Lectures on Human Myology

are occasionally aided by derivatives from those upon the dorsal aspect, which incline to the palmar or plantar surface, and so acquire a flexor action. The movements effected by the two sets of muscles, though antagonistic, are similar; and there is, accordingly, a general similarity in the arrangement of the flexor and extensor muscles. Each set consists, for the most part, primarily, of intrinsic fibres-that is, of fibres passing from the segment of the limb next above the joint to the segment next below it; secondarily, of the fibres from a higher segment, which, in the upper region of the limb, are often members of the deep ventro-appendicular or girdle-sheath ; and, thirdly, of extensions of the superficial ventro-appendicular sheath, which, as before said, often clothe the whole, or nearly the whole, surface of the limb. There are thus usually three strata more or less blended together ; and the dorsal or extensor divisions, and the palmar and plantar, or flexor divisions, of the three strata are, in each instance respectively, most closely associated or blended. Still, as just intimated, members of the dorsal division of a stratum are not unfrequently blended with the palmar or the plantar division of its own or of another stratum ; and we shall also find instances, though they are not so common, of the converse-that is, in which members of the palmar or plantar division pass upon and blend with the dorsal muscles, and associate themselves with them in their action. It will be borne in mind that in the primitive limb-that, for instance, of Lepidosiren-all the strata are blended in an uniform, or rather an undivided, muscular investment, extended over the whole limb. With regard to the flexors of the fore-arm, the examination of various animals shows that the coraco-brachials, the biceps, and the brachialis anticus, form one continuous series, composed of deep ventro-appendicular and intrinsic fibres, extending, from the coracoid, upon the humerus and the upper part of the radius and ulna. They are covered to greater or less extent by, and in some instances are blended with, the pectorals, or palmar sector of the superficial ventro-appendicular sheet; and, in their turn, they extend upon, and to some extent are blended with, the flexors of the wrist and fingers. In Man, they are segmented into the three muscles-the coraco-brachialis, the biceps, and the brachialis anticus-acting respectively upon the humerus, the radius, and the ulna. Still, even in Man, the one, or coracoid head of the biceps, continues to be united with the coraco-brachialis ; and the other, or glenoid head, arises in great measure from the base of the coracoid, where that bone contributes to the glenoid cavity. Hence the biceps remains essentially a coraco-brachial muscle; and it partly expands into the fascia of the fore-arm. Moreover, the brachialis anticus, the representative of the intrinsic stratum, though usually separated in great measure from the others in Man, yet retains some connection, either directly or through the internal intermuscular septum, with the coraco-brachialis; and it sometimes retains also a connection with the biceps, by means of slips detached from the ulnar side of the huumerus, which constitute what are called the " humeral origins" of the biceps. The fact of the biceps becoming, by virtue of its insertion into the tubercle of the radius, a powerful supinator, renders it a good example of somewhat opposite or antagonistic movements being performed by the same muscle, and of that nice balancing of antagonistic actions by virtue of which the steady movements of the limbs