. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. HISTOLOGY. 21 sue, which arises from a modification of the mesothelium. Except in the case of the muscles of the heart, the striped tissue is under control of the will; it usually occurs in larger masses than does the smooth, and is capable of rapid contraction. It differs structurally from smooth muscle. Instead of distinct, uninucleate cells there are long cylindrical elements (fig. 13, B), the primitive fibres, each with several nuclei in the interior in lower vertebrates, on its periphery in the higher. Mos
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. HISTOLOGY. 21 sue, which arises from a modification of the mesothelium. Except in the case of the muscles of the heart, the striped tissue is under control of the will; it usually occurs in larger masses than does the smooth, and is capable of rapid contraction. It differs structurally from smooth muscle. Instead of distinct, uninucleate cells there are long cylindrical elements (fig. 13, B), the primitive fibres, each with several nuclei in the interior in lower vertebrates, on its periphery in the higher. Most of the protoplasm of the fibre has been altered to minute contractile fibrillae, each crossed by lighter and darker bands, and as these come opposite each other in the different fibrillae, they give the fibre its characteristic cross-banded appearance. ^~^.^_^=. FIG. 13.—A, smooth muscle cell; B, striped muscle. The primitive fibres rarely branch at their extremities. Each is surrounded by a structureless envelope, the sarcolemma, while num- bers of fibres are bound into bundles and muscles by connective tissue (perimysium) which carries nerves and blood-vessels. At the ends of the bundles the perimysium continues into the tendons which attach the muscles to other parts. The heart muscle also arises from the mesothelium, is cross-banded, but is removed from control of the will. The cells are usually short (usually with a single nucleus); they branch, the branches connecting adjacent muscle cells. Connective Tissues. The tissues grouped here arise from the mesenchyme and are distinguished from all other tissues by the great amount of intercellular substance produced by the cells themselves. This substance or matrix varies in character and determines the variety of tissue. Frequently it is dense and hence the connective tissues may give the body support, and in fact they are sometimes called supportive Please note that these images are extracted from scanned page im
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