CONNECTIVE TISSUE. The most widely distributed tissue of the body. It. originates in the middle or mesoblastic layer of the embryo, and the differentiation which occurs and which distinguishes the different forms of connective tissue takes place mainly in the intercellularsub stance. Thus the intercellular substance may be soft and gelatinous, as in mucous connective tis sue, or dense and filly, as in fascia and tendon, or hard, as in bone. The cells of connective tis sue begin as small round mesoblastic cells. Either directly from these cells or under their influence, there is formed between the cells an intercellular substance, which, as stated, varies in character, and during the formation of which various changes take place in the cells them selves.
The principal types of connective tissue are as follows: (1) White fibrous connect-ive tissue. (2) Yellow elastic connective tissue. (3) Devel opmental forms of connective tissue, (a) mucous and (b) embryonal. (4) Cartilage. (5) Bone and dentine. (6) Adipose tissue or fat. (7) Neuroglia, the connective tissue of the nervous system. Of these forms of connective tissue, fat, cartilage, hone, and neuroglia (see NERVOUS Sys TEar) represent the more highly specialized types and will be found described in articles under their respective names. The remaining represent those forms of connective tissue to which the term usually refers. W hile Fibrous Connective Tissue.—This constitutes the subcutaneous con nective tissue and intermuscular septa, where it is known as areolar tissue; it also forms the ligaments, tendons, and the framework of all the organs. Its cellular elements consist of fixed con
nective-tissue cells and the so-called wandering cells. The fixed cells are mainly irregular or fusiform in shape, with very little cell-body. Much less numerous are the so-called plasma cells of Waldeyer and the granule cells. Some connective-tissue cells, such as many of those found in the choroid coat of the eye, are densely pigmented. The wandering connective-tiasue cells are probably identical with the white blood-cor puscles. (See BLoon.) In the intercellular sub stance two kinds of fibres are found, white fibres and yellow elastic fibres. The former occur in broad wavy bundles composed of minute fibrils; the elastic fibres are narrow, glistening, appar ently homogeneous hands which branch and anas tomose. There is much variation in the relative number of cells and fibres, the softer tissues be ing more cellular, the more dense tissues. such as tendon, being almost entirely composed of fibres. Yellow Elastic Tissue.—This may occur almost pure in some parts of the body, as in the liga ment um. nudity. (See .Nticz.) In such tissue, instead of the fine delicate fibres described above, the fibres are large and coarse. The Develop mental Forms of Connective Tissue.—The mu cous tissue constitutes the Wharton's jelly of the umbilical cord, the embryonal connective tissue found in fetal life. In mucous tissue the cells are stellate; with long branching processes which anastomose with those of other cells. The intercellular substance is gelatinous, with only a few fibres.