van Gehuchten1). These teach that the isotropous material is a network of fine fibrillæ pervading the whole of the contractile substance; the interstices between the fibrillæ are filled up by the less solid, doublyrefracting substance. A typical animal cell is a mass of protoplasm containing a nucleus. It may or may not have a cell wall; it generally has not. The nucleus consists of a network of nucleoplasmic fibres, and a nuclear matrix, a homogeneous substance that pervades the whole nucleus; the protoplasm of the cell also contains a network of fine fibrillæ, and the unfibrillated stroma in which this fibrillar network is situated is called the enchylema (Carnoy). A muscular fibre is an animal cell; each one is developed from a typical animal cell; the fully-formed muscular fibre is, however, an animal cell which has become specialised in certain points both of structure and action; it possesses, like protoplasm, contractility, but its contractility does not come into play so as to produce movements in all FIG. 68. Part of a muscular fibre of Water-beetle. The fibre has been prepare with gold chloride, and is splitting into discs which show networks of fine lines. (B. Melland.) FIG. 69.- Living muscle of Water-beetle highly magnified (E. A. Schafer). s, sarcolemina; a, dim stripe; b, bright stripe; c, row of dots in bright stripe which seem to be the enlarged ends of rodshaped particles d. The transverse filaments connecting these dots are not shown. directions, as in the amoeba or white blood corpuscle, but is limited so as to produce shortening in one direction only; then in structure it is a cell which has become elongated, and of which the nuclei have increased in number and become peripheral in position; it is a cell with a well-marked cell wall, the sarcolemma; and, lastly, it is a cell in which the fibrillar network is no longer irregular, but is arranged with 1 Van Gehuchten, Anat. Anzeiger, vol. ii. p. 792 (1887); also in La cellule (Louvain), vol. ii. p. 293 (1886); vol. v. (1888), p. 247. In the last-mentioned paper, and also in Quain's Anatomy, a bibliography of this subject will be found. D D longitudinal and transverse strands quite regularly, as denoted in fig. 68; the interfibrillary substance is the doubly refracting substance. The longitudinal strands extend throughout the length of the fibre, and the cross strands connect these in the centre of what appears in a resting muscle to be the light stripe (Dobies' line). Under polarised light with crossed nicols the fibrillar network is dark, i.e. because it is singly refracting, the enchylema is bright because it is doubly refracting. The question arises, how, then, is the ordinary appearance of the alternate striping of a muscular fibre produced? No doubt this is an optical effect; an oil globule examined in water appears surrounded with a halo of light; a row of such globules would have a bright line on each side of it; so the cross strands of the network which are not of equal thickness, but have minute thickenings at the points where the fibres join together, produce a similar effect, and thus the enchylema on each side of the transverse strands appears bright in comparison with the rest. On contraction the longitudinal strands become shorter, and the cross strands thicker, and the granules in the cross strands larger; hence the cross strands now appear dark, while the rest of the enchylema appears bright in comparison. This is the explanation of the apparent interchange in position of the light and dark striæ on muscular contraction. This view of muscular structure and contraction is much simpler than the complicated theories formerly advanced; it brings muscular fibres into the general category of cells, and shows that the optical appearances that vary with the focus of the microscope and the state of contraction of the muscle may all be explained easily on the supposition that the fibrillar network has different optical properties from those of the interfibrillar stroma or enchylema which it pervades. Though by means of the microscope and polariscope it is thus possible to distinguish the existence of two substances, it is not possible to say whether the changes that occur on contraction are active in both substances, or whether the movements of one, e.g. the isotropous material, are active, and those of the anisotropous material are merely passive, or vice versa.' It is also not possible to say whether there is a transference of any material, e.g. water from one to the other during contraction. One must, however, be very careful to recognise that both substances are merely semi-fluid; there is no justification for supposing that anything in the nature of a solid, firm network pervades the interior of the fibre; the nematoid worm seen by Kühne in the interior of a fibre had no difficulty in progressing in any direction. 1 Rollett (Arch. f. mikr. Anat. 1888, p. 233), for instance, regards the anisotropous material as the actively contractile part of the muscle, and looks upon the network stainable by gold chloride, which Marshall and Melland consider to be the actively contractile part, as merely interfibrillar material. Haycraft's theory (Brit. Med. Journ. ii. 1890] 405) comes into the same category as Rollett's. In macroscopic as opposed to microscopic chemistry, it is not possible to say whether any one of the constituents of the muscle-plasma corresponds to one or other of the two optically different substances; but by microchemical methods, the question of the chemical composition of these substances has been the subject of research by several investigators. Brücke was the first to determine that muscle does contain two substances which act differently on polarised light; and he assumed that the doubly refracting substance is made up of innumerable positive doubly refracting particles with the properties of uniaxal crystals, to which he gave the name disdiaclasts. Ebner considers that the action of polarised light does not prove that the two substances are chemically different, but merely that there are alternating differences in the elastic tension of different parts of the muscle-substance. Others, again, have supposed that the only difference chemically is a difference of water, the enchylema being the more watery of the two substances; while others, again, have endeavoured to determine what constituent it is in the muscle-substance that produces the double refraction. Thus O. Nasse 2 believes that the anisotropous (doubly refracting) substance is myosin; the precipitate produced by adding alcohol to a saline solution of myosin is thready like fibrin, and, like fibrin, these threads refract light doubly. C. Schipiloff and A. Danilewsky3 find that the more myosin is dissolved out of muscular fibres by saline solution, the less do they refract light doubly ; they consider that the double refraction of muscle is chiefly produced by myosin, but also partly by lecithin. Myosin is converted into acidalbumin or syntonin very easily by the action of hydrochloric acid; Danilewsky speaks of the substance formed in this way as HCl-myosin ; by neutralising the acid he states that he once more obtains true myosin; but this is somewhat contradicted by the fact that it no longer doubly refracts light. He has, therefore, advanced the hypothesis, that myosin may exist in one of two conditions-doubly refracting myosin and singly refracting myosin. The doubly refracting myosin he also calls crystalloid myosin; this is the form in which myosin exists in the muscle, and is apparently the same thing as Brücke's disdiaclasts.5 1 Stricker's Handbuch, chap. vi. p. 170. 20. Nasse, Zur Anat. u. Physiol. der quergestreiften Muskelsubstanz, Leipzig, 1882 (Vogel; Biolog. Centralbl. 1882, ii. No. 10. 3 Catherine Schipiloff and A. Danilewsky, Zeitschr. f. physiol. Chem. v. 349. These observers also consider that the action of acids and gastric juice in splitting up muscular fibres into discs is due to the solution of lecithin, which they consider to be especially abundant in the centre of the light stripe. This, however, does not appear to me to be proved by their experiments. Danilewsky, Zeitschr. f. physiol. Chem. v. 158. 5 For Bernstein's views on this subject, see p. 435. None of these experiments, however, prove that the isotropous material contains no myosin; they only show that the anisotropous material contains myosin, or the myosin precursors. The red variety of voluntary muscular fibres.-W. Krause was the first to notice that certain muscles in the rabbit (soleus, semi-tendinosus, crureus, &c.) were redder in colour than the rest. Similar red muscles have since been described in other mammals and in fishes (rays). These fibres differ from ordinary voluntary muscular fibres in having a longer latent period and a slower contraction; they differ histologically in being more distinctly striated longitudinally, in possessing numerous nuclei which are not confined to the sarcolemma, and in the arrangement and size of their capillary blood vessels. Chemically the only important difference is the existence in the interior of the muscular fibres of a larger quantity of hæmoglobin than is present in the pale muscles. Cardiac muscular fibres.-These are quadrate cells without sarcolemma, and with one nucleus in the centre of each. They are branched, and the branches of neighbouring fibres are united by cementing substance which is stained brown by silver nitrate, as is the cementing substance between epithelial cells. The fibres show a well-marked longitudinal striation, an imperfect cross striation, and by polarised light a similarly imperfect fibrillar network is seen to be present throughout the enchylema; the latter is doubly refracting, as in voluntary muscle. Unstriated or plain muscular fibres.--Voluntary muscular fibres are so much altered from the condition of a primitive cell that the resemblances require to be carefully sought for. Cardiac muscular fibre may be regarded as in an intermediate condition of specialisation, while plain muscular fibres have lost very few of the histological characteristics of primitive cells. They are spindle-shaped, or in the blood vessels sometimes have jagged extremities; each possesses a single nucleus, which is rod-shaped, and has the characteristic structure of nuclei. Each possesses a fine sheath; each exhibits faint longitu dinal striation; and by appropriate reagents the protoplasm can be shown to consist of an enchylema pervaded by a fibrillary network. The fibrils run in a longitudinal direction within the fibre. They never show any double refraction either during life or after death; perhaps the anisotropous substance is absent; myosin is, however, present, as we shall see later on. Perhaps, as Hoppe-Seyler says, the axes of the particles which 1 Anatomie des Kaninchens, 1863. The cement substance between these fibres is stained brown by silver nitrate. 3 Physiol. Chemie, p. 669. produce double refraction (Brücke's disdiaclasts) are differently arranged, so that the light passes through their principal axis, and is thus singly refracted. This seems improbable, however, as there is no double refraction in whatever direction the fibres are viewed. CHEMICAL COMPOSITION OF MUSCLE A muscle may be considered as composed of two parts, the supporting connective tissue often containing fat in small quantities, and the muscular fibres themselves, each of which again consists of two parts, the sarcolemma and the contractile substance which it encloses. The connective tissue of muscle resembles connective tissue elsewhere; the gelatin and fat obtained in analyses of muscles are derived from this tissue. The sarcolemma is a substance which resembles elastin very closely in its solubilities.' The contractile substance is during life of semi-liquid consistency, and contains a large percentage of proteids and smaller quantities of various extractives and inorganic salts. By the use of a press, this substance can be squeezed out of perfectly fresh muscles, and it is then called the muscle-plasma. After death muscle-plasma like bloodplasma coagulates (thus causing the stiffening known as rigor mortis). The solid clot corresponding to the fibrin of blood-plasma is called myosin, and the liquid residue is called the muscle-serum. Living muscle has in the resting condition an alkaline reaction; after extreme activity, and after death, the reaction becomes acid; this is due to the formation of sarco-lactic acid. There are other changes that occur on contraction and on death of muscle, but the details will be considered later. In round numbers, muscle consists of The percentage of water varies somewhat in different animals : Man 72-74 per cent. com 2 Birds Pecten (a mollusc) 1 Recent experiments on the solubilities of elastin, sarcolemma, and basement membranes have been made by Ewald (Zeit. Biol. xxvi. 1). 'Schlossberger, Chemie der Gewebe, Leipzig u. Heidelberg, 1856, p. 169; GorupBesanez, Lehrbuch, 1878, p. 676; Hoppe-Seyler, Physiol. Chemie, p. 636. |