CHAPTER XLI 

 REPAIR OF INJURIES 



699. Injuries. Many causes outside the body operate 

 upon its cells to injure them. Excessive heat or cold may 

 impair their vitality or cause their death. A sudden change 

 from heat to cold is a common cause of injury. Blows and 

 cuts may kill whole armies of cells. Above all, bacteria 

 may cause injury and disease. In a few hours, the injured 

 part shows a change, which is apparently due to an in- 

 crease of the injury, but which is really caused by nature's 

 attempt to repair the part. 



700. Congestion. After an injury has been received 

 the first step in its repair is to dilate the arteries so as 

 to permit more blood to flow through the part. Then 

 more plasma will penetrate into the lymph spaces. This 

 produces redness and some swelling and is called conges- 

 tion. Congestion is a sign of attempted repair. This 

 alone may be sufficient to heal the injured part. 



701. Inflammation. If the injury is greater, there is 

 a change in the behavior of the white blood cells. Ordi- 

 narily they tend to flow more in the outer part of the 

 blood stream, but when the arteries enlarge as a result of 

 injury they adhere to the sides of the smallest blood tubes 

 and some pass entirely through their walls and lodge in the 

 lymph spaces. There they envelop and digest the injured 

 parts and carry them away with the lymph. The lymph 

 and blood cells have great power of absorbing blood and 



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