vi FORM AND FUNCTION 71 



the veins of the limbs arc valves which prevent any 

 backward flow. Every movement must tend by 

 pressure to move the blood forward or backward, and 

 it will be urged forward since no other course is open. 

 The places where these valves are in the human arm or 

 hand can be seen if a finger be pressed upon a vein 

 and then passed downwards along it in the direction 

 of the capillaries, thus tending to cause a backward 

 current. Little knots will be seen at intervals, 

 marking the places where the passage of the blood 

 is checked by the pouchlike valves. Birds have 

 fewer of these valves than mammals, but more than 

 reptiles. 



Besides the veins there are other channels in all 

 parts of the body along which a current is setting 

 towards the heart. These are the lymphatics, so 

 called because they contain a pale watery fluid. They 

 differ from veins (1) in that the capillaries from which 

 they spring end blindly, i.e. do not connect with 

 arteries, (2) in having in their course numbers of 

 glands through which their contents must pass. Their 

 main function seems to be to assist the veins in 

 carrying on the drainage of the body. Some of the 

 lymphatics, however, have a special function and a 

 special name. They are called lacteals from the 

 milky nature of their contents, due to food containing 

 fat, and their duty is to carry the chyle to the heart 

 from the smaller intestine round which their capillaries 

 form a network. Like the veins they are provided 

 with frequent valves preventing any backward flow* 

 and eventually they pour their contents, in birds, into 

 the two trunk veins which bring; the blood from the 



