gelatin, whereas the yellow elastic fibre is quite insoluble under
these conditions. The white fibres swell when treated with weak
acetic acid, and are readily dissolved by peptic digestion but
not by pancreatic. The yellow elastic fibres, on the other hand,
are unaffected by acetic acid and resist the action of gastric
juice for a long time, but are dissolved by pancreatic juice.
 Fig. 1.—Connective tissue, showing cells, fibres and ground- |
In physical properties the white fibres are inextensible and extraordinarily strong, even being able, weight for weight, to carry a greater strain than steel wire. The yellow elastic fibres, on the other hand, are easily extensible and very elastic, but are far less strong than the white fibres. Their elasticity is exhibited by their straight course when viewed in a stretched preparation of areolar tissue, and this contrasts markedly with the wavy course of the bundles of white fibres seen in the same preparation.
The Cells of Areolar Tissue.—Several types of cells are found in the spaces of this tissue and are usually classified as follows. (1) Lamellar cells. These are flattened branching cells which usually lie attached to the bundles of white fibres or at the junction of two or more bundles. The branches commonly unite with similar branches of neighbouring cells. (2) Plasma cells. These are composed of a highly vacuolated plasma, are not flattened but otherwise vary greatly in shape. (3) Granular cells. These are spherical cells densely packed with granules which stain deeply with basic dyes. (4) Leucocytes. These are typical blood corpuscles which have left the blood capillaries and gained the tissue spaces. They vary much in amount and in variety.
Adipose or Fatty Tissue.—This consists of rounded vesicles closely packed together to form a dense tissue, found for instance around an organ, along the course of the smaller blood vessels, or in the areolar tissue beneath the skin. This tissue is formed from areolar tissue by an accumulation of fat within certain of the cells of the tissue. These are especially the granular cells, though some regard the fat cells as specific in character, and to be found in large numbers only in certain parts of the body. The fat is either taken in as such by the cell, or, as is more commonly the case, manufactured by the cell from other chemical material (carbohydrate chiefly) and deposited within it in the form of small granules. As these accumulate they run together to form larger granules and this process continuing, the cell at last becomes converted into a thin layer of living material surrounding a single large fat globule. The use of fatty tissue is to serve as a storehouse of food material for future use. In conformity with this it is packed away in parts of the body where it will not interfere with the working of the different tissues and organs, and in several positions is made use of as packing to fill up irregular spaces, e.g. between the eyeball and the bony socket of the eye.
Reticular Tissue.—This is a variety of connective tissue in which the reticulum of white fibres is built up of very fine strands leaving large interspaces in which the cells typical of the tissue are enclosed. The ground substance of the tissue is reduced to a minimum. Many connective tissue cells lie on the fibres which may in places be completely covered by them. This tissue therefore forms a groundwork holding together the main parts of an organ to form a compact whole. It may thus be demonstrated in lymphatic glands, the spleen, the liver, in mucous membranes and many other cellular organs.
Fig. 2.—Tendon of rat’s tail, stained with gold chloride and
showing cells arranged in rows between the bundles of fibres.
 |
| Fig. 3.—Transverse section of portion of a tendon showing arrangement of white fibres in large bundles bounded by connective tissue, with tendon cells between the fibres. 𝑎, tendon cells; 𝑏, tendon bundles. |
White Fibrous Tissue.—This is the form of tissue in which the white fibres largely preponderate. The fibrous bundles may be all arranged parallel to one another to form a dense compact structure as in a tendon.
It is found wherever great strength combined with flexibility is required and the fibres are arranged in the direction in which the stress has to be transmitted. In other instances the bundles may be united to form membranes, and in such cases the main number of bundles run in one direction only, which is again that in which the main stress has to be conducted. Such are the ligaments around the joints or the fasciae covering the muscles of the limbs, &c. In other positions, e.g. the dura mater, the fibrous bundles course in all directions, thus forming a very tough membrane. The cells of such tissues lie in the spaces between the bundles and are found flattened out in two or three directions where they are compressed by the oval fibrous bundles surrounding them (see figs. 2 and 3). The cells thus lie in linear groups running parallel to the bundles, presenting a very characteristic appearance.
Yellow Elastic Tissue.—This is the form of connective tissue mainly composed of elastic fibres. It is found in those positions where a continuous but varying stress has to be supported. In some positions the elastic tissue is in the form of branching fibres arranged parallel to one another and bound together by white fibres, e.g. ligamentum nuchae (fig. 4). In other cases it may be in the form of thin plates perforated in many directions to form a fenestrated membrane. In this type a series of such plates are arranged round the larger arteries forming a large proportion of the artery wall.
 Fig. 4.—Isolated elastic fibres of ligamentum nuchae. Branching fibres |
All the connective tissues are vascular structures though as the number of cells present is not great, and further as those cells are not as a rule the seat of a very active metabolism, the number
