226 HUMAN PHYSIOLOGY 



tion until it may block the channel. The obstruction of 

 a blood-vessel may be fatal if it is in the brain or the 

 heart but in many cases it has no serious results. This is 

 because most parts of the body are not dependent upon 

 single sources of blood-supply. 



Sometimes a clot formed in one place is later detached 

 and moves to another locality where it may do more 

 damage than it did in the first region. Thus during 

 convalescence from pneumonia there is the ugly possi- 

 bility that blood-clots from the temporarily obstructed 

 veins of the lungs may work loose and be carried away 

 in the stream. One of these may reach an important 

 artery of the brain and by stopping it cause sudden death. 

 The course followed in such instances will be more clear 

 after the next chapter. 



