*Where the Lymph enters the Blood.*—The fact that the lymph is poured into the blood at but two places, and these very close to each other, requires a word of explanation. As a matter of fact, it is impossible for the lymph to flow into blood vessels at most places on account of the blood pressure. This would force the blood into the lymph vessels, instead of allowing the lymph to enter the blood. The lymph can enter only at some place where the blood pressure is less than the pressure that moves the 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. [Fig. 31] Fig. 31—*Vessel* with an upright membranous partition for illustrating osmosis. *Osmosis.*—The term "osmosis" is used to designate the passage of liquids through some partition which separates them. Thus, if a vessel with an upright membranous partition be filled on the one side with pure water and on the other with water containing salt, an exchange of materials will take place through the membrane until the same proportion of salt exists on the two sides (Fig. 31). The cause of osmosis is the motion of the molecules, or minute particles, that make up the liquid substance. If the partition were not present, this motion would simply cause a mixing of the liquids.