Its particles (molecules),
instead of being completely separated, still cling together, forming
little masses that are too large to penetrate the membrane. Since,
however, it has the appearance, on being mixed with water, of being
dissolved, it is called a _colloidal solution_. The crystalloid substance,
on the other hand, completely separates in the water and forms a _true
solution_--one which is able to penetrate the partition or membrane.
*Osmosis not a Sufficient Cause.*--The passage of materials through animal
membranes, according to the principle of osmosis, is limited to
crystalloid substances. But colloid substances are also known to pass
through the various partitions of the body. An example of such is found in
the proteids of the blood which, as a colloidal solution, pass through the
capillary walls to become a part of the lymph. Perhaps the best
explanation offered as yet for this passage is that the colloidal
substances are changed by the cells lining the capillaries into substances
that form true solutions and that after the passage they are changed back
again to the colloidal condition.
*Summary.*--Between the cells and the capillaries is a liquid, known as the
lymph, which is similar in composition and physical properties to the
blood.
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