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THE AMERICAN NATURALIST [Vol. XLIII 



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 terras of straight and oblique 

 muscular articulations. We have seen that the trans- 

 verse 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 articu- 

 lation, 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 ; accord- 

 ing 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 ivay 

 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 micro- 

 scopical comparison of the two individual muscle bundles 

 of the synarthry with the dorsal ligament bundle of the 

 straight muscular articulation bear out this interpreta- 

 tion 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 pm- 



