vI FORM AND FUNCTION 71 
the veins of the limbs are 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, ze. 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 tu 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 
