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Fig. 1. Articular Face of a Straight Muscular Articulation ; m, muscular fossæ ; il, interarticular ligament fossæ ; di, dorsal ligament fossæ.
Fig. 2. Dorsal External View of a Straight Muscular Articulation between two Ossicles
Fig. 3. Two straight Muscular Articulations revolved through an Angle of 90°.
Fig. 4. The same, superposed so that their central Canals Coincide.
Fig. 7. Articular Face of an Oblique Muscular Articulation; m, muscular fossa ; ps, pinnule socket; il, interarticular ligament fossa ; di, dorsal ligament fossa.
bound together with ligament fibers similar to those of the dorsal ligament of muscular articulations; in synarthries (Fig. 5) these fibers are segregated into two large bundles lateral in position, separated by a dorsoventral ridge across the joint face; in syzygies (Fig. 11) the fibers are scattered over the whole joint surface, which is broken up into alternating ridges and furrows radiating outward from the central canal, which may, in certain of the more specialized forms, such as the Pentacrinitidæ, become obsolete except about the periphery of the joint face.
Muscular articulations are often doubled, thus forming an axillary from which two arms arise; this never happens in the case of non-muscular articulations; moreover, muscular articulations are primarily pinnulate, the pinnule arising from a pinnule socket in the proximal outer part of one of the muscular fossæ. The difference between straight and oblique muscular articulations was originally a difference in pinnulation. In the most primitive type of crinoid arm found among the recent forms, occurring in the family Pentametrocrinidæ, we find the following sequence of articulations: (1) straight muscular, uniting the radial to the first post-radial joint, (2) synarthry, uniting the first two post-radial joints, (3) oblique muscular, uniting the second and third post-radial joints; all the succeeding articulations are oblique muscular, except for the interpolation of occasional syzygies. The first oblique muscular articulation bears the first pinnule; the addition of a pinnule socket on one side of the joint face causes a certain amount of crowding, and a conse
Fig. 8. External Dorsal View of three oblique Muscular Articulations.
Fig. 9. Two Oblique Muscular Articulations revolved through an angle of 90°.
Fig. 10. The same, superposed so that their central Canals Coincide.
Fig. 12. External Dorsal View of a Syzygy between two Ossicles; note how its course across the arm is the mean of the course of the preceding and succeeding oblique muscular articulations.
Fig. 13. A Biserial Crinoid Arm.
FIG. 14. A Monoserial Crinoid Arm, showing the triangular joints, the indices of an ancestral biserial arrangement.
quent depression dorsally of that half of the transverse ridge, which is compensated by a ventral depression of the opposite half; this alteration in one half of the transverse ridge is a necessary consequent of any alteration in direction of the other half, for the transverse ridge is the fulcrum upon which the bending of the arm occurs, and the fulcrum must always be a straight line to admit of any motion at all. Although primarily pinnulate, in certain rare cases oblique muscular articulations are sometimes found non-pinnulate, as in Atelecrinus, Hypalometra, Cyllometra, and Perometra; but this is a purely secondary condition, and one peculiarly prone to reversion, showing it to be somewhat unstable. In all cases, the position of the first oblique muscular articulation is the second articulation beyond the last straight muscular articulation of the arm. The oblique muscular articulations always alternate in the position of their diagonal transverse ridges, and the transverse ridges of succeeding joints form angles of approximately 90° with each other; therefore, a single brachial has proximally an articular face with the transverse ridge from a left ventro-lateral to a right dorso-lateral point, and distally an articular face with the transverse ridge running from a left dorso-lateral to a right ventro-lateral position. The pinnule socket always occurs on the side on which the end of the transverse ridge is dorso-lateral in position; hence, pinnules occur on alternate sides of the arm at succeeding articulations. In reality, of course, the alternation of the pinnules is the fundamental cause of the alternation in the direction of the transverse ridge, but, from the absence of pinnules on oblique muscular articulations in certain recent types, it is more convenient to speak of it as if the reverse were the case.
Non-muscular articulations are never doubled, are never pinnulate, and moreover, never affect the pinnulation in any way; the pinnule on the next succeeding muscular articulation is on the opposite side from that of the preceding muscular articulation, just as if the non-mus
cular articulation were not there, but the two joints connected by it merely a single joint.
Of the non-muscular articulations, the synarthry occurs in the proximal part of the arm, the last synarthry immediately preceding the first oblique muscular, and immediately succeeding the last straight muscular articulation; all the non-muscular articulations succeeding the first oblique muscular articulation are always syzygies. In the simple arms of the Pentametrocrinidæ we find a straight muscular articulation, a synarthry, and then a series of oblique muscular articulations, interspersed with occasional syzygies. In this family the first brachial immediately follows the radial; but in all the other comatulids and in the recent species of the Pentacrinitidæ (except in the genus Metacrinus) the first brachial is separated from the radial by one or more interpolated division series, each composed of a reduplication of the first two brachials interpolated between the primitive first brachial and the radial. In these, however, the structure is the same; a series of synarthries alternating with straight muscular articulations occurs up to the first oblique muscular articulation, beyond which are found only oblique muscular articulations and syzygies. This is the primitive arrangement of the comatulid and pentacrinite arm, no matter how many times division may occur; but in certain specialized types, as Endoxocrinus and the Zygometridæ, one or more of the synarthries may be secondarily replaced by syzygies.
From the above discussion it is evident that (1) nonmuscular articulations are morphologically radically different from muscular articulations; and (2) that there is a distinct interrelation between the two types of muscular articulations and the two types of non-muscular articulations; that is, that proximal to the first oblique muscular articulation only straight muscular articulations and synarthries are found, while distal to the first oblique muscular articulation occur only oblique muscular articulations and syzygies; moreover, the synarthries always alternate with the straight muscular articulations, while the occurrence of the syzygies is more or less, and often very, irregular.
Bearing these facts in mind, we are able to reach a definite concept of the morphological significance of the synarthries and syzygies, in terms of straight and oblique muscular articulations. We have seen that the transverse ridges of succeeding oblique muscular articulations are always approximately at right angles to each other, and we may from this infer a fundamentally alternate position in all muscular articulations. The first articulation, uniting the radial to the first post-radial joint is straight muscular, with the transverse ridge at right angles to the dorso-ventral axis of the joint faces; according to what we found to be the case in oblique muscular articulations, the next articulation should be straight muscular, with the transverse ridge at right angles to that of the first, or coinciding with the dorso-ventral axis; but such an arrangement would leave the muscles and the interarticular ligaments on one side of the arm, and the dorsal ligament on the other, which would be manifestly absurd; but we actually find a transverse ridge running along the dorso-ventral axis of the joint face, with on either side of it a dorsal ligament bundle, in every way the same as the dorsal ligament bundle of the preceding straight muscular articulation. The synarthry, then, appears to consist fundamentally of the dorsal ligaments of two straight muscular articulations, abutting upon a common transverse ridge, which is at right angles to the transverse ridge of the preceding straight muscular articulation (Figs. 3 and 4). Not only does the microscopical comparison of the two individual muscle bundles of the synarthry with the dorsal ligament bundle of the straight muscular articulation bear out this interpretation of the origin of the synarthry, but the morphological effect of the synarthy upon the arm structure is at once explained. Non-muscular articulations never bear pin