microscopists, and much still remains to be done with reference to it, the writer may be excused for entering upon ground already so much trodden, for the purpose of stating briefly the conclusions he has drawn from his own observations, as well as from the labours of others in this field of inquiry. The muscular system has been hitherto divided into two great groups after the example of Bichat, those, namely, of "animal life", which minister to the animal or relational functions, and are characterized by their red colour, transverse and longitudinal striæ, by being for the most part connected with tendon, deriving their nervous supply from the cerebro-spinal system, and functionally, by being under the dominion of the will, whence they are also sometimes named "voluntary muscles"; and secondly, those of "organic life", which enter into the structure of the organs of nutrition, and are distinguished from the former by their pale colour, absence of superficial striation and of tendons of connection, in being supplied with nerves from the sympathetic system, and not subject to the control of the will, from which circumstance they have been designated as "involuntary”, in contradistinction to the former. The muscles of animal life, however dissimilar in size and figure, all consist of an aggregation of similar constituent parts, the muscular fibres or "primitive fasciculi" of Müller, which, being disposed in bundles insulated by an atmosphere of connective tissue known as the perimysium internum, constitute secondary, tertiary, and quaternary fasciculi; the entire muscle being enveloped by a condensed layer of fibrous tissue, named the sheath or perimysium externum, and supplied with bloodvessels and nerves which course in the interstices between the fibres, but do not penetrate them. The study of such an organ, therefore, resolves itself into that of one of its fibres, and to this we shall next direct attention. A muscular fibre of animal life, or primitive fasciculus, represents, in the state of complete development, a solid cylinder with many flat surfaces, and of an average diameter in man of 3 part of an inch, but varying in this respect at the different periods of life. Thus, in the mature foetus and in the infant, it averages of an inch in thickness, and thence onward to adult age gradually attains the standard diameter just mentioned. It likewise varies in the different classes of animals, being largest in fish (of an inch), and smallest in birds (7 of an inch), according to the measurements of Bowman. In the entire vertebrate, and likewise in those classes of the invertebrate sub-kingdom, which are characterized by active habits or energetic muscular movement, the fibre is distinguished by surface markings or striation in two directions, namely, in the line of its axis, and at right angles to it. The distance between the stria will vary with the state of the muscle as to extension or contraction; thus, in the state of extreme extension the distance between the transverse striæ is greatest, whilst it is least in that of maximum contraction. As regards the longitudinal striæ, the rule is precisely the reverse, i.e. they are farthest apart in complete contraction, and least so in full extension. The fibre is closely invested by a transparent, elastic, and structureless membrane,' the sarcolemma, which is studded on its internal surface at irregular intervals with oval nuclei affecting a linear arrangement; these may be brought into view by the addition of acetic acid, as shown in Figs. 2 and 3. The bulk of the fibre is composed of minute oval particles, the sarcous elements, which measure in man about go part of an inch in their long diameter, i.e. in the length of the fibre, and from 18000 to 20000 of an inch transversely. In the ox I have found them to be 1200 of an inch in the short diameter. The sarcous elements are disposed in linear order in the course of the fibre, and are closely coherent at their points of mutual contact, where they are slightly compressed. A single series or file of these particles, detached in the length of the fibre, as they readily may be, after boiling or maceration in chromic acid, would constitute a fibril or primitive fibre, which presents a beaded or varicose appearance, as shown in Fig. 1. It is to the Muscular fibre of ox boiled and partially ruptured, showing fibrilla separated. lateral apposition of a number of fibrils of this construction within the sarcolemma, the varicosities and constrictions on which ! Bowman states that he has observed something like a fibrous structure in this membrane; he does not, however, venture a positive opinion on the subject. I have never been able to detect even a resemblance to special structure in the sarcolemma. respectively correspond, that the transverse and longitudinal striæ on the fibre are due (see Fig. 2). From the constitution of Fig. 2. Striated muscular fibre of ox boiled and treated with acetic acid, showing long and transverse striæ and nuclei. Magnified 222 D. the fibrils as now described, it follows that their length must coincide with that of the fibre which they compose, and their width with that of the sarcous elements which compose them, and that the interval between the constrictions which they exhibit, corresponds equally to the long diameter of the sarcous elements, and to the distance between the transverse striæ on the fibre. After maceration in strong nitric or hydrochloric acid, or in a solution of carbonate of potash, the fibres may be partially fractured by gentle traction, in the line of their transverse striæ. The appearance resulting from such a partial disintegration of the fibre resembles not very remotely a pile of coins, and suggested to Bowman, who first observed it, the idea of regarding the fibre as composed, indifferently, of a pile of discs, or a bundle of fibrils (see Fig. 3). Fig. 3. Striated muscular fibre of ox boiled, and treated with hydrochloric acid, exhibiting transverse striæ and partial fracture of fibre into discs. The sarcolemma is raised at certain points by imbibition, and the nuclei are shown. Magnified 2224 D. The elementary particles seem to be held together by a viscid fluid which admits of their separation in both directions, not, however, with equal facility, as shown by Kölliker, with whose observations my own agree; the union into fibrils would appear to be the natural arrangement, and the formation of discs to result from the chemical reagent employed. Thus, after simple boiling, no appearance of transverse fission can be produced, although the fibre may, with great facility, be torn up in its length into fibrils. That the sarcous elements are cellular bodies is pretty generally conceded by physiologists, but owing to their extreme minuteness all attempts hitherto made to demonstrate their structure have been unsuccessful. Yet it is upon an exact knowledge of the constitution of these minute bodies, as the essential and potent elements of muscle, that a full comprehension of the nature of muscular contraction must be based; and until science shall have been enabled to determine this important point by the use of higher powers of the microscope than have been hitherto employed in the investigation of this department of minute anatomy, there is reason to believe we must rest content with such a knowledge of the contractile force as may be deduced from its manifestations and effects. 48 36 In the present state of our knowledge of the albumenoid bodies, no rational formula can be proposed for any of them, and least of all for fibrin. There is no need, however, of representing their composition by such complex empirical formula as that of Mulder, in which 882 equivalent simple molecules are supposed to be combined to form one compound molecule of fibrin. The formula CHaN.Os, in which some phosphorus and sulphur substitute some of the constituents, is perhaps the empirical formula which best represents the composition of all the principal albumenoid bodies (albumen, fibrin, casein). Mulder's ingenious theory of protein, by which these various bodies were sought to be connected, being no longer admissible, chemists are as yet wholly unable to explain their constitution; nay more, the numerous forms of the same type are inexplicable upon chemical principles; and so numerous are they in the case of fibrin, that physiologists and chemists have proposed to assume the existence of a primitive fibrin (molecular fibrin-Zimmermann; para- and brady-fibrin-Polli; pseudo- or neo-fibrin of Magendie), from which all the others are derived. Liebig gave the name of syntonin to the substance precipitated from a solution in hydrochloric acid of muscular fibre. This substance has been found by Lehmann to have sensibly the same composition, whether obtained from the middle coat of the aorta ascendens, descendens, or carotis, the tunica dartos, the bladder, etc.; and if we add to Lehmann's analyses those made for Liebig by Strecker, of syntonin from ox flesh, flesh of fowl, etc., the observation will equally apply. It contains about 15.8 per cent. of nitrogen, and from 1.02 to 1.21 of sulphur. As the fibrilla of the muscle alone dissolve in hydrochloric acid, the sarcolemma, fat, etc., not dissolving, the composition of syntonin represents that of fibrin. That the former can only be looked upon as a modification, if not a product of decomposition in the first degree, of the latter, is proved by the fact, that it does not act towards deutoxide of hydrogen like true fibrin. In other respects fibrin and syntonin do not appear to differ in their chemical reactions, except that the latter is difficultly soluble in nitrate and carbonate of potash, while it is soluble in dilute hydrochloric acid. According to Kölliker, it is insoluble in the salts just mentioned; but this is not strictly correct, for although seeming at first to be insoluble, it ultimately dissolves. In the human embryo the muscles are first distinctly observable about the end of the second month; at this time they are pale, transparent, and gelatinous, and the fibres present the appearance of elongated bands with nodular enlargements containing oval nuclei. They are homogeneous or slightly granular in structure, and, according to the observations of Kölliker, developed from a single cell, not from a number of cells which assume a linear arrangement, and become fused into a tube by removal of their contiguous walls, as maintained by Schwann. The nucleus of the primordial cell multiplies by endogenous growth and fission, whilst the cell itself increases in length and assumes the form of a tube. Coincidently with the elongation of the cell and the multiplication of its nucleus, its granular contents assume a linear disposition in the length of the nascent fibre, and constitute the rudimentary fibrils. In consequence of this change, which takes place in the superficial part of the fibre-cell about the fourth month, the fibre presents feebly marked striæ in the two direc tions, namely, in its length and in its width; its central portion or axis, however, is still occupied by a transparent and slightly granular plasma, which gives to the fibre the appearance of a tube on transverse section, and affords an explanation of the error into which Mr. Skey' has fallen, in attributing a tubular character to muscular fibre in its fully developed state. From the fourth month to the time of birth the fibre increases in thickness, and the process of fibrillation progresses from circumference to centre, so that at the latter period the fibre is a solid cylinder, of a rounded polygonal form, longitudinally and transversely striated, and sheathed in an elastic and structureless membrane, the representative of the original cell-wall. From the period of birth to adult age, the fibre acquires an increase in length in proportion to the growth of the body, and in width the ordinary rate of its increase is such that it doubles its own diameter for every decade between these two periods. In this last respect, however, the growth of the fibre is subject to considerable variation, dependent upon the greater or less activity of the process of nutrition. Thus, exercise is known to increase the volume of a muscle by promoting its nutrition; prolonged inaction, on the contrary, induces atrophy or wasting by partially arresting the nutritive process; and in either case the altered size of the muscle depends upon a corresponding alteration in its 2 Philosoph. Transact., 1837. |