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animals to apply their travelling surfaces to the media on which they are destined to operate at any degree of obliquity so as to obtain a maximum of support or propulsion with a minimum of slip. If the travelling surfaces of animals did not form screws structurally aud functionally, they could neither seize nor let go the fulcra on which they act with the requisite rapidity to secure speed, particularly in water and air.

Ligaments.—The office of the ligaments with respect to locomotion, is to restrict the degree of flexion, extension, and other motions of the limbs within definite limits.

Effect of Atmospheric pressure on Limbs.—The influence of atmospheric pressure in supporting the limbs was first noticed by Dr. Arnott, though it has been erroneously ascribed by Professor Müller to Weber. Subsequent experiments made by Dr. Todd, Mr. Wormald, and others, have fully established the mechanical influence of the air in keeping the mechanism of the joints together. The amount of atmospheric pressure on any joint depends upon the area or surface presented to its influence, and the height of the barometer. According to Weber, the atmospheric pressure on the hip-joint of a man is about 26 lbs. The pressure on the knee-joint is estimated by Dr. Arnott at 60 lbs."1

Active organs of Locomotion. Muscles, their Properties, Arrangement, Mode of Action, etc.—If time and space had permitted, I would have considered it my duty to describe, more or less fully, the muscular arrangements of all the animals whose movements I propose to analyse. This is the more desirable, as the movements exhibited by animals of the higher types are directly referable to changes occurring in their muscular system. As, however, I could not hope to overtake this task within the limits prescribed for the present work, I shall content myself by merely stating the properties of muscles; the manner in which muscles act; and the manner in which they are grouped, with a view to moving the osseous levers which constitute the bony framework or skeleton of the animals to be considered. Hitherto, and by common consent, it has been believed that whereas a flexor muscle is situated on one aspect of a limb, and its correspond

1. Bishop, op. cit.

ing extensor on the other aspect, these two muscles must be opposed to and antagonize each other. This belief is founded on what I regard as an erroneous assumption, viz., that muscles have only the power of shortening, and that when one muscle, say the flexor, shortens, it must drag out and forcibly elongate the corresponding extensor, and the converse. This would be a mere waste of power. Nature never works against herself. There are good grounds for believing, as I have stated elsewhere, that there is no such thing as antagon

FIG. 5. Shows the muscular cycle formed by the biceps (a) or flexor muscle,

and the triceps (b) or extensor muscle of the human arm. At i the centripetal or shortening action of the biceps is seen, and at the centrifugal or elongating action of the triceps (vide arrows). The present figure represents the forearm as flexed upon the arm. As a consequence, the long axes of the sarcous elements or ultimate particles of the biceps (i) are arranged in a more or less horizontal direction; the long axes of the sarcous elements of the triceps (1) being arranged in a nearly vertical direction. When the forearm is extended, the long axes of the sarcous elements of the biceps and triceps are reversed. The present figure shows how the bones of the extremities form levers, and how they are moved by muscular action. If, e.g., the biceps (a) shortens and the triceps (6) elongates, they cause the forearin and hand (h) to move towards the shoulder (d). If, on the other hand, the triceps (b) shortens and the biceps (a) elongates, they cause the forearm and hand (h) to move away from the shoulder. In these actions the biceps (a) and triceps (b) are the power; the elbow-joint (g) the fulcrum, and the forearm and hand (h) the weight to be elevated or depressed. If the hand represented a travelling surface which operated on the earth, the water, or the air, it is not difficult to understand how, when it was made to move by the action of the muscles of the arm, it would in turn move the body to which it belonged. d Coracoid process of the scapula, from which the internal or short head of the biceps (a) arises. e Insertion of the biceps into the radius. f Long head of the triceps (6). 9 Insertion of the triceps into the olecranon process of the ulna.-Original.

ism in muscular movements; the several muscles known as flexors and extensors; abductors and adductors; pronators and supinators, being simply correlated. Muscles, when they

1 Lectures on the Physiology of the Circulation in Plants, in the Lower Animals, and in Man.”— Edinburgh Medical Journal for January and February 1873.

act, operate upon bones or something extraneous to themselves, and not upon each other. The muscles are folded round the extremities and trunks of animals with a view to operating in masses. For this purpose they are arranged in cycles, there being what are equivalent to extensor and flexor cycles, abductor and adductor cycles, and pronator and supinator cycles. Within these muscular cycles the bones, or extraneous substances to be moved, are placed, and when one side of a cycle shortens, the other side elongates. Muscles are therefore endowed with a centripetal and centrifugal action. These cycles are placed at every degree of obliquity and even at right angles to each other, but they are so disposed in the bodies and limbs of animals that they always operate consentaneously and in harmony. Vide fig. 5, p. 25.

There are in animals very few simple movements, i.e. movements occurring in one plane and produced by the action of two muscles. Locomotion is for the most part produced by the consentaneous action of a great number of muscles; these or their fibres pursuing a variety of directions. This is particularly true of the movements of the extremities in walking, swimming, and flying

Muscles are divided into the voluntary, the involuntary, and the mixed, according as the will of the animal can wholly, partly, or in no way control their movements. The voluntary muscles are principally concerned in the locomotion of animals. They are the power which moves the several orders of levers into which the skeleton of an animal resolves itself.

The movements of the voluntary and involuntary muscles are essentially wave-like in character, i.e. they spread from certain centres, according to a fixed order, and in given directions. In the extremities of animals the centripetal or converging muscular wave on one side of the bone to be moved, is accompanied by a corresponding centrifugal or diverging wave on the other side; the bone or bones by this arrangement being perfectly under control and moved to a hair's-breadth. The centripetal or converging, and the centrifugal or diverging waves of force are, as already indicated, correlated.1 Similar remarks may be made regarding the different parts of the body

i Muscles virtually possess a pulling and pushing power; the pushing

of the serpent when creeping, of the body of the fish when swimming, of the wing of the bird when flying, and of our own extremities when walking. In all those cases the moving parts are thrown into curves or waves definitely correlated.

It may be broadly stated, that in every case locomotion is the result of the opening and closing of opposite sides of muscular cycles. By the closing or shortening, say of the flexor halves of the cycles, and the opening or elongation of the extensor halves, the angles formed by the osseous levers are diminished; by the closing or shortening of the extensor halves of the cycles, and the opening or elongation of the flexor halves, the angles formed by the osseous levers are increased. This alternate diminution and increase of the angles formed by the osseous levers produce the movements of walking, swimming, and flying. The muscular cycles of the trunk and extremities are so disposed with regard to the bones or osseous levers, that they in every case produce a maximum result with a minimum of power. The origins and insertions of the muscles, the direction of the muscles and the distribution of the muscular fibres insure, that if power is lost in moving a lever, speed is gained, there being an apparent but never a real loss. The variety and extent of movement is secured by the obliquity of the muscular fibres to their tendons; by the obliquity of the tendons to the bones they are to move; and by the proximity of the attachment of the muscles to the several joints. As muscles are capable of shortening and elongating nearly a fourth of their length, they readily produce the precise kind and degree of motion required in any particular case.

The force of muscles, according to the experiments of Schwann, increases with their length, and vice versa. It is a curious circumstance, and worthy the attention of those interested in homologies, that the voluntary muscles of the power being feeble and obscured by the flaccidity of the muscular mass. In order to push effectually, the pushing substance must be more or less rigid.

1 The extensor muscles preponderate in mass and weight over the flexors, but this is readily accounted for by the fact, that the extensors, when limbs are to be straightened, always work at a mechanical disadvantage. This is owing to the shape of the bones, the conformation of the joints, and the position occupied by the extensors.

superior and inferior extremities, and more especially of the trunk, are arranged in longitudinal, transverse, and oblique spiral lines, and in layers or strata precisely as in the ventricles of the heart and hollow muscles generally. If, consequently, I eliminate the element of bone from these several regions, I reproduce a typical hollow muscle; and what is still more remarkable, if I compare the bones removed (say the bones of the anterior extremity of a quadruped or bird) with the cast obtained from the cavity of a hollow muscle (say the left ventricle of the heart of the mammal), I find that the bones and the cast are twisted upon themselves, and form elegant screws, the threads or ridges of which run in the same direction. This affords a proof that the involuntary hollow muscles supply the type or

hollow muscle Toth the cast obtained extremity of a

FIG. 6.-Wing of bird. Shows how the bones of the arm (a), forearm (h), and

hand (c), are twisted, and form a conical screw. Compare with Figs. 7
and 8.-Original.

Fig. 7.

Fig. 8.
Fig. 7. -Anterior extremity of elephant. Shows how the bones of the arm (a).

forearm (q's), and foot (0), are twisted to form an osseous screw. Compare

with Figs. 6 and 8.-Original. FIG. 8. -Cast or mould of the interior of the left ventricle of the heart of a deer. Shows that the left ventricular cavity is conical and spiral in its nature. a Portion of right ventricular cavity ; b, base of left ventricular cavity ; x, y, spiral grooves occupied by the spiral musculi papillares ; j 9, spiral ridges projecting between the musculi papillares. Compare with Figs.

6 and 7.-Original. pattern on which the voluntary muscles are formed. Fig. 6 represents the bones of the wing of the bird; fig. 7 the bones of the

1 “On the Arrangement of the Muscular Fibres in the Ventricles of the Vertebrate Heart, with Physiological Remarks,” by the Author.-Philo. sophical Transactions, 1864.

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