226 E VOL UTION 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, and 

 consequences, often disastrous, arise. 



