Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/912

Rh 858 imbedded in a &quot; ground substance,&quot; that forms the alter nating dim and bright discs, which substance he believes to be anisotropous, whilst the muscle rods arc isotropous. He regards the ground cabstance as the true contractile part of the fibre. Each transversely striped fibre is invested by a homo geneous membrane, the sarcolemma or myolemma, which is so transparent as to allow the characteristic transverse striae to be distinctly seen through it. The sarcolemma is so closely incorporated with the periphery of the fibre, that its isolation and demonstration as a distinct membrane are attended with some difficulty, but when water is added to a living fibre it is absorbed, and elevates the sarcolemma from the sarcous contractile particles. If acetic acid be added to a muscular fibre the transverse strife become less distinct, and a number of oval bodies come into view. These are especially to be seen next the periphery of the fibre in relation to the inner surface of the sarcolemma, though some apparently lie deeper in the substance of the fibre. These bodies have long been known as the nuclei of the striped fibre. More recent investigations have, how ever, shown that each nucleus lies in a little finely-dotted protoplasm, which often extends in a fusiform manner beyond the ends of the nucleus. These nuclei, with their investing protoplasm, have the anatomical characters of nucleated cells, and are called the muscle corpuscles. Some peculiar modifications of the striped muscular fibre are met with in certain localities. As a rule, this form of fibre does not branch ; but in the muscles of the tongue and lip, and other muscles of the face, these fibres usually branch prior to their insertion, and the branches taper off to finely attenuated ends. In the heart also the fibres branch ; and the branches of adjacent fibres anastomose, so that the muscular wall of this organ consists of a com pact network of fibres. The individual fibres are smaller than those of the voluntary muscles, the transverse stria- tion is much less distinct, and it is doubtful if an investing sarcolemma be present. Some difficulty has been experienced in determining the exact mode of connection of the fibres of the belly of a muscle with those of its terminal tendons. By some it has been supposed that the fibres of the one are directly con tinued into those of the other ; whilst Weismann has de scribed the muscular fibre as terminating in a sharply- defined, rounded, or pointed extremity, to which the fibres of the tendons are closely apposed. Both the striped and non-striped forms of muscle are well provided with blood-vessels, which ramify in the sub stance of the muscle lying in the areolar connective tissue that separates the fasciculi and fibres from each other. The capillaries form an elongated network, the principal strands of which lie parallel to the muscular fibres, but never penetrate the sarcolemma. Hence, though the belly of a muscle is a highly vascular organ, its individual fibres are extra-vascular. The vascularity of the fleshy belly is much greater than that of the terminal tendons of attach ment, and the nutritive changes are much more active in it than in them. Develop- T nc contractile fibro-cells of the non-striped muscular ment of fibre are formed by the gradual elongation of the rounded muscle. cells of the middle germinal layer of the embryo into spindle-shaped cells, the oval nuclei at the same time be coming elongated, so as to assume a rod-shaped form. Usually the spindle cells which lie in the same linear series become cemented together into the smooth fibres of this form of muscle. The mode of development of the striped fibre is more difficult to follow out, and various statements have been made as to the successive stages of its formation.&quot; Schwarm believed that a fibre was built UD of the embryonic cells of [NERVOUS the part, which arranged themselves in linear series, coalesc ing with each other at their surfaces of contact; that the contents of the cells then became transversely striated, and that the cell walls formed the sarcolemma. Savory and Lockhart Clarke maintained that a formation of blastema took place around free nuclei, and that this blastema gradually assumed the striated character. Remak, Kolliker, Wilson Fox, and Frey have, however, by studying the earliest stages of development in the very young embryo, established the fact that the striped fibres are developed from the cells of the embryo, though not in the manner described by Schwann. The process, briefly stated, is as follows : The embryonic cells elongate, the nucleus may remain single, but more usually it divides and subdivides, so that many nuclei appear in the interior of the elongated cell. The nuclei lie in linear series, and may either be separated from each other, or two or more may be in contact, and they may lie either near the periphery of the elongated cell, or in its axis. With this multiplication of the nuclei, the cell increases in length and assumes the form of a fibre. The cell protoplasm, both in the single and many-nucleated fibres, then differentiates into the sarcous particles of the transverse stria?, and as this progresses the fibre assumes its characteristic striped appearance. The whole amount of the protoplasm does not, however, assume the transversely striped appearance, for a small quantity remains around each nucleus and forms with it a muscle corpuscle. The differentiation of the protoplasm occasions an anatomical and chemico-physical change in the fibre, and confers on it the property of energetic contractility. W. Engelmann has endeavoured to show that the opaque anisotropic discs of the fibre are those in which the power of contractility resides, and that the clear isotropic discs possess only elastic properties. The mode of development of the sarcolemma is still somewhat obscure. By some it is regarded as the wall of the embryonic cell, which has become metamorphosed into a muscular fibre ; by others it is regarded as a special differentiation of the protoplasm at the periphery of the fibre taking place at the time when the transverse striae are being formed ; whilst by others it is considered to be a special modification of connective tissue formed around the fibre. In the development of the muscular fibres of the heart, the cells of the embryo heart branch and anasto mose, and the nuclei multiply. By the transverse striation of the protoplasm of these cells the branched muscular fibres of the heart arc produced. In the growth of a muscle the individual fibres increase in size, so that they are bigger in the adult than at the time of birth. The observations of Budge, Weismann, and Beale show that new fibres may also form in a muscle. Weismann believes that this increase may be due to a longitudinal splitting of a pre-existing fibre ; but Beale maintains that the new fibres are produced in the muscle in the same manner as the original fibres of the part. NERVOUS SYSTEM. The Nervous System consists of a number of organs which are named respectively Nerve Centres, Nerves, and Peripheral End-organs. The largest and most important Nerve Centres are the brain and spinal cord, which together constitute the cerebro-spinal nervous axis, and are lodged in the cranial cavity and spinal canal. But, in addition, numerous small bodies, usually oval in form, technically called ganglia, are situated in the axial part of the body, and form smaller nerve centres. The Nerves are white cords which traverse the different regions of the body, both axial and appendicular, for a greater or less distance, for the purpose of connecting together the other sub-divisions of the nervous system. The Peripheral End-organa are