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I think it does not vary the case, that its oblique motions and inclinations are often motions in a diagonal, produced by the joint action of muscles lying in ftraight directions. But, whether the pull be fingle or combined, the articulation is always fuch, as to be capa ble of obeying the action of the muscles. The oblique muscles attached to the head, are likewife fo difpofed, as to be capable of steadying the globe, as well as of moving it. The head of a new-born infant is often obliged to be filleted up. After death the head drops, and rolls in every direction. So that it is by the equilibre of the mufcles, by the aid of a confiderable and equipollent mufcular force in conftant exertion, that the head maintains its erect pofture. The mufcles here fupply, what would otherwise be a great defect in the articulation for the joint in the neck, although admirably adapted to the motion of the head, is infufficient for its fupport. It is not only by the means of a moft curious ftructure of the bones that a man turns his head, but by virtue of an adjusted muscular power, that he even holds it

:

up.

As another example of what we are illuftrating, viz. conformity of ufe between the

bones

bones and the mufcles, it has been obferved of the different vertebræ, that their proceffes are exactly proportioned to the quantity of motion which the other bones allow of, and which the respective muscles are capable of producing.

II. A mufcle acts only by contraction. Its force is exerted in no other way. When the exertion ceafes it relaxes itself, that is, it returns by relaxation to its former ftate; but without energy. This is the nature of the mufcular fibre: and being fo, it is evident that the reciprocal energetic motion of the limbs, by which we mean motion with force in opposite directions, can only be produced by the inftrumentality of opposite or antagonist muscles; of flexors and extenfors answering to each other. For instance, the biceps and brachiæus internus muscles placed in the front part of the upper arm, by their contraction, bend the elbow; and with fuch degree of force, as the case requires, or the ftrength admits of The relaxation of thefe muscles, after the effort, would merely let the fore arm drop down. For the back froke therefore; and that the arm may not only bend at the elbow, but also extend and ftraighten itself with force, other muscles, the longus and brevis

K4

brevis brachiæus externus, and the anconæus, placed on the hinder part of the arm, by their contractile twitch fetch back the fore arm into a ftraight line with the cubit, with no less force than that with which it was bent out of

it.

A

They act like two oppofite pull and

The fame thing obtains in all the limbs, and in every moveable part of the body. finger is not bent and straightened, without the contraction of two mufcles taking place. It is evident therefore that the animal functions require that particular difpofition of the muscles which we defcribe by the name of antagonist muscles. And they are accordingly fo difpofed. Every muscle is provided with an adverfary. fawers in a pit by an nothing furely can more ftrongly indicate defign and attention to an end than their being thus ftationed; than this collocation. The nature of the muscular fibre being what it is, the purposes of the animal could be answered by no other. And not only the capacity for motion, but the afpect and fymmetry of the body is preserved by the muscles being marfhalled according to this order, e. g. the mouth is held in the middle of the face, and its angles kept in a state of exact correspond

ency,

ency, by two muscles drawing against, and balancing, each other. In a hemiplegia, when the muscle on one fide is weakened, the mufcle on the other fide draws the mouth awry.

III. Another property of the muscles, which could only be the refult of care, is their being almost universally fo difpofed, as not to obftruct or interfere with one another's action. I know but one inftance in which this impediment is perceived. We cannot easily swallow whilft we gape. This, I understand, is owing to the muscles employed in the act of deglutition being fo implicated with the mufcles of the lower jaw, that, whilst these last are contracted, the former cannot act with freedom. The obstruction is, in this inftance, attended with little inconveniency; but it shews what the effect is, where it does exift; and what loss of faculty there would be, if it were more frequent. Now when we reflect the number of muscles, not fewer than four hundred and forty-fix in the human body, known and named*, how contiguous they lie to each other, in layers, as it were, over one another, croffing one another, fometimes em

upon

* Keill's Anat. p. 295, ed. 3d.

bedded

bedded in one another, fometimes perforating one another, an arrangement, which leaves to each its liberty and its full play, muft neceffarily require meditation and counsel,

IV. The following is oftentimes the cafe with the muscles. Their action is wanted where their fituation would be inconvenient. In which cafe the body of the muscle is placed in fome commodious pofition at a distance, and made to communicate with the point of action, by flender ftrings or wires. If the muscles, which move the fingers, had been placed in the palm or back of the hand, they would have fwelled that part to an awkward and clumfy thickness. The beauty, the proportions, of the part, would have been deftroyed. They are therefore difpofed in the arm, and even up to the elbow; and act by long tendons, ftrapped down at the wrist, and paffing under the ligament to the fingers, and to the joints of the fingers, which they are feverally to move. In like manner, the mufcles which move the toes, and many of the joints of the foot, how gracefully are they difpofed in the calf of the leg, inftead of forming an unwieldy tumefaction in the foot itfelf? The obfervation may be repeated of the mufcle which draws

the

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