Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/442

 396

anatomy

tain but little protoplasm, and in consequence they easily suffer numerous lymph canals ; and (3) fibres, which may be of necrosis. two kinds, either white, non-extensile, arranged in more or All forms of connective tissue begin as an indifferent less wavy bundles of parallel fibres, or yellow, nig i y ex embryonic mesenchyma, whose subsequent differentiations tensile and resilient, arranged in frequently dividing and are reactions due to the influence of environing chemistry uniting bundles. . ., conditions, which are usually definite and recog- ofconaecThere are many varieties of connective tissue m the Hence the history of the connective t.vet.ssue. animal body (see i. 849 and xii. 6). The typical form is nizable. tissues of the body forms an important chapter in dynamical that called areolar tissue (Fig. 3). This consists of irregu- ontogeny. The chemical relations of the different forms larly-interlaced wavy bundles of white and yellow fibres in connective tissue are generally characteristic. The a copious ground-substance, through which are scattered a of number of flatfish or branched cells. When immature the white fibrils and ground-substance of areolar and white ground-substance is continuous and abundant; but in the fibrillar tissue yield gelatine on boiling, cartilage yields adult it diminishes and becomes broken up by the forma- chondrin, and yellow tissue consists of a substance which tion of gaps or spaces, which give it a somewhat spongy resists acetic acid and boiling water, and is named elastm. texture, so that it can be inflated with air or infiltrated The animal part of bone in most respects agrees with that with fluid. This tissue is almost universally diffused in of white fibrillar tissue. The process of. ossification has the body, varying in texture in each part according to been described in vol. i. p. 855, where it is noted that it local requirements. Under each layer of epithelium it may take place either in cartilage or in membrane. This forms a dense, almost structureless, basement membrane. is an important consideration in the determination of bony In some organs its meshes are numerous and fine, contain- homologies, for a bone which was originally cartilaginous ing entangled in them large masses of white lymph-cor- is morphologically of a different order from one which puscles, which congregate and multiply in these spaces. started as a membrane. The reason is not far to seek. It This form is known as adenoid tissue. In some regions is not that the processes differ intrinsically, but the early groups of connective tissue - cells are closely packed stages of the ancestral history recapitulated in its embryotogether, and their cell-contents become charged with oily geny must be diverse as the embryonic tissue has become or fatty material. Masses of this nature make up adipose so diversely differentiated. The history of the progress of tissue. In other cases the fibre elements, either white or ossification has been greatly elucidated by the use of the yellow, preponderate. The white fibrillar is the toughest Kontgen rays, whereby the bony nuclei can be detected at tissueinthebody; a thread of it, measuring 1 square milli- their first formation, and their growth and union can be metre in section, bore without rupture a weight of 8940 traced in the living adolescent. In some cases in which large clefts occur in masses of grammes, while a cord of yellow tissue broke with a weight of 4040 grammes. Cartilage and bone are special varieties areolar tissue the tissue forming the boundaries of such clefts of connective tissue. In the former the tissue makes solid consists mainly of flatfish connective cells, which pormation r masses, in which the cells, either separate or in small in some cases are close enough to simulate pave- 0° ™vitieSw clusters, are embedded in a hyaline matrix of ground-sub- ment epithelium. Such a layer is called endothestance with or without fibrous elements. In some inverte- lium to distinguish it from the true epithelium. This term, brates and lower vertebrates a form of cartilage exists though etymologically objectionable, is in the main conwhich contains mucin instead of chondrogen, recognized by venient, and is in general its staining yellow with thionin. Bone is characterized by use. These clefts are of the calcification of its ground-substance, which four kinds—joint cavities, Bi)ae ' becomes infiltrated with lime salts, while for the bursae, sheaths of tendons, most part the fibres and the branched connective corpuscles and serous cavities. In remain uncalcified (see vol. i. p. 852). The physical pro- joint cavities the surfaces Ty, perties of bone fit it in an eminent degree to form the of the bone are covered skeleton of the animal body. When its specific gravity, with cartilage, and have its breaking limit with tensile strain, its crushing limit no endothelial coating, but when subjected to pressure, and its modulus of elasticity the ligamentous side-wall are compared with those of other materials, this superiority of the joint is lined with becomes apparent. (See Lesshaft, Theoret. Anat. Leipzig, a specially smooth connective layer, named syn1892.) ovial membrane (Fig. 5). In the following table the numbers in the second and third columns are the weights in pounds avoirdupois required to break On the surface there are or crush a square rod of the material 1 mm. in diameter. The often patches of endothemodulus of elasticity is the weight in pounds which, if these sub- lium, although never a ■ & stances were perfectly elastic, would be required to lengthen the continuous layer of such square rod to twice its original length. To compare these so as to cells. Groups of endodetermine the skeleton-building suitability of each material, the number in each column should be divided by the specific gravity of thelial cells, however, are the material:— often clustered on fringes at the margin of the bones Substance. Sp. Gr. Tensile. Crushing. Elasticity. in these cavities, and these 7-816 225 46,300 319 secrete a mucilaginous Fio. 5.—Vertical section through kneeSteel. . Wrought-iron 7-788 48 90 41,887 exudation called synovia joint, with the bones somewhat drawn apart, showing the synovial cavity of the 160 28 Cast-iron 7-207 (vol. i. p. 833). Bursae joint (j) and the burs* (h) in its vicinity.. 1-870 33 26 5,’291 Bone. . are clefts between planes 0-845 14 10-5 2,425 Oak. . of firm areolar or fibrillar tissues which glide on each other.. The important factor that the skeleton must present a surface of These are also lined by synovial membrane, but never sufficient extent to give attachment to the muscles of the body present a continuous endothelium ; indeed, in many bursae makes it necessary that the material of which it is made must be these are only represented on a few of the synovia-secreting of comparative lightness. Bone is the most vascular form of con- processes. The same is true of the sheaths of tendons. nective tissue and the most active in nutritive changes. In general, connective tissues are low in vitality ; their elements con- Serous membranes form a separate category, for, while in