4 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



direct attachment or indirectly by spanning its cranial and man- 

 dibular articulations. Motion of the quadrates has a direct effect on 

 motion of the upper jaw and on the lower jaw as well; motion of the 

 lower jaw may in turn induce shifting of the quadrates through 

 features of the jaw articulation and ligaments. Understanding the 

 functional properties of the complex kinetic mechanism is thus very 

 difficult, but it is essential for the interpretation of variation of these 

 structures in birds. 



The depressor mandibulae has considerable potential for evolu- 

 tionary development of functional-anatomical variation. Because of 

 its superficial position, its origin may expand posteriorly over the 

 neck muscles or dorsally and anteriorly over the skull and adductor 

 muscles providing variation in both size and angle of pull of the muscle 

 in different birds. The muscle may play a role in protraction of the 

 upper jaw as well as in depression of the lower jaw, and therefore, it 

 might be expected to show modifications for feeding methods that 

 require forceful opening of both jaws or the upper jaw alone, or for 

 resisting forces on the bill. 



During the spring of 1963, Ulrich Kalkofen, then a senior honors 

 student under my supervision at the University of Maine, undertook 

 a series of pilot experiments to test hypotheses about the role of the 

 postorbital ligament in kinesis (Kalkofen, 1963). The data obtained 

 form the basis for the following section on jaw action. I wish to thank 

 Paul C. Harris for providing the chickens used for this study and 

 Jon Greenlaw for his help in performing the experiments. Financial 

 assistance was provided by the Coe Research Fund (R 625-49) of 

 the University of Maine. The final organization of the paper bene- 

 fited greatly from the constructive criticisms of George E. Watson 

 and Paul Slud. I am indebted to Walter J. Bock for providing me 

 with a translation of the 1958 paper by Yudin. 



Experiments on Jaw Action 



Methods. — Live birds used for experiment were the evening 

 grosbeak (Hesperiphona vespertina) and the domestic chicken (Gallus 

 domesticus) — hens of parentage female barred Plymouth Rock X 

 male Rhode Island red. These species were especially suitable 

 because they were readily obtainable and easily kept in captivity, 

 and because the postorbital ligament is poorly developed in the 

 grosbeak and strongly developed in the chicken. 



For each species a wooden head support was constructed to conform 

 with the contour of the mid-dorsal surface of the cranium; each 

 support was held rigidly in place by metal clamps that penetrated 

 under the supraorbital rims of the skull. Birds were first weighed 



