The Brachiæus Internus is a small fleshy Muscle, lying hid under the Biceps, which takes its rise in the upper and fore-part of the Humerus, and is implanted in the upper and inner-part of the Radius, to bend the Elbow with the Biceps.
The first of the four Extenders is the Longus having two Sources, viz. one situated in the lower Rib of the Omoplata, near its Neck, and the other descending to the hinder-part of the Arm, till it is tyed to the Olecranum or Ancon, by a strong Aponeurosis, which is common thereto, with the Brevis, and the Brachiæus Externus.
The Brevis or short Muscle of the Elbow arising from the hinder and upper-part of the Humerus, is fasten'd to the Olecranum with the Longus.
The Brachiæus Externus is a fleshy Muscle which proceeds from the hinder part of the
Humerus, and adheres to the Olecranum with the Brevis and the Longus.
The Anconeus or Cubitalis being situated behind the Fold of the Cubitus, is the least Muscle of all; it springs from the Extremity of the Arm-Bone, at the end of the Brevis and the Longus, and in descending is inserted between the Radius and the Cubitus or Ulna, three or four Fingers breadth below the Olecranum.
How many Muscles hath the Radius, and which are its Motions?
The Radius is endu'd with a twofold Motion by the means of four Muscles: Of these the Rotundus and Quadratus cause that of Pronation, as the Longus and the Brevis that of Supination.
The Pronator Superior Rotundus, or round Muscle of the Radius, commenceth from the inner Apophysis of the Shoulder-Bone, in a very fleshy Stock, and is terminated obliquely by a Membranous Tendon in the middle and exterior part of the Radius.
The Pronator Inferior Quadratus, springing forth from the bottom and inside of the Cubitus, is fixt in the lower and outward part of the Radius by a Tail as large as its Head. This Muscle lying hid under the others near the Wrist, is that which jointly with the Rotundus, turns the Arm with the Palm of the Hand downward, which is the Motion of Pronation.