344 ANIMAL PHYSIOLOGY. 
In the mammals no such special helps for the propulsion of 
lymph exist. 
There is little doubt that the blood-pressure is always 
higher than the lymph-pressure, and when the blood-vessels 
Fic. 288.—Thoracic duct (Mascagni). 1, thoracic duct; 2, great lymphatic duct ; 3, recep- 
taculum chyli; 4, curve of thoracic duct just before it empties into the venous system. 
are dilated the fluid within the perivascular lymph-channels is 
likely compressed ; muscular exercise must act on the lymph- 
channels as on veins, both being provided with valves, though 
themselves readily compressible; the inspiratory efforts, espe- 
cially when forcible, assist in two ways: by the compressing 
effect of the respiratory muscles, and by the aspirating effect 
of the negative pressure within the thorax, producing a similar 
aspirating effect within the great veins, into which the large 
lymphatic trunks empty. The latter are provided at this point 
with valves, so that there is no back-flow; and, with the posi- 
tive pressure within the large lymphatic trunks (thoracic duct, 
etc.), the physical conditions are favorable to the outflow of 
lymph or chyle. 
