48

SARCOLEMMA-SARCOUS ELEMENTS.

movements of muscular fibres. The muscular and the nervous textures admit of little modification, retaining nearly the same structural character under all kinds of circumstances. The third texture, the filamentous, being merely the connecting medium of the several component parts, may be regarded as suffering various modifications, or, at least, as representing various other tissues, particularly membrane, bone, and cartilage.

The Muscular Texture.-Two kinds of muscular fibre are known in the animal kingdom, and these, in the higher animals, are well distinguished from each other. One of these occurs in the voluntary muscles, and is named, from conspicuous cross markings, the striped muscular fibre; the other, found in the alimentary canal, the womb, and the bladder, being destitute of such cross markings, is termed the unstriped. In the heart and the gullet both kinds are met with. The elementary striped muscular fibres are arranged in sets parallel to each other; the unstriped muscular fibres, on the

contrary, cross each other at various angles, and interlace, being arranged like membranous organs enclosing a cavity, which, by their constriction, is contracted.

The striped fibres are usually as long, or nearly as long, as the muscle in which they exist. They vary in diameter from one-sixtieth to one-fifteen-hundredth of an inch; they are of the greatest breadth in crustaceous animals, fishes, and reptiles, and of least breadth in birds. Their average width in the human body is one-fourteen-hundredth of an inch. They are not cylindrical, but more or less flattened. This primitive fibre consists of a great number of primitive particles, or sarcous elements, enclosed in a tubular organ, termed sarcolemma.

Fragment of elementary fibre of an eel partially contracted in water-magnified 300 diameters. a, uncontracted part; b, the contracted part.

The ordinary diameter of the unstriped fibre is from one-two-thousandth to onethree-thousandth part of an inch. It is doubtful if they possess a sarcolemma. The absence of cross stripes seems to arise from a less uniform arrangement of their interior particles, or sarcous elements.

In the lower animals, the distinctive characters of these two kinds of primitive muscular fibre begin to be confounded, especially when the fibres become much reduced in size. The transverse stripes become irregular, not parallel, and interrupted; and sometimes a fibre shows the transverse stripes near its centre; in short, as the fibres become extremely minute, these anatomical characters are lost; and this may be the reason why in infusory animalcules, the wonderful movements of which they are capable cannot, even with the best microscopes, be referred to the presence of muscular structure.

Each primitive muscular fibre is properly regarded as a distinct organ complete in itself; and there are instances in the animal kingdom of a striped muscle consisting of a single fibre, and this fibre containing only a single file of sarcous elements.

Whenever a primitive muscular fibre preserves a rectilineal direction from end to end, the movement it undergoes is simply rectilinear; but the compound organs, termed muscles, in the human body, and in the larger animals, consist of many thousands of these primitive muscular fibres: still, however, the result must be described as a mechanical traction, compounded of the rectilineal motion, in a number of minute fibres, or parts of fibres, as to length, that original rectilineal motion being the effect of molecular movement of the sarcous elements within the primitive fibres.

EFFECTS OF MUSCULAR CONTRACTION.

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These primitive muscular fibres are plainly extravascular; that is, the minute bloodvessels which nourish them and replace their substance, continually reduced to inert chemical products by the exercise of living action, do not enter the fibre, but merely convey the blood to its exterior surface, whence the nutrient matter is attracted into its interior. Of the nervous filaments supplying the primitive muscular fibre, a like remark may be made as respects all those animals in which nervous filaments can be traced to the component fibres of a muscle. The primitive tubules of a nerve pass among the fibres of a muscle, and touch the sarcolemma as they pass; but, as far as present researches have informed us, they are entirely precluded by this structure from all contact with the contractible material, and from all immediate intercourse with it."-Physiological Anatomy, by Todd and Bowman, vol. i. p. 168.

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Contractility.-The property of a muscular fibre to shorten itself on the application of a stimulus, and, by a quick alternation, again to return to its former length, is contractility. When, then, the contractility of a muscular fibre is spoken of, the term is to be understood in this special sense, or as indicating the quick alternation of shortening and lengthening. In the works of Haller, the greatest of physiologists, this special property of muscular fibre is termed irritability. But as irritability may be sometimes employed in a larger sense, contractility appears to be the more appropriate term. At the same time, it cannot be denied that irritability includes contractility; that is to say, that contractility of muscular fibre is a species of irritability, and the same thing may be said of excitability. The contractility of a muscular fibre, in the sense here indicated, is a species, or form, of its excitability.

The stimulants which call the contractility of a muscular fibre into activity, are either mechanical, as irritation with a sharp instrument; chemical, like some acid chemical fluid; electrical, like a shock of galvanism; or psychical, like the human volition.

When a muscular fibre, the opposite extremities of which are attached, for example,

b

to adjacent points of two bones, is made to shorten itself forcibly by the application of a stimulus, the more moveable point is drawn nearer to the more fixed point; and this is the great law on which locomotion by muscular fibres depends. Thus the fore-arm is bent upon the arm by a muscle, b, which arises from the top of the latter, and which is inserted at e, at a short distance from the elbowjoint. A very slight contraction will raise the hand, but a considerable increase of power is required to overcome a resisting force.

BONES OF ARM, HOLDING WEIGHT.

Tonicity. There is another form of muscular contraction, which may or may not be the result of the same property, modified by a difference of circumstances. In past times, however, it has been regarded as a different property, and is known by the name of tonicity. The character of this so-called property of the muscular fibre is better taught by examples than by description. If a muscle in the living body be cut right through, each portion, after a few quivers, begins slowly to shorten itself in a permanent manner, so that an

ORGANIC NATURE.-No. II.

D

50

SHELL OF THE LOBSTER A SKELETON.

empty space is left between the two cut extremities. There being no tendency in these two shortened portions to return to their former length during an indefinite term, this effect has usually been ascribed to a property different from contractility, under the name of tonicity. Whenever, by any change of the relative natural position of the parts of the skeleton, as by fracture or dislocation, the points to which the opposite ends of a muscle are attached are brought nearer to each other, the muscle becomes permanently shortened by the same so-called tonicity. Again, if the muscles which extend or straighten a joint become paralysed, without a corresponding loss of power in the antagonistic muscles which bend that joint, then the flexor muscles, as they are termed, become shortened by their tonicity, and the joint remains permanently bent. This explains the permanent bent state of the elbow-joints in the paralysis of the upper extremities attendant on the painter's colic, to which all artisans are exposed whose occupations bring them into daily contact with preparations of lead.

Some forms of permanent lock-jaw seem to be of the same character; the muscles closing the jaw, which correspond to flexors, remaining in full vigour, while their antagonists have lost their power.

Muscular Texture-The muscular flesh constitutes a large proportion of the soft parts of the animal frame. In the higher animals nearly the whole of the muscles are attached to the skeleton, or are skeleton-muscles. In common quadrupeds there is a peculiar subcutaneous muscle-the panniculus carnosus-by which these animals are enabled to move the integuments, so as to shake off from their skin insects and other annoyances. In the human body there is a muscular expansion occupying the neck, corresponding to the subcutaneous muscles in quadrupeds, which anatomists term platysma myoides. The platysma myoides and panniculus carnosus, in higher animals, are conceived to represent an entire system of muscles, which, in its full development, belongs to a different part of the animal kingdom. For example, in the crab and lobster, the muscles which move the limbs are inserted into the shell, which is plainly the integument of these animals, though in them it takes the place of a skeleton. Thus the muscles of locomotion in the crab and lobster are a highly developed system of subcutaneous muscles, corresponding to the platysma and panniculus, or the hypodermal system in mammals, and which, as opposed to the skeleton system of muscles, belongs in general, under its developed state, to all animals, with the exception of the vertebrata. As organs of motion, the ciliary processes, or cilia, might be spoken of with the muscular tissue; but will be referred to elsewhere.

Nervous Texture. The nervous matter exhibits two forms, the vesicular and the fibrous. The vesicular nervous matter is gray, or cineritious, in colour, and granular in texture; it contains nucleated nerve-vesicles. The fibrous nervous matter is white and tubular; in some parts, however, it is gray, and its fibres are solid. When both these kinds of nervous matter are united into a variable-shaped body, that body is termed a nervous centre; and the threads of fibrous matter which pass to and from it, are termed nerves. The office of the latter is called "internuncial ;" that is, they establish a communication between the several parts of the body and the nervous centre, and between the nervous centre and the several parts of the body.

Of all the solids, the nervous matter comes nearest to the fluid condition. It contains from three-fourths to seven-eighths of its weight of water. In general terms, its chemical analysis may be thus given: albumen, seven parts; fatty matter, five parts; water, eighty parts; while the remainder consists of inorganic matter, the chief of which is phosphorus, if not free, in the state of phosphoric acid.

The fibrous nervous matter is most extensively diffused throughout the animal body. It enters largely into the nervous centres, and is the chief constituent of the nerves, which extend in every direction. Besides the tubular fibre, or nerve-tube, there is also what is termed the gelatinous fibre; the latter is much less abundant, being found chiefly in the great sympathetic nerve. In the tubular fibre, there is externally the tubular membrane, analogous to the sarcolemma of the striped muscular fibre. A white substance, called the white substance of Schwann, forms an interior tube, and within that the material is transparent. The nerves-tubes lie parallel to each other, and never branch. In the cut, a represents a nerve tube in water. The delicate line on its exterior indicates the tubular membrane. The dark, double-edged inner one, is the white substance of Schwann, slightly wrinkled. b is the same in ether. coalesced in the interior, and round the exterior of the tube. in part disappeared.

Several oil-globules have
others have accumulated
The white substance has

NERVE TUBES OF THE EEL, in water and ether-after Todd and Bowman. Magnified 300 diameters.

The vesicular matter exists in the nervous centres; but is never found in nerves. It essentially consists of vesicles or cells, containing nuclei and nucleoli. The wall of each vesicle is formed of an extremely delicate membrane, containing a soft but tenacious finely granular mass. The prevailing form is globular; but that figure is liable to be changed by packing. There is also a kind of nerve-vesicle, termed caudate, from exhibiting one or two tail-like processes.

A nerve is a leash of nerve-fibres, surrounded and connected by areolar tissue. The areolar tissue surrounding the nerve-fibres is called the neurilemma: from the internal surface of which, processes are sent inwards, to form partitions between the smaller leashes and the individual fibres. The blood-vessels are distributed upon the investing neurilemma and its partition-like processes-and thus the individual nerve-fibre is, like the ultimate fibres of the muscles, extravascular. The nerve-fibres within the sheath lie in simple juxtaposition, the several fibres being parallel to each other. These fibres, which in the cerebro-spinal nerves are chiefly of the tubular kind, while varying considerably, do not exceed the one-fifteen-hundredth of an inch in man and the mammalia.

Areolar Tissue, Membranes, &c.-The areolar tissue of recent authorities has a very perplexing number of names. Among the newer names applied to this tissue, is that of filamentous tissue. It is the tela cellulosa, the cellular tissue of the older authorities, called also cellular substance; but, in its ultimate structure, it appears to be of a fibrous character, and hence the term cellular is inappropriate. The areolar tissue is most extensively diffused over the animal body, connecting the other component parts of the frame in such a manner as to allow of a greater or less freedom of motion between them. Owing to this manifest use of the areolar tissue, the additional name connexive tissue" has been proposed for it. It is placed in the interstices of other textures in greater or less abundance, and in a more or less lax state, according to the exigencies of the case. It every where surrounds the blood-vessels, and is hardly absent in parts supplied with blood. In the more solid parts of bone, in teeth, and cartilage, it does not exist; nor

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DISTRIBUTION OF AREOLAR TISSUE.

scarcely in the substance of the brain, except around the larger blood-vessels. In the muscles it connects the elementary fibres together, yet does not penetrate the sarcolemma, or touch the contractile elements within. It is remarkable, that abundant as it is in the muscles at large, it is in very sparing proportion within the substance of the heart. It exists largely immediately beneath the skin; and hence it is this lax layer of areolar texture which is the seat of the dropsy termed anasarca, and of the occasional accumulation of air termed emphysema.

The areolar texture, moreover, surrounds all the organs, particularly those, like the pharynx, gullet, lumbar colon, bladder, &c., which have no free surface. It dips also into the interior of organs, and connects their proper anatomical elements together. It appears, however, that the importance of the areolar tissue in the parenchymatous organs, as they are named- the lungs, the liver, &c.-has been overrated. It always attends the distribution of the blood-vessels in such organs; "but wherever, either from the intricacy of the interlacement of the capillaries with the other essential elements of the particular organ, or the greater strength of these elements themselves, the firm contexture of the whole is provided for, while little or no motion is required between its parts, this intersticial filamentary tissue will be found to be confined to the larger blood-vessels, and to the surface of the natural subdivisions of the organ."-Todd and Bowman, vol. i. pp. 77, 78.

Under the microscope, the areolar tissue presents an inextricable interlacement of tortuous and wavy threads, intersecting one another in every direction. Of these

threads there are two kinds, the white fibrous element, and the yellow fibrous element. The threads of the former are inelastic, of unequal thickness, forming bands with the marks of longitudinal creasing, the largest of the bands being often onethree-hundredth part of an inch in width. The threads of the latter are long, single, elastic, branched filaments, disposed to curl when not put upon the stretch, and for the most part about the one-eight-thousandth part of an inch in thickness. They interlace with those of the white fibrous element, but there appears to be no continuity of substance between them. By the crossing in endless succession of these microscopic filaments, and of their fasciculi, there results a most intricate web, the interstices of which are most irregular in size and shape, while all necessarily communicate with one another. These interstices are not cavities possessed of definite limits, since they are, in fact, formed out of a mass of tangled threads. It appears at once, then, that the term cell is inappropriate to these interstices. In certain parts, however, of this texture, secondary cavities, not inappropriately termed cells, occur, particularly in the subcutaneous cellular tissue in which fat accumulates. These secondary cavities, or cells, often visible to the naked eye, have a somewhat determinate shape and size.

a, white, and b, yellow fibrous tissue, after Todd and Bowman. Magnified 320 diameters.

The fatty or adipose tissue has the like office of filling up interstices with the areolar tissue; and hence, being found almost constantly associated with that tissue, it has been too commonly regarded by anatomists as merely one of its modifications. The adipose and areolar tissues, however, appear to be altogether distinct and independent. It has, indeed, long been remarked that there are many situations in which areolar tissue