DISTURBANCES OF CIRCULATION 135 



Local anemia, or ischemia, is the condition in which a 

 part of the body contains less than the normal amount of 

 blood. Pressure is the most frequent cause of this con- 

 dition, though an artery may be obstructed by sclerosis 

 of the vessel walls or by emboli or thrombi. The anemic 

 area is pale, reduced in size and temperature, and func- 

 tionally less active. 



Hemorrhage is the escape of blood from the blood- 

 vessels. It is termed arterial, venous, or capillary, ac- 

 cording to the vessel from which the flow of blood occurs. 

 It may occur by diapedesis or extravasation through 

 an intact vessel wall or by rupture of the walls of the 

 vessel. 



Diapedesis. Under certain abnormal conditions red 

 blood-corpuscles may pass through an intact vessel wall 

 and collect in the tissues. This is known as diapedesis. 

 Altered states of the vessel walls and of the blood itself 

 induced by infectious diseases give rise to diapedesis. 

 The small collections of blood in the tissues are called 

 petechiae. The petechiae of purpura and the rose spots 

 of typhoid fever are examples of diapedesis. 



Emigration. Under normal conditions a certain 

 number of white blood-corpuscles escape from the cap- 

 illaries and wander about in the tissues; this is a normal 

 process and is known as emigration. 



Hemorrhage from rupture of the vessel walls may occur 

 from 



(i) Traumatism, knife wounds, crushing injuries, etc. 



