In the deeper coloured muscular fibres of those animals which, like the rabbit, possess two kinds of voluntary muscles, the transverse loops of the capillary network are dilated far beyond the size of the ordinary capillaries.

Fig. 344.

CAPILLARY VESSELS

The number of capillaries in a given space of a muscle, or their degree of closeness is partly regulated by the size of the fibres; and accordingly in the muscles of different animals it is found that, when the fibres are small, the vessels are numerous and form a close network, and vice versa: in other words, the smaller the fibres, the greater is the quantity of blood supplied to the same bulk of muscle. In conformity with this, we see that in birds and mammalia, in which the process of nutrition is active, and where the rapid change requires a copious supply of material, the muscular fibres are smaller and the vessels more numerous than in cold-blooded animals, in which the opposite conditions prevail.

Lymphatics. So far as is known there are no lymphatic vessels in the voluntary muscles, although there is an abundant supply in their connective tissue sheaths and tendons, and the lymphatic vessels here would seem, as pointed out by Ludwig and SchweiggerSeidel, to serve the purpose of collecting and conveying away the lymph from the muscular substance.

Nerves. The nerves of a voluntary muscle are of considerable size. Their branches pass between the fasciculi, and repeatedly unite with each other in form of a plexus, which is for the most part confined to a small part of the length of the muscle, or muscular division in which it lies. From one or more of such primary plexuses, nervous twigs proceed, and form finer plexuses composed of slender bundles, each containing not more than two or three dark-bordered nerve-fibres, whence single fibres pass off between the muscular fibres and divide into branches which are finally distributed to the tissue. The mode of final distribution will be described with the general anatomy of the nerves.

DEVELOPMENT OF VOLUNTARY MUSCULAR TISSUE,

Most of the voluntary muscles of the body are developed from a series of portions of mesoblast which are early set aside for this purpose in the embryo and are termed the muscle-plates (see Embryology, p. 159). When the muscular fibres are about to be formed the cells become elongated, and their nuclei multiplied so that each cell is converted into a long multi-nucleated protoplasmic fibre. At first the substance of the fibre is not striated but is merely granular in appearance, but presently it becomes longitudinally striated along one side (fig. 345, A), and about the same time a delicate membrane, the sarcolemma, may be discovered bounding the fibre. The longitudinal striation, which is the first indication of the proper muscular substance, extends along the whole length of the fibre, but at first as just intimated affects only a small part of its breadth, the rest being formed by a highly glycogenic protoplasm containing the nuclei. In due time, however, this conversion into the proper muscular substance, further shown by the appearance of cross striæ (fig. 345, B and c), extends round the greater part of the circumference of the fibre, and finally gradually involves its whole thickness, except along the axis, which for some time remains occupied by the nuclei embedded in undifferentiated protoplasm. Eventually, however, the nuclei take up their permanent position.

Schwann considered each fibre to be formed by the linear coalescence of several cells; but the researches of Kölliker, Wilson Fox, and others, tend to establish the

view, originally promulgated by Remak, that the fibres are produced as above described by the elongation of single cells, with differentiation of their contents and multiplication of their nuclei.

Growth. The muscular fibres, after having acquired their characteristic form and structure, continue to increase in size till the time of birth, and thenceforward up to adult age. In a full-grown foetus most of them measure twice, and some of

Fig. 345.-DEVELOPING MUSCULAR FIBRES. HIGHLY MAGNIFIED.

A, elongated cell with two nuclei and a striation beginning down one side of the cell (from foetal sheep, Wilson. Fox).

B, from foetus of 2 months; p, granular protoplasm; g, glycogenous substance; n, nucleus; 8, commencing sarcolemma, with striated muscular substance developing immediately beneath it.

C, from foetus of 3 months, displayed so as to show the contractile substance collected at one side of the fibre, and partially enclosing the unaltered substance of the fibre, g; f, fibrils. B and C from Ranvier.

them three or four times their size at the middle of foetal life; and in the adult they are about five times as large as at birth. This increase in bulk of the individual fibres would, in a measure, account for the enlargement of the entire muscles.

B

A

It is uncertain how far there may be a multiplication or new formation of muscular fibres during the growth of a muscle; but it is probable that during growth at least, if not in adult life, a new formation of fibres within the muscles does occur. The new fibres appear to be in part formed by the longitudinal splitting of the existing fibres, a process which was described by Weissmann, and which seems to be of very general occurrence at a certain period of growth. The splitting is preceded by the multiplication of the muscle-nuclei, which form a longitudinal series in the part which is about to become separated. The bundle of fine muscular fibres is found to be enclosed within a thick connective tissue sheath formed of several layers, continuous with the layers of Henle's sheath of the nerve-fibre, and since these layers are more developed at the point of entrance of the nerve into the bundle than elsewhere, the whole structure has a spindle-shape, and on this account has received the name of muscle-spindle (Kühne). The fibres of the muscle-spindle appear gradually to enlarge and eventually to form a bundle of ordinary muscle-fibres. The nerve and nerve-ending must, of course, participate in the cleavage, since each fibre is ultimately found to be furnished with a nerveending. But another important mode of new formation is by the transformation of cells (sarcoplasts) which lie between the muscle fibres, and which are presumably undifferentiated cells derived from the original muscle-plate. These cells enlarge and become elongated, and striated muscular substance becomes formed within them. usually at one side, and often forming oval masses, which as they grow become longer and more spindle-shaped, the middle part being bulged out and the ends tapering off to fine terminations. At this stage of development the name of muscle-spindle has also been applied to these fibres. The middle bulged part receives a nerve fibre, which is provided with a terminal ramification, as with other muscular fibres, and a multiplication of nuclei, and formation of sarcolemma having already taken place, the development of the fibre may now be looked upon as completed. Such fibres in

various stages of development have been described in muscles at all stages of growth, and also in the adult condition, so that it has been conjectured that new muscular fibres may be formed in this way even after the development of the muscle is completed. Nothing is known of the manner in which absorption of pre-existing fibres is effected to make room for the newly-formed fibres; if, indeed, such change occurs at all.

According to Mayer, many of the structures which have been described, under the name sarcoplasts and muscle-spindles, as fibres in course of development, are in reality fibres undergoing degeneration. Kerschner, on the other hand, from their relatively large nerve supply, believes the muscle-spindles to be sensory end-organs.

Regeneration. It was formerly thought that after removal by the knife or by disease striated muscular tissue is not regenerated, but that any breach of continuity which may occur in a muscle is filled up by a growth of connective tissue. It would appear, however, that the breach is, after a certain lapse of time, bridged across by muscular substance, but how the new muscular tissue is formed is not fully understood.

PLAIN OR UNSTRIPED MUSCULAR TISSUE.

This is made up of cells, named contractile fibre-cells, which were first distinguished as the true elements of the tissue by Kölliker. The cells may form

fibrous bundles, and strata,

or may be less regularly arranged, and the tissue occurs either almost pure or mixed with other tissues in varying proportion. The cells are of an elongated fusiform shape (figs. 346 and 347), usually pointed at the ends. They are generally prismatic in transverse section, but are sometimes more flattened. The cells vary greatly in length according to the part or organ in which they are found. Some occur which are cleft or forked at one or both FROM ends. Their substance is finely vacuolated and exhibits also a faint longitudinal fibrillation. It is doubly refracting. Each cell has a nucleus (a, a), rarely more than one, which is always elongated and either oval or rod-shaped. Towards each end of the nucleus the substance of the cell usually contains a few distinct granules arranged in linear series.

SMALL

(E. A. S.)

A, a complete cell, showing the nucleus with intranuclear network, and the longitudinal fibrillation of the cell-substance, with protoplasm between the fibrils.

B, a cell broken in the process of isolation; the delicate enveloping membrane projects at the broken end a little beyond the substance of the cell (B is from a draw. ing by Mr. R. Boxall).

Fig. 347. MUSCULAR FIBRE-CELLS FROM
HUMAN ARTERIES, MAGNIFIED 350 DIA-
METERS. (Kölliker.)

The nucleus shows the usual

a, a, nucleus; b, a cell treated with structure, having an intranu

acetic acid.

clear network (fig. 346, A). The involuntary fibre-cells possess an exceedingly delicate homogeneous sheath (fig. 346, B), and like the sarcolemma of voluntary muscular fibres, this sheath is apt to be wrinkled when the fibre is

contracted, so that an indistinct appearance of striation may thus be produced. The cells are united by a small amount of intercellular cementing substance which becomes stained by nitrate of silver. In some parts intercellular spaces may occur, bridged across as in epithelia by fine denticulations of the cells.

They are generally collected into larger and smaller fasciculi, which in many cases cross one another and interlace. The fasciculi are attached at their ends by connective tissue to the membranous and firmer parts where they occur. In some cases the attachment of the plain muscular cells takes place by means of elastic fibres which bifurcate at the end of the muscular cell. The two branches extend along either side of this and are firmly attached to it. In other cases again, according to Watney, the attachment may take place through the medium of connective tissue corpuscles, the branches of which embrace the muscular cell in like manner.

Distribution. The plain muscular tissue is for the most part disposed in the coats of the membranous viscera. It is met with in the lower half of the gullet, the

Fig. 348.-MUSCULAR FIBRE-CELLS FROM THE UTERUS THREE WEEKS AFTER DELIVERY,
TREATED WITH ACETIC ACID, MAGNIFIED 350 DIAMETERS.

a, nuclei;, fat-granules.

(Kölliker.)

stomach, and the whole intestinal canal; that is, both in the muscular coat of the alimentary canal, and also as a layer in the tissue of the mucous membrane, and in the villi; in the trachea and bronchial tubes, in the bladder and ureters, and the ducts of the larger glands generally, in the uterus and its appendages, in the corpora cavernosa of both sexes, in the prostate gland, in the spleen, in the muscle of Müller at the back of the orbit, and in the ciliary muscle and iris. The middle coat of the arteries, the coats of many veins and those of the larger lymphatics contain plain muscular tissue. In the skin it is present in the tubules of the sweat glands, in the form of minute muscles attached to the hair-follicles, and in the dartos or subcutaneous tissue of the scrotum. Numerous nerves, chiefly of the pale variety, are supplied to this tissue; before their ultimate distribution they frequently come into connection with microscopic ganglia. The tissue receives blood-vessels, but these are far fewer in proportion than those of voluntary muscle. In some situations, as in the wall of the stomach and intestine, abundant lymphatic plexuses are found in close relation to the muscular layers.

Development.-The elements of the plain or unstriped muscular tissue are derived from embryonic nucleated cells, consisting of the usual granular-looking protoplasm. These cells become lengthened out, pointed at the ends, and flattened with elongation of the nucleus, whilst their substance becomes longitudinally fibrillated and anisotropous, and acquires its permanent condition and characteristic properties.

The great increase in the muscular tissue of the uterus during gestation takes place both by elongation and thickening of the pre-existing fibre-cells of which that non-striated tissue consists, and it is said also by the development of new muscular fibre-cells from small nucleated, granular cells lying in the tissue. In the shrinking of the uterus after parturition the fibre-cells diminish to their previous size; many of them become filled with fat granules (fig. 348), and eventually many are doubtless removed by absorption.

Regeneration of plain muscle after artificially-produced lesions has been seen to be accompanied by karyokinetic multiplication of the muscle-cells adjacent to the lesion (in the newt by Stilling and Pfitzner).

CARDIAC MUSCULAR TISSUE.

The fibres of the heart (figs. 349, 350) differ remarkably from those of involuntary muscular organs in general, inasmuch as they present transverse striæ. The striæ, however, are less strongly marked, and less regular, and the fibres are smaller in diameter than in the voluntary muscles. They differ also from these in being made up of distinct quadrangular cells (fig. 349) joined end to end and often presenting a branched or forked appearance near one extremity (c). Each cell has a

(E. A. S.)

Fig. 349.-SIX MUSCULAR FIBRE-CELLS FROM THE HEART, MAGNIFIED 425 DIAMETERS. a, line of junction between two cells; b, c, branching of cells. From a drawing by Mr. J. E. Neale.

Fig. 350.-MUSCULAR FIBRES FROM THE HEART, MAGNIFIED, SHOWING THEIR CROSS STRIE, DIVISIONS, AND JUNCTIONS. (Schweigger-Seidel.)

The nuclei and cell-junctions are only represented on the right-hand side of the figure.

single clear oval nucleus situate near the centre; occasionally two nuclei are seen. The cell substance is striated longitudinally as well as transversely, its substance appearing to be composed of a number of parallel columns (sarcostyles), which on transverse section are seen as small polygonal areas.

An investing membrane or

tions with neighbouring fibres being effected by the medium of the cell-offsets above noticed.

In the frog and lower vertebrata generally the muscular fibres of the heart are formed of elongated spindle-shaped cells, resembling in shape the cells of plain muscular tissue, but exhibiting distinct transverse striation.