Cellular Tissue, a name given by the older anatomists to a tissue formed by a mixture of white and yellow fibres, extensively diffused in the animal body under the names of cellular, fibro-cellular, areolar, and fibrous tissue. The best name is areolar tissue, derived from the appearance of areola, or meshes, left between the intricate crossings of the component fibres; these were formerly mistaken for cells or cavities; the old term cellular tissue, however, is so well and universally understood, that, though inaccurate, it will probably long be employed in this application. Its principal use seems to be to connect other tissues, allowing at the same time more or less freedom of motion between them; it supports the vessels and nerves in their minutest branches; it is abundant under the skin and the mucous and serous membranes; it enters largely into the formation of membranes, hence often called cellular membranes, protecting the organs and cavities by their toughness and elasticity. The spaces of the cellular tissue are continuous throughout the body, as may be proved by artificial inflation by the blowpipe, and as is frequently seen in cases of emphysema and anasarca, where air or fluid is effused into its meshes.
Under the microscope this tissue presents two kinds of fibres, inextricably mingled in various proportions. The one is white and inelastic, disposed to a waved or zigzag arrangement in bands of unequal thickness, creased longitudinally by numerous streaks; the largest of these bands are often 1/500 of an inch wide; the component fibres do not branch, according to Ilassall; this is the white fibrous tissue. The other kind of fibre is elastic, of a yellowish color, composed of branched filaments disposed to curl when not put on the stretch; they are generally about 1/8000 of an inch thick, interlacing with the others without becoming continuous with them; this is the yellow fibrous tissue. These two elements of the cellular tissue may be at once distinguished by submitting it to the action of dilute acetic acid, which instantly causes the former to swell up and become transparent and soft, while it causes no change in the latter. Cellular tissue is especially abundant in parts which enjoy free motion, as in the face about the eyes and cheeks, the anterior part of the neck, the armpit, the flexures of the joints, the palm of the hand, and the sole of the foot; the superficial and most movable muscles are separated by thicker layers than the deep-seated ones, and the constituent fibres are held together by it during contraction; almost every part of the vascular system is held in place by this tissue, whose elasticity protects the vessels during the necessary movements of the body; even its own minute but numerous vessels are conducted and enveloped by this all-pervading tissue.
It is difficult to say where cellular tissue is not found, unless it be in the teeth, in bone, in cartilage, and in the cerebral substance, where its presence would be manifestly useless. The internal vital organs most exposed to external violence are protected by large quantities of this substance, as the pancreas, kidneys, colon, and genito-urinary apparatus; every organ has its investing covering of cellular tissue, and its processes of the same penetrating and holding together its component parts. It is especially abundant just under the skin, to facilitate its movements, and it exists in uncommon quantity about and in the interior of the mammary glands. Thus this tissue seems to serve as a bond of union between parts, as an element of strength and protection rather than as a substance of primary importance in itself; wherever elasticity is required, the yellow fibrous tissue is most abundant, while the white fibrous tissue prevails in parts demanding resistance and tenacity; and the openness of the meshes is in proportion to the amount of mobility needed.
The amount of cellular tissue varies with age and temperament, being greatest in youth and least in old age; the plumpness and roundness of the arms in children and females depend to a great extent on the presence of this substance around the joints, which in man are prominent and angular. Like other soft solids, it contains a small quantity of serous fluid in its interstices, which is favorable for the free movement of the fibres; an unnatural increase of this fluid in the subcutaneous cellular tissue causes the form of dropsy called anasarca, so common about the feet and ankles, and indicated by the skin pitting under the pressure of the finger. In the English training process it is rapidly lessened, with a remarkable diminution of the bulk of the body; its natural and slow disappearance is seen in old age and in chronic disease, in which the skin, especially about the face and neck, becomes wrinkled and flabby. Its power of reproduction is great, and it is rapidly formed both in healthy and morbid growths; it undergoes the putrefactive process slowly, and when boiled yields gelatine from its white fibrous element. - So extensive a tissue as this must of necessity become involved in many diseases; it is subject to all the effects of inflammation, with suppuration and mortification; to the infiltration of blood, serum, air, and urine; to induration, tumors, and unnatural increase and degeneration.
In common inflammation of this tissue, the capillaries become congested, and a part of their contents escapes, more or less tinged with blood; the coagulable lymph thus effused causes the hardness of circumscribed inflammation; this may be removed by absorption, or may become softened by the deposition of purulent matter, constituting an abscess, whose walls are formed by an indurated layer of the tissue which prevents the pus from spreading indefinitely. When an abscess is formed, the cellular tissue between it and the surface of the skin is removed by ulceration or absorption, or the pus is evacuated by the knife; when from excess of inflammation or other cause the capillary circulation is permanently suspended, the vital properties of the tissue are destroyed, and mortitication takes place, the dead parts being removed in offensive fiuicls and pulpy shreds. In chronic inflammation the cellular tissue becomes indurated. In debilitated conditions of the system, after poisoned wounds, and in certain epidemic alterations of the air, the usual barrier of circumscribing lymph is not effused, and the products of inflammation spread extensively through the areola of the subcutaneous and internal cellular tissue; this is familiarly seen in phlegmonous erysipelas, and constitutes a most dangerous disease from the extensive suppuration and sloughing of the tissues.
In wounds of the lungs a communication is often established between the air passages and this tissue, when the integuments are variously raised by the infiltration of air in the areola), constituting external emphysema; a similar condition is artificially produced by the butcher when he blows up his meat. It grows with such rapidity that tumors, often of large size, are developed from it; most so-called "fibrous" tumors are composed of this tissue; in such cases the microscopist is able to detect the fusiform cells and fibres characteristic of the natural tissue.