III.  Bone and Dentine of Teeth.

20.  White Fibrous Tissue.  This tissue consists of bundles of very delicate fibrils bound together by a small amount of cement substance.  Between the fibrils protoplasmic masses (connective-tissue corpuscles) are found.  These fibers may be found so interwoven as to form a sheet, as in the periosteum of the bone, the fasciae around muscles, and the capsules of organs; or they may be aggregated into bundles and form rope-like bands, as in the ligaments of joints and the tendons of muscles.  On boiling, this tissue yields gelatine.  In general, where white fibrous tissue abounds, structures are held together, and there is flexibility, but little or no distensibility.

[Illustration:  Fig. 6.—­White Fibrous Tissue. (Highly magnified.)]

21.  Yellow Elastic Tissue.  The fibers of yellow elastic tissue are much stronger and coarser than those of the white.  They are yellowish, tend to curl up at the ends, and are highly elastic.  It is these fibers which give elasticity to the skin and to the coats of the arteries.  The typical form of this tissue occurs in the ligaments which bind the vertebrae together (Fig. 26), in the true vocal cords, and in certain ligaments of the larynx.  In the skin and fasciae, the yellow elastic is found mixed with white fibrous and areolar tissues.  It does not yield gelatine on boiling, and the cells are, if any, few.

[Illustration:  Fig. 7.—­Yellow Elastic Tissue. (Highly magnified.)]

22.  Areolar or Cellular Tissue.  This consists of bundles of delicate fibers interlacing and crossing one another, forming irregular spaces or meshes.  These little spaces, in health, are filled with fluid that has oozed out of the blood-vessels.  The areolar tissue forms a protective covering for the tissues of delicate and important organs.

23.  Adipose or Fatty Tissue.  In almost every part of the body the ordinary areolar tissue contains a variable quantity of adipose or fatty tissue.  Examined by the microscope, the fat cells consist of a number of minute sacs of exceedingly delicate, structureless membrane filled with oil.  This is liquid in life, but becomes solidified after death.  This tissue is plentiful beneath the skin, in the abdominal cavity, on the surface of the heart, around the kidneys, in the marrow of bones, and elsewhere.  Fat serves as a soft packing material.  Being a poor conductor, it retains the heat, and furnishes a store rich in carbon and hydrogen for use in the body.

24.  Adenoid or Retiform Tissue.  This is a variety of connective tissue found in the tonsils, spleen, lymphatic glands, and allied structures.  It consists of a very fine network of cells of various sizes.  The tissue combining them is known as adenoid or gland-like tissue.

[Illustration:  Fig. 8.—­Fibro-Cartilage Fibers. (Showing network surrounded cartilage cells.)]