CHAPTER XI. 



PATHOLOG Y— continued. 



CHANGES IN THE B'LOOJ)— continued. 



HYPEREMIA OR CONGESTION. 



The liability of the blood to be variously altered and modified 

 having been described in the preceding chapter, it now becomes 

 necessary to enter into a consideration of certain changes which 

 may occur in its circulation, arising from an alteration in the 

 condition and properties of the blood-vessels, and causing an in- 

 terference with its natural flow. 



By hyperosmia is meant an excess of blood in the vessels of 

 a part. It is of two kinds, namely, active or arterial, and 

 mechanical or venous. 



Active hyperaamia is an excess of blood in the arteries, with, 

 most commonly, an increased rapidity of the flow, and arises 

 from increased blood pressure and from diminished arterial 

 resistance. 



Hyperemia from increased blood pressure occurs from inter- 

 ruption of the main current of blood in any particular part, 

 owing to which increased pressure is thrown upon the collateral 

 vessels, which thus become dilated, the amount of blood in 

 them being increased, and its flow accelerated. This kind of 

 congestion is seen after the main current of a part has been 

 obstructed from any cause, and is known as collateral hyperccmia. 



Congestion of the vessels of one part from an obstruction to 

 the flow of blood in another, is also well exemplified iu con- 

 gestion of the internal organs from contraction of the superficial 

 capillaries, so often produced by exposure to cold. 



In some instances, the intropulsive operation of cold is so great 

 that the internal capillaries become loaded with blood to such 



