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I think it does not vary the cafe, that its oblique motions and inclinations are often motions in a diagonal, produced by the joint action of muscles lying in straight directions. But, whether the pull be single or combined, the articulation is always such, as to be capable of obeying the action of the muscles. The oblique muscles attached to the head, are likewise so disposed, 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 muscles, by the aid of a considerable and equipollent muscular force in .constant exertion, that the head maintains its erect posture. The muscles here supply, 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 insufficient for its support. It is not only by the means of a most curious structure 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 illustrating, viz, conformity of use between the
bones bones and the muscles, it ha* been observed of the different vertebra, that their processes are exactly proportioned to the quantity of motion: which the other bones allow of, and which the respective muscles are capable of producing.
\l. A muscle acts only by Contraction. Its force is exerted in no other way. When the exertion ceases it relaxes itself* that is, it re-< turns by relaxation to its former state j but without energy. This is the nature of the muscular fibre: and being so, 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 instrumentality of opposite or antagonist muscles; of flexors and extensors 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 such degree of force, as the cafe requires, or the strength admits of. The relaxation of these muscles, after the effort, would merely let the fore arm drop down. For the back Jlroke therefore; and that the arm may not only bend at the elbow, but also extend and straighten itself with force, other muscles, the longus and
K 4 brevis brevis brachiseus externus, and the anconæus, placed on the hinder part of the arm, by their contractile twitch fetch back the fore arm into a straight line with the cubit, with no less force than that with which it was bent out of it. The fame thing obtains in all the limbs, and in every moveable part of the body. A finger is not bent and straightened, without the contraction of two muscles taking place. It is evident therefore that the animal functions require that particular disposition of the muscles which we describe by the name of antagonist muscles. And they are accordingly so disposed. Every muscle is provided with an adverfary. They act like two fawers in a pit by an opposite pull: and nothing surely can more strongly indicate design and attention to an end than their being thus stationed; 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 aspect and symmetry of the body is preserved by the muscles being marshalled 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 correspondency,
ency, by two muscles drawing against, and balancing, each other. In a hemiplegia, when the muscle on one side is weakened, the muscle on the other side draws the mouth awry.
III. Another property of the muscles, which could only be the result of care, is their being almost universally so disposed, as not to obstruct or interfere with one another's action. I know but one instance in which this impediment is perceived. We cannot easily swallow whilst we gape. This, I understand, is owing to the muscles employed in the act of deglutition being so implicated with the muscles of the lower jaw, that, whilst these last are contracted, the former cannot act with freedom. The obstruction is, in this instance, attended with little inconveniency; but it shows what the effect is, where it does exist; and what loss of faculty there would be, if it were more frequent. Now when we reflect upon the number of muscles, not fewer than four hundred and forty-six in the human body, known and named*, how contiguous they lie to each other, in layers, as it were, over one another, crossing one another, sometimes em
* Keill'6 Anat. p. 395, ed. 3d.
bedded bedded in one another, sometimes perforating one another, an arrangement, which leaves to each its liberty and its full play, must neceflarily require meditation and counsel.
IV. The following is oftentimes the case with the muscles. Their action is wanted where their situation would be inconvenient. In which case the body of the muscle is placed in some commodious position at a distance, and made to communicate with the point of action, by slender strings or wires. If the muscles, which move the fingers, had been placed in the palm or back of the hand, they would have swelled that part to an awkward and clumsy thickness. The beauty, the proportions, of the part, would have been destroyed. They are therefore disposed in the arm, and even up to the elbow; and act by long tendons, strapped down at the wrist, and passing under the ligament to the fingers, and to the joints of the fingers, which they are severally to move. In like manner, the muscles which move the toes, and many of the joints of the foot, how gracefully are they disposed in the calf of the leg, instead of forming an unwieldy tumefaction in the foot itself? The observation may be repeated of the muscle which draws