MUSCULAR TISSUE.

The muscular tissue is that by means of which the active movements of the body are produced. It consists of fibres, which are for the most part collected into distinct organs called "muscles," and in this form it is familiarly known as the flesh of animals. These fibres, from a characteristic appearance which they exhibit under the microscope, are usually known as "cross-striped" or "striated;" they are many of them under the control of the will, and are hence often spoken of as "voluntary" muscles. Another kind of muscular tissue is disposed around the blood-vessels and most of the hollow viscera, often forming a distinct coat or coats to these. In this kind the fibres do not exhibit the same cross-striated appearance, and they have therefore been termed in contradistinction "plain" or "non-striated" muscular fibres. Most of these are entirely withdrawn from the control of the will, and they are therefore also termed involuntary. The muscular tissue of the heart, although having a cross-striated appearance, differs in many respects from that of the skeletal muscles: it is therefore described separately under the term "cardiac " muscular tissue. Muscular fibres are endowed with contractility, by virtue of which they shrink or contract more or less rapidly under the influence of certain causes which are capable of exciting or calling into play the property in question, and which are therefore named stimuli.

STRUCTURE OF CROSS-STRIATED OR SKELETAL MUSCLES.

The skeletal muscular fibres are for the most part gathered into distinct organs or muscles of various sizes and shapes, but most generally of an oblong form, and furnished with tendons at each extremity, by which they are fixed to the bones.

The fibres are in the first place collected into bundles, of greater or less thickness, named fasciculi or lacerti (fig. 322). The fibres are parallel in the fasciculi ; and the fasciculi extend continuously from one terminal tendon to the other, unless in those instances, like the rectus muscle of the abdomen and the digastric of the inferior maxilla, in which the fleshy part is interrupted by interposed tendinous tissue. The fasciculi also very generally run parallel, and, although in many instances they converge towards their tendinous attachment with various degrees of inclination, yet in the voluntary muscles they do not interlace with one another.

An outward investment or sheath of areolar tissue (epimysium) surrounds the entire muscle, and sends partitions inwards between the fasciculi; furnishing to each of them a special sheath (perimysium).' The areolar tissue extends also between the fibres (endomysium), but does not afford to each a continuous investment, and therefore cannot be said to form sheaths for them. Every fibre, it is true, has a tubular sheath; but this, as will be afterwards explained, is not composed of areolar tissue. The perimysium contains elastic as well as white fibres; but the elastic element is found principally in its investing, as distinguished from its penetrating, portion. In the endomysium numerous plasma-cells are found. The chief uses of the areolar tissue are to connect the fibres and fasciculi together, and to conduct and support the blood-vessels and nerves in their ramifications between the parts. The relation of these different subdivisions of a muscle to each other, as well as the shape of the fasciculi and fibres, is well shown in transverse section (figs. 322 and 323).

1 The term perimysium has usually been employed to designate the external investment of the whole muscle as well as the special sheath of each fasciculus. I have ventured, however, to introduce the word epimysium for the general sheath, since the terms will then have a close analogy with those which are applied to the connective tissue investments of a nerve (see p. 324).

Fasciculi. The fasciculi are of a prismatic figure, and their sections have therefore an angular outline (fig. 322). The number of fibres of which they consist varies, so that they differ in thickness, and a large fasciculus may be divisible into two or three orders of successively smaller bundles, but of no regularly diminishing magnitude. Some muscles have large, others only small fasciculi; and the coarse or fine texture of a muscle, as recognized by the dissector, depends on this circum

Fig. 322.-A, SMALL PORTION OF MUSCLE, CONSISTING OF LARGER AND SMALLER FASCICULI, NATURAL
SIZE; B, THE SAME MAGNIFIED 5 DIAMETERS, SHOWING A TRANSVERSE SECTION. (Sharpey.)
Fig. 323.-A FEW MUSCULAR FIBRES, BEING PART OF A SMALL FASCICULUS, MORE HIGHLY

MAGNIFIED. (Sharpey.)

b, b, fibres; a, end view; c, a fibre splitting up into longitudinal elements.

stance. The length of the fasciculi is not always proportioned to the length of the muscle, but depends on the arrangement of the tendons to which their extremities are attached. When the tendons are limited to the ends of a long muscle, as in the sartorius, the fasciculi, having to pass from one extremity to the other, are of great length; but a long muscle may be made up of a series of short fasciculi attached obliquely to one or both sides of a tendon, which advances some way upon the surface or into the midst of the fleshy part, as in the instances of the rectus muscle of the thigh, and the tibialis posticus. Many short fasciculi connected thus to a long tendon, produce by their combined operation a more powerful effect than a few fasciculi running nearly the whole length of the muscle; but by the latter arrangement the extent of motion is greater, for the points of attachment are moved through a longer space.

Fibres; their figure and measurement.-In shape the fibres are cylindrical, or prismatic with rounded angles. Their diameter varies greatly even in each muscle, although for the most part a prevailing standard is found to exist in every muscle. The largest fibres in human muscles average about inch (0-1 mm.) in diameter, the smallest are only about one-tenth that width.

250

The eye-muscles are mainly composed of small fibres; and the muscles of the limbs mainly of larger ones, but there is no constant relation between the size of a muscle and that of its constituent fibres. The fibres tend to be thicker in the male than in the female (for the same muscles); the differences between different muscles are not evident in infancy, but only manifest themselves in the process of growth (Schwalbe and Mayeda).

The fibres composing a muscle are of limited length, generally not exceeding one inch and a half; and accordingly in a long fasciculus a fibre does not reach from one tendinous attachment to the other, but ends with a rounded or tapering extremity, invested with its sarcolemma, and cohering with neighbouring fibres. Unless when

either is fixed to a tendon, both extremities of the fibre terminate in the way described, so that it has a long cylindrical shape, but when provided with tapering ends it becomes somewhat spindle-shaped. In some muscles, e.g., the sartorius, fibres have been measured which are much longer than the dimension above given.

Generally speaking the fibres neither divide nor anastomose; but this rule is not without exception. In the tongue of the frog the muscular fibres (fig. 324) as they approach the surface divide into numerous branches, by which they are attached to the under surface of the mucous membrane (Kölliker). The same thing has also been seen in the tongue of man and various animals : and the fibres of the facial muscles of mammals divide in a similar manner where they fix themselves to the skin (Busk and Huxley).

Structure of the fibres; sarcolemma.-A muscular fibre may be said to consist of a soft substance enclosed in a tubular sheath. The latter is named the sarcolemma. It consists of transparent and apparently homogeneous elastic membrane, and, being comparatively tough, will sometimes remain entire when the included muscular substance is ruptured, as represented in figs. 325, 326. It is especially well seen in fish and amphibia, for in these it is thicker and stronger than in mammalian muscle, in which it is more difficult to render evident but nevertheless always exists (fig. 326). Nuclei are found on the inner surface of the sarcolemma, but these belong rather to the contractile substance than to the inclosing membrane, and will be afterwards more fully described.

Muscular substance.-When viewed by transmitted light even with a comparatively low power of the microscope, the fibres, which are clear and pellucid in aspect, appear marked with parallel stripes or bands alternately light and dark passing across them with great regularity (fig. 327), and this not only at the surface but,

Fig. 324.-A BRANCHED MUS

CULAR FIBRE FROM THE FROG'S TONGUE, MAGNIFIED 350 DIAMETERS. (Kölliker.)

Fig. 326.-SARCOLEMMA OF MAMMALIAN MUSCLE, HIGHLY MAGNIFIED. (E. A. S.) The fibre is represented at a place where the muscular substance has become ruptured and has shrunk away, leaving the sarcolemma (with a nucleus adhering to it) clear. The fibre had been treated with serum acidulated with acetic acid.

as may be seen by altering the focus of the microscope, throughout its substance also. In a moderately extended fibre about eight or nine dark and as many light bands may be counted in the length of of an inch, which would give about 700 inch as the breadth of each. But whilst this may be assigned as their usual breadth in human muscle, they are in different parts found to be much narrower, so that not unfrequently there are twice as many in the space mentioned. This closer approximation may generally be noticed in thicker and apparently

contracted parts of the fibre. The cross-striped appearance, which is very characteristic, is found in all the skeletal muscles; but it is not altogether confined

to them, for it is seen in the fibres of the heart, and striped fibres are also found in some other viscera, such as the pharynx and upper part of the gullet.

When the muscular fibres are deeply focussed, the appearance of the striæ becomes somewhat altered, and a fine line, often dotted, is seen passing across the middle of each light band, (see fig. 328). This has been termed Dobie's line or the stria of Amici (disque mince, Zwischenscheibe), and it has been supposed to represent a membrane stretching across the fibre and attached at the surface to the sarcolemma (see below). The line is, however, not visible in the most superficial planes of the fibre, and although there certainly are membranes crossing the fibre at about this situation, they are only seen when the fibre is treated with acids and certain other reagents. There is reason to believe that the appearance of a dotted line in this situation in the fresh fibre is due to the peculiar optical conditions of the tissue.

A fine clear line is sometimes to be seen in the middle of each dark band. This was first noticed by Hensen, and named the line or disk of Hensen.

Fig. 327.-MUSCULAR FIBRE OF A MAMMAL EXAMINED FRESH IN SERUM, HIGHLY MAGNIFIED.

(E. A. S.)

This figure was drawn with the surface layer of muscular substance accurately focussed, the lateral portions having been added by gradually sinking the focus.

The nuclei are seen on the flat at the surface of the fibre, and in profile at the edges.

Fig. 328.-PORTION OF A HUMAN MUSCULAR FIBRE SHOWING DOBIE'S LINE IN THE MIDDLE OF THE CLEAR BAND. (Sharpey.)

The proper substance of the fibre presents, besides the transverse bands, an appearance of longitudinal striation. On separating the fibre with needles, especially after hardening in alcohol, it may be broken up longitudinally into fine longitudinal elements, of a rounded or angular section, which run from end to end of the fibre; these may be conveniently termed muscle-columns (Kölliker) or sarcostyles. Each sarcostyle appears to consist of a row of elongated prismatic particles with clear intervals. These particles may be termed sarcous elements (Bowman). The sarcostyles are united into the fibre by a variable amount of intercolumnar substance to which the name sarcoplasm has been given (Rollett). In some muscles the sarcostyles can be made out to be longitudinally striated, an appearance which has led same authors to believe that they are composed of still finer elements or fibrils, a term which has also been employed to denote the muscle-columns themselves.

Under certain circumstances the fibres show a tendency to cleave across in a direction parallel to the bands, and even to break up into transverse plates or

1 σαρξ, muscle ; στῦλος, a column.

disks, which are formed by the lateral cohesion of the sarcous elements of adjacent sarcostyles (fig. 329). To make up such a disk, therefore, every sarcostyle contributes a particle, which coheres with its neighbours on each

side, and this with perfect regularity.

From a consideration of these facts Bowman was led to conclude the sub-division of a fibre into "fibrils" (sarcostyles) to be merely a phenomenon of the same kind as the separation into disks, only of more common occurrence, the cleavage in the latter case taking place longitudinally instead of transversely; accordingly, he came to the conclusion that the "fibrilla" (sarcostyles) have no existence as such in the fibre, any more than the disks; but that both the one and the other owe their origin to the regular arrangement of the particles of the fibre (sarcous elements) longitudinally and transversely, whereby, on the application of a severing force, it cleaves in the one or in the other direction. That this conclusion was erroneous, however, is shown by the fact that a fibre can be split into longitudinal elements after death even without the action of any reagents, but never into disks; and also by the circumstance that in certain muscular fibres (those which move the wings of many insects) a separation into longitudinal elements (sarcostyles) can be seen to preexist even in the living and contractile condition of the fibre. Moreover, in these muscles, in consequence of the large amount of interstitial substance between the sarcostyles, the whole of a fibre never cleaves across into disks.

Fig. 329.-MUSCULAR FIBRE FROM

THE LEG OF A BEETLE, TREATED
WITH DILUTE ACID, SHOWING A
TENDENCY TO BREAK ACROSS

INTO DISKS. (E. A. S.)

The sarcoplasm is in the form of fine longitudinal lines with dotlike enlargements. The ordinary cross-stripes of the tissue are not

seen.

If a transverse section of a muscular fibre, or the surface of a separated disk (fig. 330 A), is examined with a high power, it appears to be marked out into small polygonal areas separated by fine lines which, in acid preparations, have the

Fig. 330.-TRANSVERSE SECTIONS OF MUSCLE FIBRES. (E. A. S.)
A.-TRANSVERSE SECTION OF A MAMMALIAN MUSCULAR FIBRE SHOWING COHNHEIM'S AREAS.
Alcohol preparation. Three nuclei are visible under the sarcolemma.

B. AN ISOLATED DISK OF LEG-MUSCLE OF A BEETLE TREATED WITH DILUTE ACID. The disk is seen partly on the flat, partly in profile, and exhibits the net-like appearance of the sarcoplasm in the transverse section of the fibre: the meshes represent the areas of Cohnheim.

C.-TRANSVERSE SECTION OF MUSCULAR FIBRE OF LEG OF WASP, SHOWING A RADIAL DISPOSITION OF THE SARCOPLASM. ACID PREPARATION.

appearance of a network (fig. 330, B). These areas represent sections of the muscle-columns; they are known as Cohnheim's areas, and the lines between them represent the intercolumnar substance or sarcoplasm. The network is coarser near the surfaces of such a disk, because, as will immediately be explained, the sarcoplasm is increased in amount at regular intervals, corresponding with the bright striæ; by alteration of the focus, however, a fine network can be made out through the whole thickness of the disk.