Outlines of zoology / by J. Arthur Thomson.

Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: Outlines of zoology / by J. Arthur Thomson. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    {b) Connective Tissue. I am afraid that this term is somewhat like the title worms. It- includes too many different kinds of things to mean much. It represents a sort of histological lumber-room. The embryologists help us a little, for they have shown that all forms of connective tissue are derived from the mesoderm or middle layer of the embryo. As this mesoderm usually arises in the form of outgrowths from the gut, or from ( mesenchyme ) cells liberated at an early stage from either (?) of the two other layers of the embryo (ectoderm or endoderm), we may say that connective tissue is primarily derived from epithelium. .Sometimes, e.g., in the lancelet (Aviphioxiis), this origin is very evident. Moreover, in the .Sponges and .Stinging-animals which have no strict mesoderm, there is usually a middle stratum ( mesogloea), of a gelatinous character in jellyfishes, with more abundant cells in .Sponges. It must be included as a form of connective tissue, and it is derived from the outer and inner layeis which enclose it. The general function of connective tissue is to enswathe, to bind, and to su]:)port, but the forms assumed are very various. (a) The cells may be close together, without any intercellular mortar or matrix. They may contain large vacuoles, and thus pro- duce the appearance of a network, or they may be crammed with fat or with pigment. (b) In other cases the cells of the connective tissue lie in a matrix, which they exude, or into which they in part die away. .Such cells are very often irregular in outline, and give off in most cases fine processes, which traverse the matrix as a network. The fibrous tissue of tendons and the different kinds of gristle or cartilage, are good illustrations of connective tissue with much matrix. Cartilage is sometimes hardened by the deposition of lime salts in its substance, and then has a slight re- semblance to another kind of connective tissue—bone. But bone, which is restricted to Vertebrate animals, is quite different from the cartilage which it often succeeds and replaces. It is made by strands or layers of special bone-forming cells (osteoblasts), which may rest on a cartilage foundation, or may be quite independent. These osteoblasts form the bone matrix, and some of them are involved in it, and become the permanent bone-cells. These have numerous radiating branches, and are arranged in layers, usually around a cavity or a blood-vessel. There are no blood-vessels in cartilage). The matrix becomes very rich in lime salts (especially phosphate); and the cartilage foundation, if there was one, is quite destroyed by the new formation. Here we may also note two important fluid tissues, the floating corpuscles or cells of the blood, and tho.se of the body-cavity or perivisceral fluid, which is often abundant and important in backbonele.ss animals. {c) Muscular Tissue. Origin.—The single-celled Anmba moves by flowing out on one side and drawing in its substance on another. It is diflusely contractile, and it has also sensitive, digestive, and other functions.