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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    In Hydra and some other Ccelenterates, the bases of the epithelial cells which form the outer layer and line the internal cavity, are pro- longed into contractile roots. By the activity of these muscular roots the Hydra elongates and contracts its body. Here then we have cells of which a special part discharges a contractile or muscular function, while the other parts of the cells retain other powers. (The external contractile cells in Hydra are often called neuro-muscular, as if they combined nervous and muscular functions. It is possible that they do, but the existence of special nerve-cells does not favour the idea in this case at least.) In other Ccelenterates, the muscular cells are still directly connected with the epithelium, but become more and more exclusively devoted to the function of contraction. In all other animals the muscular tissue is derived from the mesoderm, which, as we have already mentioned, is not distinctly present in Ccelenterates. In the majority the muscle-cells arise on the walls of the body-cavity, and their origin may often at least be justly described as epithelial. But in other cases the muscles are started by those wandering mesenchyme cells, to which we have already referred. So, as regards origin, muscular tissue may be classified as follows :— 1. In Ccelenterates, where there is no definite mesoderm, the muscle-elements arise from the ectodermic and endodermic epithelium, of which they often form a direct part. In the passive Sponges, the contractile elements are few and un- important. 2. In other animals the muscle-elements arise from the middle layer, but O. and R. Hertwig distinguish between those which have an epithelial and those which have a mesenchy- matous origin. Structure.—A distinction is usually drawn between striped and un striped muscle-fibres, but, according to the Hertwigs, this distinction is of little morphological value. .Smooth or unstriped muscle-fibres are elongated contractile cells, ex- ternally homogeneous in appearance. They are especially abundant in sluggish animals, e.g.. Molluscs, and occur in the walls of the gut, bladder, and blood-vessels of Vertebrates, where they are somewhat quaintly called involuntary. They are less perfectly differentiated than striped muscle-fibres, and usually contract more slowly. A striped muscle-fibre is a cell, the greater ]3art of which is modified into a set of parallel longitudinal fibrils, witfi alternating clear and dark transverse stripes. A residue of unmodified cell-substance, with a nucleus or with many, is often to be observed on the side of the fibre, and a slight sheath or sarcolemma forms the cell-wall. Many muscle- fibres closely combined, and wrapped in a sheath of connective tissue, form a muscle, which, as every one knows, can contract with extreme rapidity when stimulated by a nervous impulse. The contraction involves a visible change of form, associated with a chemical explosion in the cell-substance with the production of heat and waste-products, and with subtle changes of electric potential. One of the greatest marvels of animal life, is the strength and sustained power of muscles.