Fig. 2.—Section of bladder epithelium. (Hill.)
Closely allied to the areolar is the [fibrous tissue], in which the white fibers lie close together and run for the most part in one direction only. This is found in ligaments, joints and tendons, as also in such fibrous protective membranes as the periosteum, dura mater, the fasciæ of muscles, etc. Fibrous tissue is silvery white in appearance and is very strong and tough, yet pliant. It is not extensile.
Elastic tissue, on the other hand, has a large predominance of yellow elastic fibers and is very extensile and elastic, though not so strong as the fibrous. It is found in the walls of the blood-vessels, especially the arteries, in the walls of the air tubes, in the ligaments of the spine, etc.
[Fatty or adipose tissue] is formed by the deposit of fat in the cells of the areolar tissue and is found in most parts where the areolar tissue occurs, though it varies largely in amount in different parts. It is found pretty generally under the skin, fills in inequalities about various organs and about the joints, and exists in large quantities in the marrow of the long bones. In moderate amounts it gives grace to the form and constitutes an important reserve fund.
Fig. 3.—Adipose tissue (Leroy): a, Fibrous tissue; b, fat cells; c, nucleus of fat cells; d, fatty acid crystals in fat cells.
Cartilage consists of groups of nucleated cells in intercellular substance. It is very firm, yet highly elastic, and serves in the joints to break the force of concussion of the harder and less elastic bones. Except when it occurs at the end of a bone, it is covered with a membrane called the perichondrium, which carries its blood supply. In the nose, ear, larynx and trachea it serves to give shape, to keep the passages open, and to afford attachment for muscles. Most of the skeleton of the fetus consists of cartilage, which later develops into bone.
Bone.—In [bone] the intercellular tissue is rendered hard by the deposit of mineral salts, the resulting material being of great strength and rigidity. The texture may be close and dense like ivory or open and spongy, the difference lying merely in the fact that the one has fewer spaces between the solid particles than the other. There is usually a hard, compact layer on the exterior of the bone, as that is where the greatest cross-strain comes, especially in the long bones, while within is the cancellous or spongy tissue, which gives lightness to the bone and is capable of withstanding enormous pressure, though it can bear little cross-strain.
Fig. 4.—Cross-section of compact bone tissue. (After Sharpey.)