Interconnections of muscles in the adductor mandibulae complex of birds

In addition to a variety of connections at or near the attachment of muscles, some of the adjacent bellies in the adductor mandibulae complex of birds merge with one another across the whole or most of their contiguous sides. Such continuous intermuscular transitions are found between the adductor posterior and the deep part of the adductor externus, the posterior adductor and quadratomandibularis muscle, the adductor posterior and pterygoideus muscle and between the quadratomandibularis and pseudotemporalis muscles. Since a complete breakup of the adductor premuscle mass occurs early in development and there is no indication of adult intermuscular transitions in any of the studied avian embryos, the muscles must coalesce at some stage of growth. Therefore, the intermuscular transitions of adult birds cannot be derived with any certainty from the primary connections in the adductor mandibulae of early gnathostomes. The coalescence of developmentally discrete muscles is invariably seamless and appears to be contingent not only upon the contiguity of muscles but also upon the congruent arrangement of their fibers.