Volume 1

The collected papers of Sir W. Bowman, Bart., F.R.S / edited for the committee of the "Bowman Testimonial Fund" by J. Burdon-Sanderson and J. W. Hulke.

  • Bowman, William, Sir, 1816-1892.
Date:
1892
    than in the foetus, while their number relatively to the bulk of the fibre, at these two epochs, remains nearly the same. Muscles grow by an increase, not of the number, but of the bulk of their elemen- tary fibres : there is reason to believe that the number of fibres remains through life as it was in the foetus, and that the spare or muscular build of the individual is determined by the mould in which his body was originally cast. Of the Unstriped Fibres.. This variety possesses less interest than the other, in consequence of the apparent simplicity of its structure. The fibres consist of flattened bands (fig. 298, p. 245 infra), generally of a pale colour, bulged at frequent intervals by elongated corpuscles, similar to those of striped muscle and capable of being displayed by the same process.* The tex- ture of these fibres seems to be homogeneous. By transmitted light, they have usually a soft very finely mottled aspect, and without a darkly-shaded border. Sometimes the mottling is so decided as to appear granular, and occasionally these granules are arranged in a linear series for some distance. This condition is probably an approach towards the structure of the striped fibre, for these granules are about the size of the sarcous elements already described. It is generally to be seen more or less distinctly in the gizzards of birds; and may be now and then met with in the fresh muscle of the stomach, intestinal canal, urinary bladder, and uterus of mammalia. The ordinary diameter of the unstriped fibre is from -goVotii to 201oo^,n °^ an mch- It might be expected, from this account of the appearance of these fibres, that their discrimination from other tissues would be often difficult. The peculiar texture, however, the size, the soft margin, and, above all, the presence of numerous elongated oval corpuscles with two or three granules near their centre, are characters which, when united, will seldom be mistaken. As a number of fibres commonly take a parallel course together, the bulgings occasioned by the corpuscles give rise to partial longitudinal shadows, extending for some way beyond the corpuscles in the intervals of the fibres. As these irregular longitudinal shadows occur pretty uniformly throughout a bundle of fibres, and as some of them are necessarily out of focus, while others are in focus, the whole mass commonly presents a confused reticulated appearance, which has given rise to an almost universal notion that the fibres interlace one with another. This idea, however, is, in most cases, erroneous. It is doubtful whether these fibres are invested by a sarcolemma: none has hitherto been detected in an unequivocal manner. It is also still a matter of speculation how they terminate, or whether they in all instances have a termination. In the case of the transverse * In some specimens, however, of both varieties of fibre, they may be discerned without the addition of an acid.
    fibres ol the intestine, for example, it is uncertain whether each fibre surrounds the canal once, returning into itself as a ring, or more than once, as a spiral; or whether it passes only partially round it, the circle being completed by others. Whether the areolar tissue (the representative of the fibrous), that is found in connection with these fibres, serves to give them an attachment, by union with their extremities, or by involving them in its meshes, is also altogether unknown. In the gizzard of the bird, the ends of the fibres are united to white fibrous tissue, thus making an approximation to the striped fibre, as they do in colour. But we have not been able, after diligent search, to detect the true transverse stripes, which Ficinus describes to exist in this organ.
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    ON MUSCULAR ACTION. From 'The Physiological Anatomy and Physiology of Man, Vol. I, Chapter VII, pp. 170 to 194. Published February, 1843. Of the Function of Muscles. The great property of muscular tissue is that of shortening in a particular direction, and this property is called contractility. It is not that mechanical power which elastic substances possess of shortening themselves on the removal of some force which has stretched them, but it is an endowment, responsive to appropriate stimuli, and diminishing or disappearing with the healthy state of the tissue. The distinction between the contractility, the elasticity, and the physical tenacity of a muscle may be illustrated by the following imaginary experiment: Suppose the leg of an animal so severed from the trunk as to hang by a single muscle, which, after retaining its contractility for some time, were gradually to lose it. The limb would at first be borne up by the contractile power; but, as that ceased, the muscle would elongate under the weight, and the limb would remain suspended simply by the tenacity of the part. If, now, the muscle, were stretched between the hands, we should find it to possess some slight elasticity. The elasticity and much of the tenacity of muscles are attributable to the sarcolemma, and to the capillary and areolar tissues. It does not appear that elasticity is in any degree a property of the sarcous elements, and their tenacity must be comparatively slight; but it is the sarcous tissue alone that possesses contractility. Although it is universally allowed that the muscular tissue is the contractile substance, yet the strange question has been raised, and is still warmly debated, whether it possesses this power in itself, and independently of all other tissues: some contending that nerve is necessary to confer contractility on muscle,—to charge it, as it were, with this property; others, that nerve is only necessary to call it into action; and others, that the property is the essential attribute of the tissue, and totally independent of all nerves. The time is past when the intricacies of this Q
    PASSIVE AND ACTIVE CONTRACTION. keen contest can be threaded with any benefit to the student, and we therefore refrain from attempting to follow them. We shall prefer offering him a view of the facts of the subject, as at present known, drawing our conclusions as they arise. The contractility of muscle is exhibited in two varieties of contraction, passive and active. Passive contraction is that which every muscle is continually prone to undergo, by the mere quality of its tissue, as long as it remains in its natural situation in the body. The muscles are ever kept on the stretch by the nature of their position and attachments, and cannot have their ends so approximated, by attitude or otherwise, as that their tendency to shorten themselves shall cease. If, for example, the rectus muscle of the thigh have its extremities brought as near together as can be effected artificially by posture, they would yet be found to approach still nearer on being freed from their attachment to the bones. The stimulus to this contraction may be therefore considered to be that of extension. In fractures and dislocations attended with shortening of the limb, the muscles adapt themselves permanently to their shortened state by virtue of this property. This tendency to contract has been distinguished by the term retractility, ftom its being manifested by the retraction that occurs when the belly of a muscle is cut across. But, in this instance, the retraction would appear to be in part caused by an active contraction excited by the stimulus of the injury. It has been also styled tonicity. It is well exemplified in all those contracted states of muscles which follow paralysis of their antagonists, as when the features are drawn towards the healthy side in hemiplegia. The passive contraction of muscles is continually opposed to their elongation by the active or passive contraction of antagonists, and restores them when that subsides. By it they are accommodated to an attitude artificially given, when no muscular effort is required to maintain it. When no active contraction is present in a limb, the passive contraction remains; and being brought to a state of equilibrium in all the muscles, by their mutual antagonism, the limb is said to be at rest. This is the general condition during sleep, in which the posture assumed by the limbs is determined by the relative power of antagonist muscles: the flexors overcome the extensors, and hence the limbs are bent. Active contraction is attended with those striking manifestations of power that specially characterise muscle. It is always excited by a local or partial stimulus, and is always exerted in opposition to another force within the body, which it is able more or less completely to master. The opposing force is generally the passive contraction of antagonist muscles, as well as the weight or resistance of some part upon which the muscle acts directly; but it may be the elasticity of parts, or, in the case of hollow muscles, the resistance of their own contents. Active contractions are also frequently opposed to one another in the maintenance of a fixed posture. Active contraction is partial and interrupted, both in extent and duration. It requires intervals of rest, being attended with exhaustion of the power which produces it;