THE CONNECTIVE TISSUES.
Under this term are grouped certain tissues which to all outward appearance vary greatly from one another, but which are all produced in the developing body from the same parts, and act - as packing or supuli.124 Under certain circumstances one of these tissues may be substituted for another, and in certain situations one merges into another. The following is is list of such tissues:— I. Connective Tissues Proper: 1. White Fibrous Tissue.
2. Yellow Elastic Tissue.
3. Adipose or Fatty Tissna.
4. Areolar or Cellular 'tissue. A. Adenoid, Retiform, or Lymphatic Tissue.
6. Mucous Tissue.
II. Cartilage (commonly called Gristle), see p. 64: 1. Hyaline Cartilage.
2. White Fibro•Cartilage.
3. Yellow Fibro•Cartilage.
III. Bone and Dentine of Tooth (see IT. 57 and 190).
All of these tissues consist of three elements, the proportions of which vary in each tissue, namely, (1) a ground substance or matrix, (2) cells, (3) fibres. The ground substance is best seen in hyaline cartilage (p. 64), where it is transparent and glassy looking, but in the connective tissues proper it is in small amount, and is obscured by the mass of fibres. In bone, and in tooth, this ground substance is infiltrated with salts, which give the bone its hardness and make it so seemingly different from the other tissues. In the connective tissues proper the cells are called connective tissue corpuscles, in cartilage they are called cartilage cells, and in bone, bone corpuscles. The fibres are of two kinds, one exceedingly fine, transparent, and running a wavy course in bundles--fibres of white fibrous tissue, the other, coarse, yellowish, and elastic—fibres of yellow elastic tissue.
White Fibrous Tissue.—This tissue con sists of bundles of very delicate fibrils. In each bundle the fibrils run a more or less parallel course, though wavy. The bundles are bound together by a small amount of cement substance. Associated All them are the cc nnective-tissue corpuscles, irregular masses of nucleated protoplasm, often branch ing, but determined as to shape by the pres sure exerted on them by the bundles. They lie on the bundles, in minute passages between them, when the bundles run parallel as in tendon; or they lie in spaces, inclosed by the bundles, when these cross to form a felt-work, as in subcutaneous tissue, the loose tissue under the skin. On boiling, white fibrous tissue yields gelatin, and on the addition of a dilute acid the bundles swell up and become cloudy and gelatinous.
Now this tissue is found forming part of various structures, skin, tendon, membranes, loose tissue between and over muscles,.beneath skin, &c.; and in these different situations the bundles are variously disposed, parallel in ten don, crossing and recrossing in skin and inter muscula• tissue.
Yellow Elastic Tissue (Fig. 8, 2).—The fibres which form the main portion of yellow elastic tissue are much stronger and coarser than those of white fibrous tissue. They are yellowish, and tend to split and curl up at the ends. They possess a high degree of elasticity. It is these fibres that confer Jasticity on the skin, and on the coats of blood-vessels. This tissue is the main component of the ligamen tum nuchw, the broad ligament in the back of the neck of large quadrupeds for the support of the heavy head. It does not yield gelatin on boiling; acetic acid has no effect on the fibres, and cells are few in it if any.
Cellular or Areolar Tissue.—This is a compound tissue made up of bundles of white fibrous tissue interlacing and crossing one another to form a mesh-work. Numerous elastic fibres are present, conferring elasticity. The two kinds of fibres are easily distinguished, under the microscope, by adding dilute acetic acid, when the white fibres swell up, become transparent, and the unaffected yellow fibres are revealed. The interlacing bundles inclose little spaces or areola, hence the term areolar or cellular. Attached to the bundles or lying in the spaces are connective-tissue corpuscles. There are also other cells, identical with white blood corpuscles, which have probably found their way, by anurboid movement (p. 53), from the blood-vessels abounding in the tissue. It is a tissue found in large quantities under the skin, covering the muscles, the blood-vessels, and nerves, and in various parts forming a kind of protective covering for delicate and important organs. It is because of its general distribution, and because of its binding various; structures together, that it is called connective; The little spaces are filled up with fluid that has oozed out of the blood-vessels.
Adipose Tissue or Fat.—This is a simple modification of ordinary connective tissue. It contains fibres of white fibrous tissue, forming a mesh-work in which fat cells are embedded. These fat cells are round or oval, and consist of ordinary connective-tissue cells, in which oil drops have accumulated till the cells resemble little envelopes or sacs filled with oil. The oil may be removed and used up in the body, and then the cells are once more ordinary connective tissue cells. Adipose tissue forms a considerable layer beneath the skin, covers various internal organs, and is found in the marrow of bones and elsewhere. It is protective, in the sense of acting as a packing agent between organs; and it prevents the heat being carried off too quickly from the body, as it is a bad conductor.
Adenoid Tissue is described on p. 277.