(b) Fibers are formed of elements of organic matter which have only a passive function. They can be assimilated to little strings, or cords, tangled one with another like a mass of waste yarn, woven regularly like a cloth, or bound together like a rope. They are of two kinds--white connective tissue fibers, only slightly extensible, pliable, and very strong, and yellow elastic fibers, elastic, curly, ramified, and very dense. These fibers once created require the constant presence of fluids around them in order to retain their functional condition, as a piece of harness leather demands continual oiling to keep its strength, but they undergo no change or alteration in their form until destroyed by death. (c) Cells, which may even be regarded as low forms of life, are masses of protoplasm or amorphous living matter, with a nucleus and frequently a nucleolus, which are capable of assimilating nutriment or food, propagating themselves either into others of the same form or into fixed cells of another outward appearance and different function but of the same constitution. It is simply in the mode of the grouping of these elements that we have the variation in tissues, as (1) loose connective tissue, (2) aponeurosis and tendons, (3) muscles, (4) cartilage, (5) bones, (6) epithelia and endothelia, (7) nerves. (1) Loose connective tissue forms the great framework, or scaffolding, of the body, and is found under the skin, between the muscles surrounding the bones and blood vessels, and entering into the structures of almost all the organs. In this the fibers are loosely meshed together like a sponge, leaving spaces in which the nutrient fluid and cells are irregularly distributed. This tissue we find in the skin, in the spaces between the organs of the body where fat accumulates, and as the framework of all glands. (2) Aponeurosis and tendons are structures which serve for the termination of muscles and for their contention, and for the attachment of bones together. In these the fibers are more frequent and dense, and are arranged with regularity, either crossing each other or lying parallel, and here the cells are found in minimum quantity. (3) In the muscles the cells lie end to end, forming long fibers which have the power of contraction, and the connective tissue is in small quantity, serving the passive purpose of a band around the contractile elements. (4) In cartilage a mass of firm amorphous substance, with no vascularity and little vitality, forms the bed for the chondroplasts, or cells of this tissue. (5) Bone differs from the above in having the amorphous matter impregnated with lime salts, which gives it its rigidity and firmness.