the wound is at first 
moist, due to the exudation of fluid from the divided 
tissues ; this, in conjunction with the blood clot, dries in: 
consequence of exposure to the air and forms a scab, 
This is of great use, for in many instances it her- 
metically seals the wound, preventing the entrance of 
micro-organisms. Under the scab the leucocytes unite, 
become transformed into tissue of repair, and finally, 
completely fill the gap. Should micro-organisms pos 
sessing pathogenic properties gain entrance the leuco 
cytes attack them, the inflammation becomes often 
intense, fermentation, due to the growth of micro 
organisms in the juices of the wound, takes place 
products noxious to the individual are produced, an 
consequences, often disastrous, arise. 
