Proceedings of 

 the United States 

 National Museum 



SMITHSONIAN INSTITUTION • WASHINGTON, D.C. 



Volume 123 1967 Number 3607 



The Role of the Depressor Mandibulae Muscle 

 in Kinesis of the Avian Skull 



By Richard Zusi 



Associate Curator, Division of Birds 



Introduction 



Basic structures of the facial and palatal portions of the bird's skull 

 are understandable only in terms of the kinetic property of the skull — 

 the ability to move the upper jaw with respect to the braincase. 

 Although avian kinesis was known to 18th-century anatomists and 

 has been analyzed variously in terms of muscles, ligaments, and 

 adaptive modifications, the important complex of structures consti- 

 tuting the jaw-quadrate linkage has not been studied sufficiently in 

 any species to permit a detailed evaluation of the actions of the major 

 jaw muscles. 



My purpose here is to examine a single muscle, M. depressor mandib- 

 ulae, to determine the effects upon jaw action of variations in muscle 

 configuration and variations in the jaw articulation through which 

 the muscle acts. (See fig. 1 for structures under discussion in this 

 paper.) My methods include experiments based on stimulation of 

 muscles in live birds, construction of models of the muscle-ligament 

 sj'stem, and manipulation of fresh and preserved specimens. The 



1 



