General Physiology: An Outline of the Science of Life

99, B), which flow together and under certain circumstances finally form a large lumpy mass about the nucleus. This is the complete analogue of the phase of contraction in the Rhizopoda, where the pseudopodia retract themselves so that the body assumes a more or less spherical form. Hence, the phenomena of protoplasmic streaming are, in principle, exactly the same in plant-cells as in Rhizopoda ; Max Schultze ('63) has discussed very fully the analogy of protoplasmic movement in the two cases.

The amount of work that can be performed in amoeboid movement has thus far not been ascertained, but the development of energy does not appear to be considerable. Muscular movement is the specific form of movement of the animal organism, by which apparently it is distinguished from all plants. All the gross and rapid mass-movements of the whole animal body or of single systems of organs, all those remarkable movements which of all vital phenomena produce most the impression of living, depend upon the contraction of muscle-fibres. Such movements mislead ordinary observers into ascribing to animals a higher stage of life than to the plants, the latter being considered to stand nearer to lifeless nature

In contrast to amoeboid protoplasmic movement, muscular movement is especially characterised by the fact that its factors are co-ordinated in space, in so far as the particles of a muscle-fibre shift themselves in one definite direction. Of course it can be said that in a long, straight, filose pseudopodium the particles flow likewise in a definite direction: but this direction is not continual, for, in retraction the particles mingle again with others and separate from one another in all possible directions. In contrast to this, the particles that in a muscle-fibre are the seat of contractionphenomena are constantly present as special structures in the rest of the cell-protoplasm and cannot directly mix with it. It is customary to term the whole muscle-cell a musclcfibre, and these specially differentiated contractile strips in it

FIG. 99.— Cell from a stamen-hair of Tradescantia virginica. A, Quiet protoplasmic streaming in the strands of protoplasm ; 27, the protoplasm has contracted into lumps and globules at a, 6, c, d. (After Kiihne.) muscle-fibrillce ; the fibrillge lie embedded in the protoplasm of the fibre, which can be termed with Rollett, in brief, sarcoplasm, arranged in various ways but all in the same direction. They represent specially differentiated organoids of the cellprotoplasm.

In accordance with the varying structure of the contractile muscle-fibrillaB, two groups of muscle-fibres or muscle-cells are distinguished, the smooth and the cross-striated. In smooth muscle-fibres the fibrillas, which lie embedded in the sarcoplasm parallel to one another, are completely homogeneous threads in FIG. 100.— Stentor cceruleus, a ciliate infusorian containing numerous parallel muscle-fibrillfe (myoid-fibres) in the exoplasm. A, Extended ; B, half -contracted (free-swimming) ; C, wholly contracted.

which every cross-section is like every other one. Cross-striated muscle-fibres, on the other hand, contain fibrillge that from one end to the other are divided into many segments, all of which possess a corresponding but complicated structure. The simplest forms of smooth muscle-cells are found among Infusoria. In many ciliate Infusoria, such as Stentor, the cell-body represents such a muscle-cell of the simplest kind ; it contains, embedded in the external layer of its protoplasm, smooth muscle-fibrillse, the so-called myoids, arranged approximately parallel to one another (Fig. 100). Other Infusoria, especially

the delicate Vorticella, possess a single, smooth muscle-fibre, composed of several fibrillge cemented together ; this extends outside the body as a thick strand and, surrounded by an elastic sheath to the inner wall of which it is fixed in an elongated spiral, serves the cell-body as a stalk for attachment (Fig. 101). In smooth muscle-cells that are united in the cell-community to form tissues, the protoplasmic body is reduced very much in quantity in comparison with the contractile fibrillse. It either forms merely a small sarcoplasmic mass containing the nucleus, which is enclosed by a long, spindle-shaped covering of contractile, fibrillar substance,

FIG. 101.— Vorticella. a, Extended; b, contracted (the stalk-muscle is not seen in a and &); c, stalk-sheath containing muscle-fibre, strongly magnified. FIG. 102. — Smooth muscle-cells — a, from the bladder of the frog ; 6, from the retractor muscles of freshwater Bryozoa. as in the smooth muscle-cells from the bladder of the frog (Fig. 102, a), or it lies as a small cell-body in the middle, lateral to the contractile bundle of fibrillse, as in the retractor muscles of fresh-water Bryozoa (Fig. 102, &).

The structure of cross-striated muscle-fibres is far more complex. As a type of these, which, like the smooth muscle, appear in manifold modifications, the insect muscle-fibre may serve, the structure of which has become known in minute detail, especially through the striking and extended investigations of Erigelmann and recently Rollett. The cross-striated muscle-fibre of insects is a long thin cylindrical cell, consisting of sarcoplasm, which is bounded externally by a somewhat denser layer, the sarcolemma, and contains numerous nuclei elongated longitudinally (Fig. 103). Embedded in this sarcoplasm, and extending parallel from one end of the fibre to the other, lie the regularly segmented muscle-nbrillse (Fig. 104, A). If the muscle-segments of a fibrilla be examined with very high powers, it is found that they all possess the same structure, the same arrangement of their constituents being repeated in every segment. Each segment is separated from the two adjacent segments by the so-called Dobies line [Zivischenscheibe] (Fig. 104, 2), arid contains

FIG. 103.— Cross-striated muscle-fibres. A, Two excised pieces of muscle-fibre (at the left above, the end of a fibre) ; the cross-striation is clearly to be seen, likewise many spindle-shaped muscle-nuclei. (After Schiefferdecker.) B, Two uninucleated cross-striated muscle-cells from the heart, at the left from man, at the right from the frog. (After Disse.) C, Crosssection of a muscle-fibre of an insect ; three nuclei are to be seen and, embedded in the sarcoplasm, the cross-sections of innumerable fibrillse. (After Rollett.)

two different substances, of which the one is doubly light-refracting, or anisotropic, and lies in the middle of the segment (Fig. 104, q, or q + m + q\ while the other is singly refracting or iso tropic, and in two portions borders the anisotropic substance (Fig. 104, i). In the middle of the anisotropic layer there appears more or less distinctly a clearer zone, which is termed Hensens disc or Hensen's line [Mittelscheibe] (Fig. 104, m). Finally, there occur in many muscle-fibres, but not as a constant constituent of all, one or two accessory discs [Nebenscheibe] (Fig. 104, 'n) lodged in the iso tropic substance. The general constituents of the muscle-segment are the anisotropic layer and the two isotropic layers bordering it ; of these the anisotropic substance is darker,

denser and more strongly refracting, while the isotropic substance appears richer in water, brighter, less dense, and less refracting. In every muscle-fibre similar discs of the individual fibrillse lie in the same transverse plane, so that the whole fibre appears regularly banded or cross-striated (Fig. 103, A). The cross-striated musclefibres of vertebrates often reach a very considerable length, although they represent only a single, multinucleate cell — e.g., the fibres from the long skeletal muscles of man are more than a decimetre in length, and each fibrilla in them extends from one end to the other.

In the movement of both smooth and cross -striated musclefibres, two phases can be distinguished, as in amoeboid movement — FIG. 104. — A, Two isolated muscle-fibrillge ; z, Dobie's line ; i, isotropic substance ; q, anisotropie substance. (After Ranvier.) B, Two single muscle-segments ; z, Dobie's line ; i, isotropic substance ; q, anisotropie substance containing Hensen's disc, m. The segment at the right possesses an accessory disc, n, in the isotropic substance.

that of contraction and that of expansion. Contraction consists of a shortening and thickening of the fibrillse. This process passes from the place of its origin in the form of a contraction-wave over the whole fibrilla. The particles, therefore, shift themselves in the longitudinal direction in such a manner that they come to lie beside one another in a larger cross-section. In this way the whole surface of the fibrilla becomes diminished, although not to its minimum, the spherical form, as is the case in naked , protoplasmic masses. The simultaneous contraction of the single fibrillae in either a smooth or a cross-striated musclecell evidently causes a shortening and thickening of the whole

fibre. If the contraction proceeds very rapidly, as in the fibrillse of infusorian cells and cross-striated muscle-fibres, the fibre gives a very quick twitch, the single events of which cannot be followed by the eye. Thus, the stalk of Vorticella contracts suddenly, assuming a screw-shape as a result of the spiral winding of the muscle-fibre and drawing the head of the animal tightly down to the foot of the stalk (Fig. 101, &). The smooth muscle-fibres of the tissues, on the contrary, generally contract extremely slowly and never show sudden twitches like the infusorian myoids and cross-striated muscle-fibres. But, while in the smooth musclefibrilla no further events, apart from the change of form, are to be noted microscopically, the cross-striated fibrilla, in correspondence with its complex structure, shows in the phase of contraction highly characteristic changes of its striation (Fig. 105). During the contraction of a single muscle-segment the following phenomena, which Engelmann (73, 75, 78) first carefully analysed, may be observed. The segment becomes shorter and

FIG. 105.— Single muscle-segment ; /, extended ; //, contracted ; 1, in ordinary light ; 2, in polarised light ; a, the anisotropic disc ; i, i, the isotropic discs. thicker, which is to be expected from the shortening and thickening of the whole fibrilla. Meanwhile, remarkable changes occur in the relation of the isotropic to the anisotropic substance. The latter increases in volume and the former decreases, while the volume of the whole segment remains unchanged. At the same time the anisotropic substance, which before was denser and darker, becomes less dense and lighter, i.e., less refracting, while the isotropic substance undergoes the reverse changes, becoming denser and darker, i.e., more refracting than it was before. These changes are extremely important, for they show that contraction consists in a passage of substance from the isotropic discs into the anisotropic, and, moreover, of substance that is of less consistency than that of the anisotropic disc. Recently, by means of photography Schafer ('91, 2, 3) has studied more carefully the microscopic changes in this process, and has discovered the interesting fact that in the anisotropic disc extremely fine tubes run parallel to one another and in correspondence with the direction of the fibres almost up to Hensen's disc (Fig. 106); in contraction the isotropic

substance flows into these tubes, so that the lumen of each is enlarged and the whole segment becomes broader and shorter. All these complex phenomena of contraction proceed with excessive rapidity from one muscle-segment to the following, so that one contraction-wave after another passes metachronically over all the elements of the whole muscle-fibre until the latter is completely contracted. The expansion of smooth and cross-striated muscle-fibres shows exactly the reverse of all these events observed during the contraction. The fibrillse extend, becoming gradually longer and thinner from the point where the contraction-wave previously began, so that now a wave of expansion proceeds from here over the whole fibrilla, until the latter is completely extended. In the single segment of the cross-striated fibre, also, the changes are exactly the reverse of those that appear in contraction. The segment becomes longer and thinner, the aniso tropic substance decreases in volume, and becomes darker, denser, and more highly refracting, while the isotropic substance gains in volume and becomes lighter, less dense and less refracting, until the resting state is again reached. In the expansion °f the cross-striated muscle-fibre, therefore, stance seen from above ; b, substance that possesses slioht consistency passes

muscle-segments. (After ffOVfl the amSOtrOplC 10 trie ISOWOpW CilSCS. Both smooth and cross-striated muscle - fibres are united in the cell-community into tissues, the muscles. Wherever rapid and powerful muscular effects are to be brought about, as in the skeletal muscles and the heart, the muscles are composed of cross-striated fibres, while the slow, sluggish movements of the involuntary organs, such as the stomach, the intestine, and the bladder, depend upon the activity of smooth muscle-cells. The contraction of muscle reaches its highest, and indeed, an astonishing rate in the wing-muscles of many insects, e.g., gnats, where, as Marey has shown, 300 — 400 contractions in a second can be carried out. It is evident that the effect of the contractions will be very considerable where very many fibres compose a muscle. In fact, even in relatively small muscles an enormous transformation of energy takes place. Thus, such a small muscle as the calf-muscle (gastrocnemius) of the frog, which measures scarcely a centimetre in crosssection in its thickest place, is capable, according to Rosenthal's. observations, of raising a weight of more than one kilogram. The work that the heart-muscle performs is enormous. Zuntz ('92) has calculated that the heart of a man beating normally performs in one day a work of about 20,000 kilogram-metres — a labour that would be sufficient to raise a weight of 20,000 kilograms one metre

high. It is easy from this to compute the enormous labour performed by this organ during the whole life of a man. The muscle is the most perfect dynamic machine known. Ciliary movement, finally, is no less wide-spread than the two other forms of contraction. The infusorian that bustles about actively in the water of a puddle moves by the strokes of flagella or cilia. The spermatozoon that in fertilisation strives after union with the egg-cell is driven forward by the vibrations of its flagellum. The cells of ciliated epithelium that line air-passages keep the mucous membrane clean by their activity, and by the rhythmic beating of their cilia shove to the outside foreign bodies that have come into the passages in swallowing. The host of Infusoria is numberless, flagellated spermatozoa are wide-spread among both plants and animals, and there is scarcely a group of animals whose bodies do not possess in some spot ciliated epithelium.

Like muscular movement, ciliary movement is co-ordinated — i.e., the motile particles are shifted in a definite direction. This is rendered possible by the fact that the contractile elements, as in the muscle-cell, are developed as constant differentiations of the cell-protoplasm in the form of short, hair-like appendages of the cell-body. According as the cell possesses one cilium or a few long ones, or many short ones, the term flagellated cell (Fig. 107, 0, D, JE), or ciliated cell (Fig. 107, A, B), is employed. The phenomena of ciliary motion result from the performance of vibratory movements by the flagella or cilia.

The following are the chief characteristics of ciliary motion. In contrast to most forms of muscular motion, which with few exceptions (Infusoria, heart-muscle) come about only as the result of external impulses from the nervous system, ciliary motion is automatic, i.e., the impulses that lead to it arise in the cilia themselves ; there is no known case in which the motion is at all under the influence of the nervous system. It has been determined by vivisection experiments 1 that the cause of the motion is seated in the protoplasm of the cell-body, for, if the isolated cilia possess absolutely no protoplasm at their bases, they are wholly motionless. Further, most cases of ciliary motion are distinguished by their rhythm, for except in certain flagellate and ciliate Infusoria the cilia contract always at regular intervals, at least during pronounced activity. The vibrations become irregular only during the transition to the resting-stage or under the influence of external factors. Finally, a third characteristic, which belongs only to multiciliated cells, is the metachronism of the motion of the individual cilia. The individual cilia of a row, beginning at one end, contract in exactly the same rhythm and succession, so that every beat of the first cilium is followed by a beat of the second, then of the third, 1 Of. Verworn ('90, 2),

the fourth, and so on. A cilium never contracts spontaneously out of order, it never makes a movement before the preceding cilium in the row has moved; it always begins its movement FIG. 107.— A, Three ciliated epithelium-cells from the human epididymis. (After Schiefferdecker.) B, Paramcecium aurelia, a ciliate infusorian ; C, Hexamitus inflatus, a flagellate infusorian E easing six flagella (After Stein.) D, Human spermatozoa; a, head, b, c, flagellum. sr Stohr.) E, Peranema, a flagellate infusorian, with its flagellum contracting, a> feebly, ore strongly.

immediately after the movement of the preceding cilium has begun arid before it is ended. It thus happens that, considering the row from above downward, the movement of each upper cilium slightly precedes that of each lower one (Fig. 108). In other words, the uppermost cilium gives the sign to the others ; if the uppermost one is at rest, the others rest ; if it contracts, they also contract in order ; and this is true not only of the cilia of the single cell, but,

in ciliated epithelium, of the cilia of all the cells in a row. In this manner there occurs an extremely delicate and regular play of the cilia, which has fascinated many observers and gives the impression of regular waves passing over the ciliated row, somewhat as the wind sweeps over a field of grain. When several parallel rows of cilia are present, the cilia standing beside one another in adjacent rows beat synchronously, just as the fibrillse lying beside one another in a muscle-fibre contract at the same time.

The phases of movement of the individual cilium can best be studied in the swimming-plates of the Ctenophora.1 The body of these remarkable animals consists of a delicate transparent jelly, and possesses eight stripes or ribs (Fig. 109) extending from one FIG. 109. — Beroe ovata, a ctenophore, natural size. Of the eight ribs or rows of swimming-plates extending from the upper (sense-)pole to the lower (mouth-) pole, only the four rows of one side are to be seen, two from the front, and two from the side.

pole to the other ; each rib consists of a row of plates, the swimming-plates, that lie upon one another like tiles upon a roof. Each swimming-plate is about 2 mm. long, and consists of a considerable number of cilia, cemented together, which belong to the cell-bodies lying beneath. On account of their extraordinary size, the unusual simplicity of their arrangement in rows, and the fact of the rhythm of their beat being frequently very slow, these swimming-plates serve as no other object does for experimentation and observation. As was said above, the plates are formed of many cilia cemented together, but each cilium evidently makes exactly the same movement as the whole plate, so that observations made upon the whole plate may be transferred to the conditions in a single cilium. On account of the size of the object observations can be made with the naked eye or with a weak lens. If a single swimming-plate be observed in profile, it is seen that in the resting-position it lies flat against the body, so that it shows two curves, a greater one of smaller radius immediately above the base, and a smaller one of greater radius and in the opposite direction in the upper half (Fig. 110). This

FIG. 110. — Swimming-plates of Beroe in profile, a, In the resting-position; b, in the position of is the position of rest. If now the plate performs a stroke, the lower curve beginning from the base of the cilium extends itself completely, even giving place to a slight curve in the opposite direction. Hence, in the position of extreme swing the plate stands erect with a slight curve toward the opposite side. The progressive phase of the stroke is thereby completed. Now follows the retrogressive phase, in which the plate falls back again into its position of rest, the original curve at the base gradually coming back until the plate again lies against the body. The retrogressive phase proceeds more slowly than the progressive. Because of this fact and by means of the upper curve — into the special significance of which we shall go no further — it is rendered possible that the motor effect of the progressive phase is not balanced by the retrogressive phase; otherwise the animal would remain continually in the same place in the* water. The movement of

individual cilia can be followed in Infusoria under the microscope, if the stroke be slowed by placing the objects in a thickish medium, such as a solution of gelatine. It is then found that the rest ing-position, from which the cilium performs its movements, is changeable. At one time the cilium lies more against the bodyr at another time it stands more vertical ; hence the amplitude of the swing, and thus the amount of the motor effect can be very finely graded (Fig. 111).

It follows from the change of form of the individual cilium in carrying out the stroke, that in the progressive phase a contraction, starting from the base of the cilium, takes place on the side toward which the stroke is carried out, for a simple measurement shows that this side is shortened when it passes into the position of extreme swing. At the same time the opposite side is drawn FIG. 111. — Movement of a single cilium of a ciliate infusoriau (Urostyla grandis, border-cilium) from two different resting-positions, / and 11. A. Progressive, B, retrogressive phase of the movement in several successive stages. The arrows indicate the direction toward which the body is driven.

over passively, being extended necessarily, according to simple mechanical principles. In the retrogressive phase the contracted side relaxes, and to the same extent the cilium, as a result of the elasticity of the extended side, bends back into the position of rest. The progressive phase, therefore, is the phase of contraction, the retrogressive phase that of expansion of the single stroke of the cilium. The play of the ciliary movement comes about by the rhythmic alternation of the two.