oo 
Herbert M. Evans 20 
areas of supply or drainage, the alternation of vein and artery is almost 
always present. On the whole, however, it is commoner to find that from 
either side, capillaries are given to both sides of the lymphatic. This also 
may occur in a fairly regular plan, approximating the condition shown 
om Big, 7. 
According as the blood-supply is unilateral or bilateral in character, 
we can observe in all lymphatic ducts certain differences in the plan of 
circulation which are easily correlated with the maintenance of a constant 
length of capillary bed. This will be readily seen by reference to the 
diagrams A and B in Fig. 8. In A, the duct is so chosen that its 
Fic. 9. Illustrating unilateral blood-supply of vessels one-half millimeter 
in diameter. X 50. 
diameter is approximately equal to the length of the capillary path, 7. ¢., 
from the point a to the point b. In B, the duct is of the same size, but as 
the supply is only unilateral, the existence of merely capillary paths on the 
lymphatic wall would necessitate the length of the capillary bed being 
double. Here, however, an artery of a length, half the distance from 
a to b extends out over the lymphatic wall to the point z; and a cor- 
responding vein on the other side extends to the point y. The distance 
from z to y is thus equal to that from a to b. 
Fig. 9 shows two lacteals about equal in size to that figured in Fig. 5; 
