Observations on the Structure and Distribution of Striped and Unstriped Muscle in the Animal Kingdom, and a Theory of Muscular Contraction

With Plate VI.

It is necessary first to examine the descriptions of the several observers who have described a network in the striped muscle-fibre and to consider the interpretation that they have put upon it. In the following account I have only referred to those observers who have described some form of network in the muscle-fibre.

Dr. G. Thin¹ appears to be the first observer who described an intracellular network in the fibre of striped muscle. He examined frog’s muscle treated with gold chloride in the following manner. After staining with gold chloride the muscle was exposed to light in acidulated water and then kept in strong acetic acid, at a temp, of 38°C., from 6—24 hours. By this method he demonstrated a network of fine fibres, concerning which he says “this network was composed of exceedingly fine fibres, and its meshes accurately corresponded to Cohnheim’s areas” (p. 252). He states that he demonstrated in the muscle-fibre, by the process of isolation, (1) the existence of flat cells (the muscle-corpuscles), (2) a network connected with central cellular protoplasm, and (3) parallel rows of spindle elements. Further on he states that he was “compelled to associate the transverse markings with the existence of this network, without attempting to explain the connection between them more definitely” (p. 258).

Gerlach² has written two papers on this subject. In the first paper he states that the contractile contents of the sarcolemma is traversed by a retiform substance continuous with and identical with the axis cylinder of the nerve. He thus regards the network as of a nervous nature.

In the second paper he states that (i) an intravaginal nerve network is present within the sarcolemma. (ii) In good specimens striæ are seen which behave in a similar manner to the intravaginal network, and can be traced into continuity with it. He divides muscle into an anisotropous contractile matrix, and an isotropous nerve network. His results were obtained by the following method: the gold preparations were left several days in a mixture of 1—2 parts hydrochloric acid, 20 parts glycerine, and 20 parts water. This method brought out the longitudinal striæ. On treating gold preparations as above and subsequently treating with 1 per cent, potassium cyanide, he states that the sarcolemma gives way and the contents escape, partly as fine particles and partly in larger pieces; in such pieces the transparent substance bounding Cohnheim’s areas was stained red, and was thickened at the nodal points. The muscle-corpuscles always lay in the stained substance and not in Cohnheim’s areas; they consisted of a central oval nucleus and a stained peripheral substance continuous with the stained network of longitudinal striæ. He states that the longitudinal striæ are very variable in thickness and always zigzag; never straight. He regards them as thickenings of fine sheaths of nervous matter enclosing the fibrillæ of the muscle; these sheaths corresponding to the boundaries of Cohnheim areas.

He therefore concludes that the intravaginal nerve plexus and the longitudinal striæ are continuous, and together make up the isotropous part of the muscle-fibre, and are to be considered as nervous. To them belong also the muscle-corpuscles and the nuclei of the intravaginal nerves. He regards the anisotropous matrix as the contractile part. Gerlach thus appears to view the isotropous part of the muscle, stained by the gold, as a honeycomb and not a true network of fibrils. He has apparently failed to observe the transverse networks, and does not attempt to explain the relation between the network and the transverse striation.

The existence of an intravaginal nerve plexus in the muscle-fibre, and also the continuity of the nerve end plate with the isotropous part of the muscle, have been denied by Ewald¹ and Fischer.²

Engelmann³ regarded the isotropous part of the fibre as a structure “das in Physiologisches Hinsicht von einem Nerven nicht wesentlich abweichen Wunde,” and suspected a connection with the axis cylinder.

In a later paper Engelmann¹ states that the contractility of the muscle-fibre is always connected with fibrillar elements in the fibre; and he compares these with fibrillar elements in the protoplasm of some of the Rhizopods and Infusoria, &c.

Retzius² describes very carefully a network in the muscle-fibre of Dytiscus and other forms. This network consisted of (i) transverse networks placed at regular intervals, and corresponding in position to Krause’s membranes, (ii) Longitudinal bars parallel to each other, and apparently running the whole length of the muscle-fibre, and connected with the transverse networks. His results were obtained partly by transverse and longitudinal sections, and partly by teased preparations. He also employed the following method of gold staining: the specimens were placed for twenty-five minutes in per cent, gold chloride, either with or without previous immersion in 1 per cent, formic acid; then in 1 per cent, formic acid for ten to twenty hours, and exposed to the light. He gives the following description of the muscle-fibre of Dytiscus: In the axis of the fibre there are one or more rows of muscle-corpuscles, the protoplasm of which is produced into several (2—5) processes from which finer processes arise forming the transverse networks. Each muscle-corpuscle is in connection with five or six successive transverse networks. The longitudinal bars of the network he describes rather doubtfully as consisting of rows of dots (p. 8), but he describes and figures them projecting freely in some preparations. The matrix is structureless, and is only slightly stained by the gold. The sarcolemma is apparently closely attached to, but probably independent of, the network. The nerve endings appear to be in close connection with the transverse networks. The function of the network he was unable to determine, but states that it is probably not merely a supporting framework, but actively concerned in contraction. He does not, however, regard it as the true contractile part, as, according to him, it does not undergo important changes in form during contraction and extension. He thinks that the network is probably concerned in conveying the stimulus from the nerve to the muscle-fibre. In support of this he mentions that the fibre, as a rule, contracts simultaneously throughout its whole thickness; also that the nerve-fibres are apparently connected with the transverse networks.

Retzius also examined muscle from Musca, Oestrus, Noto-necta, Locusta, Astacus, Rana, and Triton. In Locusta the transverse networks had more polygonal meshes than in Dytiscus. In Astacus, Rana, and Triton the longitudinal bars of the network were thicker than the transverse.

In some cases he states that the longitudinal bars of the network were not straight but zigzag. From the descriptions and figures it is very probable that this appearance was due to pressure, and is not normal.

Bremer¹ also describes a well-defined network in the striped muscle-fibre, evidently identical with that described by Retzius. He states that the longitudinal lines are true fibrils, and not part of cylindrical sheaths as Gerlach maintained. He further traces the axis cylinder of the nerve into direct continuity with the muscle-corpuscles.

He considers the longitudinal striæ of Gerlach to be identical with the longitudinal bars of the network, and explains the irregular dotted appearance, or “Sprenkelung,” of Gerlach’s longitudinal striæ as being due to imperfect staining.

Bremer’s results, though published subsequently to those of Retzius, were obtained quite independently.

B. Melland² has recently investigated the structure of the striped muscle-fibre, and has arrived independently at results agreeing very closely with those of Retzius and Bremer, This close correspondence between the accounts of these three observers affords satisfactory evidence of the correctness of their observations. ‘

The points of difference between the networks described by Melland and Retzius are slight, the chief one being that Melland figures the transverse networks in Dytiscus with more polygonal meshes, and furnished with nodal thickenings at the points of junction with the longitudinal bars of the network. Retzius figures these in Locusta, but in Dytiscus he describes the transverse networks as generally composed almost entirely of radial fibres with very few transverse connections; and in place of nodal dots he describes several thickenings or nodes placed irregularly, and much fewer in number than the nodal dots described by Melland. Melland does not trace any connection between the network and the muscle-corpuscles nor with the nerve endings. He shows how the optical appearances of striped muscle are caused by the network. He considers the network to be intimately connected with the sarcolemma, and to be homologous with the intracellular networks which have been described in other cells. It is evident from fig. 6 of his paper that we have to do with a true network and not a honeycomb, a fact which is not so apparent from the figures of Retzius and Bremer. I have reproduced the figure from Mr. Melland’s paper (fig. 18).

Mr. Melland’s results were obtained partly in conjunction with myself, and the object of the present paper is a continuation of this investigation. I have endeavoured to trace the distribution of this intracellular network of the striped muscle-fibre in the animal kingdom, and also, so far as possible, to determine its function.

The striation of muscle must not be confounded with a transversely striated appearance caused by a corrugated outline of the fibre, possibly due to a state of over-contraction. Such a false striation is met with occasionally in some fibres in the Echinus, Leech, &c., and is the cause of the muscles of these animals having been described as striped. I shall, therefore, only describe muscle as being striped when the striation is due to the presence of the intracellular network, described by Retzius, Bremer, and Melland.

I have examined muscle taken from representatives of the chief groups of the animal kingdom with the special object of investigating the presence of an intracellular network in the muscle-cells, either such as that of the striped muscle-fibre, or, when this does not exist, an intracellular network of any kind.

The Amœba and Hydra have been included in this investigation; for it is an important point to determine the existence of an intracellular network in such a primitive and eminently contractile cell as the Amœba; it is also important to investigate the structure of the muscular processes of the ectoderm cells of the Hydra, as they are supposed to represent the first beginning of a muscle-cell.

In all cases the outlines and main details of the figures were drawn with the camera; in most cases under the immersion objective of Beck with No. 2 eyepiece, giving a magnifying power of 1100 diameters.

Methods of Preparation.—The chief method of preparation used was the method of gold staining employed by Melland. The gold stains and renders evident the intracellular network of most cells, especially the network of the striped muscle-cell; hence it is at once a test whether the striation of the fibre is due to the presence of the network, or whether it is merely the false striation mentioned above.

Various modifications of the gold method were employed according to the delicacy of the tissue under investigation. The method employed by Melland consists in placing the muscle in 1 per cent, acetic acid for a few seconds; then in 1 per cent, gold chloride for thirty minutes; then in formic acid, 25 per cent., for twenty-four or forty-eight hours in the dark.

This answers well for vertebrate and insect muscles. But for the more delicate organisms, such as the Hydra, Daphnia, &c., and the heart muscle of invertebrates, I found a one hour’s immersion in formic acid, exposed to strong sunlight, to be the best treatment; or, in some cases, a warm chamber (40° C.) was used. A longer immersion than one or two hours in the formic acid in these cases leads to disintegration of the tissues. In addition to the gold preparations, osmic acid preparations were made in most cases, and compared with those made by the gold method. Osmic acid is well known for its property of fixing the histological elements in their natural state.

The examination of fresh tissues was in many cases of very little use; for the cells of the striped muscle of many of the animals investigated are so small that under a high power they barely appear striped, and no network can be seen at all. In these cases it is only by softening the fibre and so swelling it out, and at the same time staining the network, that the latter can be demonstrated; this is the special action of the method of gold staining.

It is necessary to mention that the results obtained by the gold method are somewhat uncertain. In some cases the network will come out distinctly, but in others, especially when the preparation has been left for a longer time than usual in the acetic acid, the network appears to consist of rows of granules instead of definite lines. This uncertainty was noticed also by Retzius, Gerlach, and Bremer, and is no doubt the cause of the different appearances described by these authors.

In order to avoid the monotony and interruption of repeatedly stating the treatment used for the muscle of each animal, the exact method used is given with the description of the figure of each animal in the plates at the end of the paper.

I shall now take the chief group of the animal kingdom in their zoological order, describing the muscle found in each case.

Klein¹ states, on Heitzmann’s authority, that under suitable conditions the protoplasm of the white blood-corpuscles can be seen to contain an intracellular network composed of fine fibrils. Dr. Klein has, however, recently informed me that he does not find an intracellular network in the Amœba, nor in the majority of white blood-corpuscles.

On examination of very large specimens of Amœba princeps in the fresh state the constant flowing movement of the protoplasm renders it difficult to conceive of any permanent intracellular network. I have, moreover, made gold preparations of these Amœbæ in the following manner:—The Amœba was placed in a drop of water with a little cotton wool under-neath the cover glass to prevent the animal being washed away by the reagents. A few drops of 1 per cent, acetic acid were then run in under the cover glass for a few seconds. Gold chloride was then run in, and the animal left in this for fifteen minutes. Formic acid was then added, and the animal left exposed to light for about one hour; by this time the gold was reduced and the animal stained. The preparation was then mounted in dilute glycerine.

Amœbæ prepared in this way showed no trace of an intra-cellular network; the protoplasm simply presenting a mottled granular appearance.

Although there is no definite intracellular network, comparable to that of an ordinary epithelial or gland cell, known to exist in any of the Protozoa, yet a vacuolated condition of the protoplasm is well known to occur in many of them. This attains a high degree of development in many forms, e. g. Noctiluca. These vacuoles are certainly not all food vacuoles, and may possibly indicate the starting point of the differentiation of an intracellular network, i. e. a differentiation of the cell into firmer and less dense parts, the former of which takes on the form of a network or reticulum. For although it is not absolutely certain that the structures described as intracellular and intranuclear networks are in all cases denser than the rest of the protoplasm of the cell, they are, I believe, generally assumed by histologists to be so, and also to be protoplasmic in nature.

The stalk of the Vorticella contains a spiral protoplasmic fibre, which is eminently contractile. This fibre, when treated with the gold staining, shows no trace of the presence of fibrils, having simply the appearance of undifferentiated protoplasm.

The peculiar ectoderm cells of the Hydra are important to investigate, since they are generally held to represent the first commencement of a muscle. Here the one cell is differentiated into two parts to perform two functions, the one portion to act as a sensory cell, the other to act as a muscle.

Hamann¹ describes, in the epithelial muscle-cells of the hydroid polypes, a network in the body of the cell, but in fibrillation in the muscular process.

My own observations on the cells of the Hydra agree with those of Hamann. Gold preparations of these cells show a network in the body of the cell, but no continuation of it into the muscular process (fig. 1).

Striated muscle has been described as occurring in the disc of Aurelia by Max Schultze, Brücke, and Virchow, and in Pelagia by Kölliker.²

In gold preparations of muscle from the disc of Aurelia I find distinct transverse striation, which, under the immersion objective, is found to be due to the presence of a network similar, in all respects, to the network described by Retzius and Melland in striped muscle (fig. 2).

Muscle taken from the base of the Actinia and treated with gold was found to consist of elongated fusiform cells, non-striped, and showing no trace of any intracellular network, or of any fibrillation.

Hence the conclusions obtained are that in the muscular process of the Hydra cell there is no form of network or fibrillation, although a network is present in the body of the cell. In the more highly organised Medusa the typical network of striped muscle is found to be present, but in the equally highly organised Actinia no network is present, nor is there any fibrillation in the muscle-cells.

These results agree with those obtained by Hamann¹ by the method of isolating the cells by maceration in various reagents. He states that the muscles of Hydroid polypes are always smooth, and quite distinct from the striated muscles of the Medusæ. Hamann thinks that where transverse striation has been described in Hydroid polypes it is probably due to the action of reagents.

The Hertwigs,² in their observations on the Actiniae, describe no fibrillation in the muscle-fibre. They investigated the tissues of Sagartia and Anthea.

The muscle of the Echinoderms has been described as striped by several observers. Firstly, by Schwalbe³ in the muscle-cells between the ambulacral plates of Ophiothrix, and more recently by Geddes and Beddard.⁴ From the figure given by the latter observers it is evident that the striation they describe is the false striation mentioned above as being due to annular constrictions. Schwalbe, however, describes double oblique striation.

I have made gold preparations of muscle taken from the “lantern of Aristotle” of Echinus, and find no trace of the network of striped muscle or of any fibrillation. The cells are remarkable for the clearness and transparency of their protoplasm.

These results again agree closely with those obtained by Hermann.¹ He describes the muscle of the Asterids as smooth, and very seldom showing fine longitudinal fibrillation. In the Holothurians he describes the muscle as non-striped, but states that longitudinal fibrillation is to be seen in it.

The muscle-fibres of the Leech are peculiar: they consist of an outer clear portion and a central granular part. In gold preparations the outer part stains the more deeply of the two portions of the cell and appears quite homogeneous, showing no trace of a network. In osmic acid preparations the outer layer appears very faintly fibrillated, but I could not identify any distinct fibrils differentiated from the rest of the cell even under the immersion objective.

Transverse sections of the muscle of the Leech show a radiating appearance of dark and light bands in the outer portion of the cell. This is, I believe, caused by the method of preparation, for in some sections the outer portion of the cell is broken up into pieces arranged in a radiating manner and corresponding to the light portions between the radiating dark lines in the better preserved specimens. I find nothing corresponding to this appearance in muscle prepared by the gold or osmic acid methods, which are the methods generally recognised as maintaining the true histological characters of cells intact.

Wagener,² from transverse sections of dried specimens of Leech, states that the muscle-cells consist of a central medullary substance round the nucleus, and a cortical substance splitting into fibrils. This is also described by Schwalbe¹ in the fibres of Aulostoma.

Gold preparations of the muscle of the Earthworm show large elongated cells which on close examination show longitudinal lines; these under the immersion objective present a dotted appearance (fig. 3). At first sight it might appear that we have here the network of striped muscle; but this is not the case. In the first place, there is no appearance of transverse striation at all; in the second place, the dots are not arranged transversely but are quite irregular; lastly, so far as I could observe, the dotted lines are superficial and do not extend into the body of the cell.

One of the so-called “hearts” of the worm was treated in the same way; the muscle-cells were found to resemble almost exactly the muscle-cells described above.

In the Polyzoa and Rotifera striped muscle is well known to occur. I have not, however, been able to determine with certainty whether the striation is due to the presence of a network or not. Nitsche¹ says that the striation in the retractor muscle of the Polyzoa is not due to any wrinkling of the sarcolemma. The retractor muscle appears to be the only muscle-that is striped from Nitsche’s observations.

Striped muscle has been recently described by Haswell² in the gizzard of Syllis one of the Polychæte worms.

According to Schwalbe,³ double oblique striated muscle is present in Solen, Ostrea, and Helix.

Preparations of the adductor muscle of the Anodon treated with gold show that the muscle consists of small elongated cells of the unstriped type, showing no fibrillation or transverse striation. The muscle treated with osmic acid shows faint fibrillation, but no distinct fibrils.

The Limpet was chosen for an investigation of the structure of the heart muscle. In gold preparations of the ventricle I find the network of striped muscle present (fig. 5).

In the heart of the Anodon I could not determine with certainty whether the network was present or not, although faint indications of it were obtained.

The adductor muscle of the Oyster consists of two portions: a white opaque portion, and a more gelatinous portion. Gold preparations were made of each of these. The cells of the “white muscle” are large, with clear outlines, and remarkable for the clearness and transparency of their protoplasm. The cells of the “gelatinous muscle” are smaller and less transparent. Neither of these showed any network or fibrillation.

Gold preparations of the muscle of the foot show that it consists of very small cells of the unstriped type densely massed together.

The muscle of the odontophore, however, shows transverse striation, which under the high power is seen to be caused by the presence of the typical network of striped muscle.

The Pecten differs from most of its class by performing rapid movements of its adductor muscle whereby it propels itself through the water. Gold preparations of the adductor muscle made by my friend Mr. J. T. Cunningham show the network of striped muscle very plainly (fig. 4). I have not observed the double oblique striation described by Schwalbe in the muscle of Molluscs and Echinoderms. As this is not seen in gold and osmic acid preparations, I think it must be an optical effect. Schwalbe, indeed, admits that in Ophiothrix the transverse striation is due to folds in the sarcolemma (loc. cit., p. 211).

Representatives of the Crustacea and Insecta, viz. the Lobster, Dysticus, and the Bee, were investigated by Mr. Melland,¹ the network (of striped muscle) being found in each case.

Gold preparations of the heart of the Crayfish show the network to be present in this muscle as in the body muscles; the network is, however, much finer and more difficult to demonstrate (fig. 6).

The muscle-fibres of the heart are intimately blended with what appear to be large masses of granular protoplasm enclosing nerve-cells; these may possibly be of the nature of nerve-endings.

As a representative of the minuter forms of Crustacea, I examined the Daphnia. The muscle-fibres of this animal, when examined in the fresh state, only show transverse striation faintly. After many attempts I succeeded in obtaining a satisfactory gold preparation, where the muscle-fibres were much softened and pressed out to many times their normal diameter. These fibres show the network very plainly (fig. 7).

In this case the animal was placed whole in 1 per cent, acetic acid for ten minutes, and left in the formic acid in a warm chamber at 40° C. for two hours.

To determine if striped muscle is present in the larval insect as well as in the imago, I made preparations from the larva of the Ermine Moth (Spilosoma lubricepeda). Muscle was taken both from the jaws and from the legs. In both cases the muscle was found to be striped, the network in the muscle of the jaw being especially well developed.

Muscle taken from the leg of the Spider and treated with gold showed the network of striped muscle.

The vertebrate animals examined by Mr. Melland were the Frog and the Rat. These will serve as examples of the Amphibia and Mammalia. I have examined the muscle of animals taken from the other chief groups, viz. the Cyclostomata, Elasmobranchia, Teleostea, Reptilia, and Aves, taking as representatives of these groups respectively: Myxine, Scyllium, Gastrosteus, Testudo, and Turdus.

Muscle taken from each of these animals was treated with the usual gold method, and in each case a network was found identical with that described by Melland. On comparing these networks with one another and with those described above in the striped muscle of the several invertebrate animals, they are found to agree in all respects.

With regard to Amphioxus, I have not had the opportunity of examining fresh specimens. The muscle has, however, been described as striped,¹ and (from analogy) I see no reason to think that the striation is due to any other cause than a network. In the Ascidian, I have examined the muscular bands of Salpa and find striped muscle present.

The heart muscle has long been described as faintly striped transversely, but whether this striation is due to the same cause as that to which ordinary striped muscle owes its striation, has not been determined with certainty.

In order to investigate this point, I made gold preparations of muscle taken from the Rat’s heart. The cells are seen to contain a network similar to that of ordinary striped muscle. The network is more delicate and with much smaller meshes than the network in the body muscle of the same animal, and is therefore more difficult to demonstrate by the gold method (fig. 9). I have also prepared muscle from the heart of the

Frog (fig. 10) and Bird (fig. 11); the network in the latter animal is much plainer than in the others.

The striation of cardiac muscle therefore appears due to an intracellular network similar to that of ordinary striped muscle.

Klein¹ describes in the unstriped muscle of vertebrates a bundle of longitudinal fibrils which are in connection with the intranuclear network. This description of the structure of the unstriped muscle-cell is as follows: “Thus we may regard the unstriped muscle-fibre as composed of a sheath with annular thickenings and a bundle of delicate fibrils which at one more or less central point forms a delicate network. This, surrounded by a special membrane—except where the network is in connection with the bundle of fibrils—represents the nucleus.” Klein regards the bundle of fibrils as the contractile part of the cell, and thinks that by their shortening the muscle-fibre is caused to contract, elongation being produced by the elastic rebound of the sheath. Flemming² has observed these fibrils in the living muscle.

Dr. Klein informs me that he regards the bundle of longitudinal fibrils as representing an intracellular network homologous with other intracellular networks. Dr. Klein has been kind enough to show me his preparations of unstriped muscle prepared from the mesentery of the newt by twenty-four hours immersion in 5 per cent, ammonium bichromate, and afterwards stained with logwood. These preparations show clearly the longitudinal fibrils and their connection with the intranuclear network.

I have made preparations, by the gold method, of muscle from the mesentery of the newt and from the bladder of the Salamander, in both of these the fibrils in the muscle-cells are very evident, but the intranuclear networks do not show at all distinctly, which is a most unusual result in this mode of preparation. However, in preparations from the mesentery of the newt, made by Klein’s method, the intranuclear networks come out very distinctly in many fibres; and in one case I could trace the connection of the intranuclear network with the fibrils of the cell. The longitudinal fibrils do not show so well in these preparations as in those made by the gold method (figs. 12, 13). It thus appears that the vertebrate unstriped muscle differs from all the invertebrate unstriped muscle that I have investigated, in that the cells contain an intracellular network in the form of longitudinal fibrils. This may perhaps represent a form of network intermediate between the typical irregular network of other cells and the highly modified network of the striped muscle-cell.

From these investigations it appears that the peculiar intra-cellular network of striped muscle is developed in all muscles which have to perform rapid or regular movements.

A brief review of the chief animals mentioned in the preceding pages will make this clear. Commencing with the Actinia and the Medusa, these are both highly organised Coelenterates, but the Actinia is a sluggish animal which exhibits slow and irregular movements, while the Medusa propels itself through the water by rapid and regular contractions of its disc. Now, in the Actinia we find no striped muscle, but in the Medusa the network is present. In the worms such as the Leech and Earthworm striped muscle is absent; these animals only performing comparatively sluggish movements. In the Polyzoa the retractor muscles of the stomach, and in the Rotifers the retractors of the trochal disc, perform rapid movements, and have been described as striated transversely; this is probably due to the network, although, as stated above, I have so far been unable to determine this myself. In the Mollusca the movements are as a rule sluggish, and unstriped muscle is the prevailing type in this group. But in the odontophore muscles of the Snails the movements are more rapid, and in these we find the network developed. Also in the hearts of these animals which perform rapid and regular contractions I find the network present, at any rate in the case of Patella.

In the Pecten we have a Mollusc which differs from the majority of its class by performing rapid movements by the contraction of its adductor muscle, and here we find the network present. This is a most important fact in favour of the view that the peculiar network of striped muscle is developed when rapid movements are to be performed; for here we have the Mussel and the Pecten, both belonging to the same division of the Mollusca, and both having adductor muscles moving the valves of the shell. In the Mussel the adductor muscles only act at irregular intervals and comparatively slowly; but in the Pecten they perform rapid and frequent contractions when the animal swims. In the Mussel we find unstriped muscle, but in the Pecten the network of striped muscle is present.

In the majority of Arthropods and Vertebrates the movements are chiefly rapid and of frequent occurrence, and in these groups there is a wide distribution of striped muscle.

It is quite possible that in some animals of sluggish habits, such as some adult insects, the presence of striped muscle may be due to inheritance.

We should expect on this view to find striped muscle present in all well-developed hearts, since they execute rapid and regular contractions. However, in the so-called “hearts” of the Earthworm the muscle is unstriped. This can, I think, be explained as follows. These so-called “hearts” represent the earliest and most primitive form of heart in the animal kingdom, being simply local hypertrophies of the blood-vessels which perform rhythmic contraction. Now, the muscle of the blood-vessels is unstriped, therefore we should scarcely expect to find striped muscle in what are simply local hypertrophies of those vessels. Moreover, the contraction of these “hearts” is slow and peristaltic in nature. It is only when we come to the more highly developed hearts, such as those of the Patella, Snail, &c., which have to perform much more rapid and regular contractions than the “hearts” of the worm that we find striped muscle developed.

I may here state that I have not yet been able to determine the nature of the connection between the network of striped muscle and the nerve end-plate, which must exist if the combined results of Retzius and Bremer are correct. This I hope to do in a subsequent paper. I have, however, recently observed the connection between the network and the muscle-corpuscles described by Retzius.

The general conclusions arrived at in the preceding part of the paper are as follows:

1. An intracellular network of a definite character is present in the fibre of striped muscle throughout the animal kingdom.

2. This network is developed where rapid and frequent movements have to be performed.

3. The striped muscle-fibre consists of sarcolemma, network, and sarcous substance; and, so far as at present determined, there is no other structure present in the fibre (excepting the muscle-corpuscles and nerve-endings).

The question now before us is to determine if possible the nature and function of the network, and what relation it bears to the contractility of the muscle-fibre.

In order to investigate this point I teased out some perfectly fresh muscle from the leg of a Dytiscus and placed it on an inverted cover-glass over a gas-chamber. Alcohol vapour was then blown over the preparation when most of the fibres contracted owing to the chemical stimulus. The vapour was passed over the muscle for about a quarter or half a minute. The fibres were then fixed in their contracted state by plunging them into 5 per cent, acetic acid for half a minute, and then treated with gold and formic acid in the usual way. Many fibres were thus obtained completely contracted and also many fixed waves of contraction.

I also made preparations of relaxed muscle from a Dytiscus killed with chloroform. However, as the fibres vary so much in appearance according as they are more or less pressed out in the gold preparation, comparisons of the muscle stimulated with alcohol vapour, with that reduced by chloroform, though they may give the general effect of the difference, are not absolutely trustworthy. The only way of really proving this point is to examine a fibre, one portion of which is in the relaxed condition and the other contracted, or, in other words, a fixed wave of contraction.

On careful examination of the network in one of these fixed waves of contraction with the immersion objective the longitudinal fibrils of the network were always straight in all parts of the fibre and appeared slightly thicker in the contracted part of the fibre although it was difficult to judge accurately of the difference in thickness. The nodal dots, however, were the same size in both the contracted and relaxed portions of the fibre. The dots appeared in many cases even smaller in the contracted than in the relaxed muscle. This is, I believe, due to their being more separated from each other laterally, whereby the refractive effects which somewhat obscure the real size of the dots in the relaxed muscle are diminished (fig. 14).

It therefore appears from gold preparations that during contraction the nodal dots do not alter in size but that the longitudinal bars of the network increase in thickness. The apparent enlargement of the nodal dots when the fibre is seen in the fresh state is due to optical effect. Moreover, if the nodal dots do not alter in size it follows necessarily that the longitudinal bars must increase in thickness; for since they keep straight during contraction if they do not increase in thickness there must be a diminution in the volume of the fibre, which is known not to occur.

These results differ from the account given by Schafer of the changes during contraction. He states¹ from observations on the living fibre that during contraction his “muscle-rods” (which correspond to the longitudinal bars of the network) become compressed in the centre and their substance tends to accumulate towards the ends, i. e. that the knotted ends of the muscle-rods, which correspond to the nodal points of the network, increase in size at the expense of the shafts connecting them. On examination of the living fibre this certainly appears to be the case, but the optical effects of reflection and refraction are so great as to obscure the real change that takes place.

In discussing the theory of contraction, I shall assume that the intracellular network of striped muscle, and the longitudinal fibrils of the vertebrate unstriped muscle, are of the same nature as other intracellular networks; and, in accordance with the views of modern histologists, that they are protoplasmic in nature, and denser than the rest of the cell.

We have first to consider the nature of intracellular networks in general, and whether the function is an active or a passive one. In the case of intranuclear networks the changes which the network undergoes in karyokinetic division of the nucleus point to their being of an active nature. The extra-nuclear network (intracellular) is apparently of the same nature as the intranuclear, since the two have been shown to be continuous in many cells; and also they have the same behaviour towards stains and reagents. Moreover, if intra-cellular networks are developed by a process of vacuolation of the protoplasm of the cell, or a division into denser and less dense parts, as described previously when treating of the Protozoa, it is obvious that in these cases the network must be the active and the contractile part of the cell.

The continuity and identity of nuclear and extranuclear networks is strongly supported by Sedgwick’s remarkable observations on the early stages of Peripatus.¹ He not only demonstrates the continuity of the extranuclear and intra-nuclear networks, but he also shows that during segmentation of the ovum the cells do not become completely separated, but remain connected by their protoplasmic networks, i. e. that the intracellular networks of all the cells are continuous.

He also states that the so-called nuclear membrane is reticular in nature and not a true membrane, being, in fact, part of the general reticulum of the cell. In the cells described by Sedgwick there is no doubt that the reticulum is the active portion of the cell, for the rest of the cell consists simply of vacuoles.

Flemming states,² as Sedgwick also noticed, that the first change observable in a cell whose nucleus is about to divide is in the extranuclear protoplasm. Strasburger³ further states that the fibrils which form the nuclear spindle originate in the surrounding “cytoplasm” at the time of division. This appears to be direct evidence of an active function in the intra-cellular network.

These considerations show that the function of intracellular networks is very probably of an active nature.

We have now to consider the networks of striped and unstriped muscle. Both these forms of network are non-essential to contraction, for we have seen that many muscle-fibres of invertebrates are devoid of a network of any kind; but that they modify the nature of the contraction is very probable.

We have seen that the network of striped muscle is developed when rapid movements are to be performed; this shows that the function of contraction is intimately associated with the presence of the network.

The chief points of difference in the contraction of striped and unstriped muscle respectively are the great length of the latent period and the long duration of the contraction in the unstriped muscle. The velocity of the contraction-wave in striped muscle is in the Frog, 3—4 metres per second, while in the unstriped muscle (ureter) the velocity is only 20—30 mm. per second.¹ This seems to indicate that the peculiarly arranged network of striped muscle may be associated with the rapidity of its contraction.

In nearly all the specimens I have examined both the transverse and longitudinal bars of the network remain perfectly straight in all conditions of contraction and relaxation of the muscle. Hence the network, or part of the network, must either contract to the full extent that the muscle-fibre does, or else be elastic and so follow the movements of contraction of the fibre.

Retzius² figures a specimen in which the longitudinal bars are zigzag. However, from his description, and from comparison with my own preparations, I believe this to be due to disturbance during the preparation and not to be a normal condition.

We have now to consider whether the network is actively contractile or merely a passively elastic structure; or whether one part of it is contractile and the other passive. That both network and sarcous substance are contractile is improbable; for if the function of the network and the sarcous substance is identical, there is no apparent reason for the presence of the network. Differentiation in structure always implies differentiation in function.

We have seen that the longitudinal bars of the network diminish in length and apparently increase in thickness during contraction, and that they always remain straight in all conditions of contraction and relaxation of the fibre. The question now before us is to determine if they are actively contractile or passively elastic. The following considerations are opposed to the latter view.

• If the longitudinal bars of the network are passively elastic they must be on the stretch in the relaxed condition of the fibre, and resemble stretched elastic threads running the whole length of the muscle-fibre. Now, when a muscle is cut out of the body, and thereby removed from its attachments, it does not contract to any considerable extent; therefore, supposing the longitudinal bars to be elastic, something must keep them on the stretch.

  • This cannot be the sarcous substance, for as it is semi-fluid in nature it can hardly keep elastic threads on the stretch.

  • It cannot be a nervous impulse, continually acting on the longitudinal bars, for if it were so a muscle would contract on section of its nerve.

  • The only force which can keep the bars on the stretch must be that of the transverse networks. On this supposition the uncontracted muscle is not in a state of rest, for there is a continual force exerted against the transverse networks by the tendency of the longitudinal bars to shorten. It is very difficult to conceive that the muscle, in its uncontracted condition, should be in a state of extreme tension, and not of comparative rest.

• In the unstriped muscle-fibre there are no transverse networks present, and hence no force to keep the longitudinal fibrils on the stretch, except the sarcolemma, which would be scarcely adequate to do so.

It therefore appears improbable that the longitudinal bars of the network are passively elastic, and if this is the case the only conclusion remaining is that they are actively contractile, and hence, presumably, the cause of contraction of the fibre. This view is also supported by the following considerations:

In the muscle-cell the part which performs the contraction is evidently the most fundamental part of the cell, and this we should expect to be differentiated first. In the embryonic development of striped muscle it is found that the longitudinal striation appears first, i. e., that the longitudinal bars of the network are differentiated before the transverse. This is also the case in regenerating muscle. Again, in tracing the phylogeny of muscle, we found that the first indication of an intracellular network was in the vertebrate unstriped muscle in the form of longitudinal bars only. Hence both the phylogeny and the ontogeny of the network favours the view that the longitudinal bars are the contractile part of the cell.

Similarly to the longitudinal bars the transverse networks always remain straight in all conditions of contraction relaxation of the fibre. Hence they become necessarily extended when the muscle-fibre contracts, and return to their original form on relaxation of the fibre. The question now remains as to whether the return of the transverse networks to their original position is due to active contractility or to elastic rebound. The following arguments, for the first of which I am indebted to Mr. Melland, are in favour of the latter view.

• An elastic thread, if stretched and then allowed to rebound, will always return to its original length, i. e. will always shorten to the same extent. The transverse networks behave in this way; they always shorten to the same extent, viz. to the normal diameter of the fibre. This speaks in favour of their being passively elastic, for if they were actively contractile there is no reason why the fibre should not be compressed to less than its normal diameter, elongation at the same time taking place; whereas the fibre always relaxes to the same extent.

• If the statements of Gerlach, Retzius, and Bremer are correct, both parts of the network are connected with the end-plate and with the axis cylinder of the nerve, the longitudinal bars being connected indirectly through the transverse networks, the latter being in direct connection with the nerve. It is therefore difficult to conceive that the transverse networks can contract actively after the longitudinal bars have begun to relax, for the nervous impulse will apparently reach the former first, and hence they must contract at the same time as or before the longitudinal bars; and yet if the relaxation of the fibre is held to be due to active contraction of the transverse networks this is what must occur.

The conclusion to which I am therefore led is that the contraction of the striped muscle-fibre is due to the active contraction of the longitudinal bars of the network, and that the transverse networks are probably passively elastic, and by their rebound cause relaxation of the muscle-fibre. That the transverse networks and the muscle-corpuscles, with which they are said to be continuous, possibly furnish paths by which the nervous impulse is conveyed from the nerve ending to the longitudinal bars. That the contraction of the unstriped muscle-fibre is due to the active contraction of its longitudinal fibrils when these are present (as in vertebrate muscle). In the case of unstriped muscle which possesses no fibrils the contraction is due to the whole protoplasm of the cell, there being no special part differentiated to perform this function.

Should these conclusions prove to be correct, we may imagine the changes that occur in the striped muscle-fibre during contraction to be as follows:

The nervous impulse reaching the end-plate of the nerve is conducted by the transverse networks to the longitudinal bars, and causes them to shorten; it does not cause the transverse networks to contract, because they are passively elastic and non-contractile. The longitudinal bars shorten according to the strength of the nervous impulse, and remain so as long as it lasts. By fluid pressure the transverse networks are extended and remain so as long as the longitudinal bars remain contracted; when these cease to contract the elasticity of the transverse networks comes into play, and they shorten to their original dimensions, and by fluid pressure extend the longitudinal fibrils to their original length, the elastic sarcolemma aiding in the process.

The alternate action of the longitudinal and transverse networks no doubt causes the special features of the contraction of striped muscle, viz. the quick response to stimulus and the rapid contraction; and we have seen that the network is developed wherever rapid movements have to be performed.

In connection with the foregoing considerations, the results of Gerlach, Retzius, and Bremer, should they prove to be correct, are of importance. I think there is little doubt that the longitudinal striæ described by Gerlach are identical with the longitudinal bars of the network figured by Retzius, Bremer, Melland, and myself. Gerlach traced these striæ into connection with the nerve endings. Retzius showed the connection between the muscle-corpuscle and the transverse striæ, and Bremer traced the axis cylinder of the nerve into direct continuity with the muscle-corpuscles. It therefore appears that the network is connected with the nerve, and that the longitudinal bars are connected with it indirectly through the transverse networks. The direct continuity of the network with the nerve does not necessarily imply that the network is itself nervous; in fact, it really supports the view that it is the part actively concerned in contraction; for we should expect a priori, that if a differentiation occurred in muscle it would be with the contractile part that the nerve would be in continuity.

On the other hand, with regard to the transverse networks it is possible that they may be in part nervous in nature, and have for their function the more rapid conveyance of the stimulus through the muscle; and that the more rapid response to stimulus, the special characteristic of striped muscle, may be partly explained in this way.

There are two obvious objections to the theory of contraction we have arrived at, which I shall proceed to discuss:

1. It necessitates a difference between the longitudinal and transverse bars of the same network. This is an objection, the real nature of which it is impossible to determine in the present state of our knowledge of the nature and import of intracellular networks in general. In unstriped muscle the longitudinal fibrils are alone present, and in the development of striped muscle the longitudinal elements of the network appear first. The transverse networks are described and figured by Retzius as direct processes of the muscle-corpuscles; the mode of their development is as yet unknown, but should they prove on further investigation to develop as processes of the corpuscles, it would follow that the two elements of the network are, in spite of their close connection in the adult, of entirely independent and different origin. And then a difference of function would become not only possible but highly probable. Further, the action of different reagents in splitting the fibre in different directions (alcohol, &c., causing longitudinal, and acids transverse splitting) lends some support to the same view. Haswell¹ in his observations on the striped muscle of the gizzard of Syllis, states that after treatment with hæmatoxylin, and then glacial acetic acid, the transverse networks are stained, but not the longitudinal; he says this may point to some difference in the substance of which they are composed.

2. This theory attributes the function of contraction to the network which forms much less of the bulk of the fibre than does the sarcous substance, the latter being far greater in amount than the network. In reference to this it should be borne in mind that contraction is not the only function performed by muscle. The muscles, as stated by Dr. Michael Foster,² are continually undergoing metabolism, giving rise to a certain amount of heat; the metabolism during rest being slow, but suddenly increasing during contraction. The energy involved in the work done in a muscular contraction is only about one tenth the total energy expended, the rest going out as heat. Hence the muscles must be regarded as the chief sources of heat of the body, and are, “par excellence, the thermogenic tissues.”

It thus appears that the thermogenic function of muscle absorbs a far greater amount of its energy than does the contractile function, and if we attribute the thermogenic function to the sarcous substance and the contractility to the network, the above objection appears to receive a satisfactory answer.

The following quotation from Prof. Michael Foster¹ is curiously in accordance with the view of the structure and function of muscle maintained above, and may fitly conclude this paper.

“It is quite open for us to imagine that in muscle, for instance, there is a framework of more stable material, giving to the muscular fibre its histological features, and undergoing a comparatively slight and slow metabolism, while the energy given out by muscle is supplied at the expense of more fluctuating molecules, which fill up, so to speak, the interstices of the more durable framework, and the metabolism of which alone is large and rapid.”

1. In all muscles which have to perform rapid and frequent movements, a certain portion of the muscle is differentiated to perform the function of contraction, and this portion takes on the form of a very regular and highly modified intracellular network.

2. This network, by its regular arrangement, gives rise to certain optical effects which cause the peculiar appearances of striped muscle.

3. The contraction of the striped muscle-fibre is probably caused by the active contraction of the longitudinal fibrils of the intracellular network; the transverse networks appear to be passively elastic, and by their elastic rebound cause the muscle to rapidly resume its relaxed condition when the longitudinal fibrils have ceased to contract; they are possibly also paths for the nervous impulse.

4. In some cases where muscle has been hitherto described as striped, but gives no appearance of the network or treatment with the gold and other methods, the apparent striation is due to optical effects caused by a corrugated outline in the fibre.

5. In muscles which do not perform rapid movements, but whose contraction is comparatively slow and peristaltic in nature, this peculiar network is not developed. In most if not all of the invertebrate unstriped muscle there does not appear to be an intracellular network present in any form, but in the vertebrate unstriped muscle a network is present in the form of longitudinal fibrils only; this possibly represents a form of network intermediate between the typical irregular intra-cellular network of other cells and the highly modified network of striped muscle.

6. The cardiac muscle-cells contain a network similar to that of ordinary striped muscle.

The investigations connected with this paper were partly carried on in the laboratories of the Owens College and partly at the Scottish Marine Station at Granton. I must here express my thanks to my brother, Professor Milnes Marshall, for his kindness in revising the paper, for much advice in its production, and for obtaining the literature of the subject; all the controversial points were discussed with him and the preparations submitted to his examination. My thanks are also due to Dr. Klein for kindly showing me his preparations and for examining several of my own. I must also thank Mr. J. T. Cunningham for the use of the Scottish Marine Station, and for obtaining several of the animals.