CIRCULATION 179 



stream of blood flows through the capillaries of a man's muscles; 

 in violent exercise, a great torrent, impelled by increased heart 

 action, brings food and oxygen and carries away waste products 

 and heat. This superfluous heat is carried by the blood to the 

 lungs and skin for elimination. The skin becomes flushed by the 

 distension of its blood vessels and heat is radiated, usually aided 

 by perspiration. Thus the body is losing heat, but we ' feel warm ' 

 because the sense organs that make us aware of temperature are 

 situated in the skin. We 'feel cold' when blood is withdrawn 

 from the skin to the internal organs and heat is being conserved. 

 This makes clear the apparent paradox — one is apt to ' feel warm 

 when catching cold.' 



So much for the paths and duties of the blood and lymph that 

 circulate through the body — just enough, perhaps, to emphasize 

 that with increase in size and complexity of the animal body there 

 goes hand in hand an elaboration of the transportation system. 

 It is gradually transformed to meet the new demands made upon 

 it, and so leaves in higher animals evidence of their origin. 



