AREOLAR TISSUE. The nature of this tissue will be best under stood if we first describe Fibrous Tissue, of which it may be regarded as a modification. Fibrous Tissue is now usually considered under two heads, namely, as the white and the yellow tissue. White Fibrous Tissue occurs in ligaments, tendons, and membranes requiring great strength. On carefully dissecting away the areolar tissue with which it is asso ciated, it seems, when examined under the microscope, to consist of extremely delicate fibrillae running parallel to one another, and taking an undulating course. There is however reason to believe it does not in reality consist of a bundle of fibrillae, but that it is simply a mass with longitudinal parallel streaks, and which has a tendency to split up in a longitudinal direction. (Fig. 2, a.) Yellow Fibrous Tissue differs in many essential points from the pre ceding form. It is remarkably elastic, is of a yellow colour, and is arranged in bundles or fibres, invested by a thin sheath of areolar tissue. In man we find it extended between the laminae of the ver tebrae, in several other ligaments, and in the tmnsversalis fascia of the abdomen. It forms the ligamentum nuchm of animals. Examined under the microscope it is seen to consist of fibres varying in diameter from the 5000th to the 10,000th of an inch. They bifurcate, or even divide into three, and freely anastomose with each other.
A Tissue is dispersed over almost every portion of the body, being the substance most commonly (but incorrectly) termed Cellular Tissue. The following are the microscopic characters of this tissue, as described by Bowman and Todd :—" When a fragment is examined it presents an inextricable interlacement of tortuous and wavy threads, intersecting one another in every possible direction. They are of two kinds. The first are chiefly in the form of bands of very unequal thickness, and inelastic. Numerous streaks are visible in them, not usually parallel with the border, though taking a general longitudinal direction. These streaks, like the Lands themselves, have a wavy
appearance, but can be rendered straight by being stretched. The streaks seen have more the marks of longitudinal creasing than a true separation into threads ; for it is impossible to tear up the band into filaments of determinate size, although it manifests a decided tendency to tear lengthways. The larger of these bands are often as wide as the 500th of an inch ; the smaller can only be detected with high powers. These are the white fibrous element. The others are long, single, elastic, branched filaments, with a dark decided border, and disposed to curl when not put on the stretch. They interlace with the others, but appear to have no continuity of substance with them. They are most commonly about the 8000th of an inch in diameter. These form the yellow fibrous element.
These two tissues may be most easily discriminated by the addition of a drop of dilute acetic acid, which at once swells up the former and renders it transparent, whilst it produces no change in the latter.
It thus brings into view corpuscles of an oval shape, which arc probably the nuclei of the cells from which the bands have been originally produced. Oval corpuscles (fig. 2, d), either altogether isolated or hav ing very delicate prolonga tions with the adjacent threads, are sometimes noticed. They seem to be either advancing or receding stages of the tissue.
In fig. 3, which repre sents the Areolar Tissue from beneath the skin of a five-months fcetus, we can perceive the cells elongating into fibres.
In a chemical point of view the leading difference between the white and yellow tissues is, that the former is acted on by acetic acid in the manner described, and yields a considerable amount of gelatine in boiling ; while the latter resists the action of acetic acid, yields little or no gelatine.