lymph. Such a place is found in the thoracic cavity. As already pointed
out (page 54), the blood pressure in the veins entering this cavity
becomes, with each expansion of the chest, negative, i.e., less than the
pressure of the atmosphere on the outside of the body. This, as we have
seen, aids in the flow of the blood into the right auricle. It also aids
in the passage of lymph into the blood vessels. The lymph is said to be
"sucked in," which means that it is forced in by the unbalanced pressure
of the atmosphere.(28) Some advantage is also gained by the lymph duct’s
entering the subclavian vein on the upper side and at its union with the
jugular vein. Everything considered, it is found that the lymph flows into
the blood vessels where it can be "drawn in" by the movements of breathing
and where it meets with no opposition from the blood stream itself (Fig.
30).

[Fig. 30]


Fig. 30—*Diagram* showing general movement of lymph from the place of
relatively high pressure at the lymph spaces to the place of relatively
low pressure in the thoracic cavity.


*Lymph Movements at the Cells.*—The double movement of the lymph from the
capillaries toward the cells and from the cells toward the capillaries is
not entirely understood. Blood pressure in the capillaries undoubtedly has
much to do in forcing the plasma through the capillary walls, but this
tends to prevent the movement of the lymph in the opposite direction.
Movements between the blood and the lymph are known to take place in part
according to a general principle, known as _osmosis_, or dialysis.