The Longus is the first of the Supinators, whose Origine is three or four Fingers breadth above the external Apophysis of the Arm-Bone; from whence it passeth along the Radius, and cleaves to the inner-part of its lower Apophysis.
The Brevis, or the second of the Spinators arising from the lower part of the Inferior Condylus, and the external of the Humerus, is twisted round about the Radius, going forward from the hinder-part till it is united to its upper and forepart. This Muscle, with the Longus, serves to turn the Arm and the Palm of the Hand upward, and produceth the Motion of Supination.
How many sorts of Motions belong to the Wrist, and what are its Muscles?
Two several Motions are perform'd by the Wrist, viz. one of Flection, and the other of Extension, three Muscles being appropriated to the former, and as many to the later: But it ought to be observed, that a strong Ligament, call'd the Annular, appears here, which, surrounding all the Tendons of the Muscles as it were a Bracelet, holds them together, and elsewhere serves to unite the two Bones of the Elbow. The three Flexors or Bending Muscles of the Wrist are the Cubitæus Internus, the Radiæus Internus, and the Palmaris.
The Cubitæus Internus derives its Original from the part of the Arm-Bone, passeth under the Annular Ligament, and is ty'd by a thick Tendon to the small Bone of the Wrist, which is plac'd above the others.
The Radiæus Internus proceeds from the same place with the Cubitæus, and is fasten'd to the first Wrist-Bone which supports the Thumb. It lies along the Radius, and passeth under the Annular Ligament.
The Palmaris is reckon'd among the Flexors of the Wrist, although situated in the Palm of the Hand. It ariseth from the inner Process or Knob
of the Arm-Bone, and is united by a large Tendon to the first Phalanges of the Fingers, slipping under the Transverse or Annular Ligament and sticking under the Skin of the Palm of the Hand.
The three extending Muscles of the Wrist are the Cubitæus Externus, and the Radiæus Externus or the Longus, and the Brevis.