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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124. No. 2, June 2012 
Falco, Ardeidae. Cochleariidae, Cuculidae, Up- 
upa. Derulrocopos, Coracias , and all passerine 
birds ilius far studied (Hudson 1948, George and 
Berger 1966. Vanden Berge 1970, Raikow 1987). 
Unlike the typical structure in galliform birds, the 
vinculum of Chinese Grouse is present as a small, 
single tendon sent from the tendon of M. flexor 
perforans digiti 111 to that of M. flexor perforans 
et perforatus digiti Ill. This kind of weak 
vinculum is probably of little functional signifi¬ 
cance and. suggests greater independent action of 
these muscles, permitting more efficient and 
subtle bending of the segments of the third toe 
while perching and moving in trees. 
A second feature is the modification of the deep 
plantar vinculum between M. flexor digitorum 
longus and M. flexor hallus longus. Birds show a 
bewildering variety of different structural rela¬ 
tionships between these two deep plantar tendons 
from complete separation to virtual separation 
except for a simple vinculum, through a variety of 
degrees and patterns oI fusion and interconnection 
in association with different functional and 
adaptive specializations of the feet (Raikow 
1987). A vinculum is present between the 
insertion tendons of M. flexor digitorum longus 
and M flexor hallus longus in Chinese Grouse, 
but is a small tendon sent from M. flexor hallus 
longus, structurally different from the typical 
vinculum in other galliform birds. This kind of 
connecting structure, like that of the M. flexor 
perforans digiti III and M. flexor perforans et 
perforatus digiti III. is of little functional signif¬ 
icance and might be correlated with greater digital 
dexterity and independence needed in arboreal 
locomotion (Owre 1967). 
A third feature involves the function of M. 
flexor perforans digiti II. The typical insertion 
of this muscle is on the plantar surface of the 
proximal phalanx of digit II and Ilexes the entire 
digit at its articulation (George and Berger 1966. 
Raikow 1985). However, this muscle in Chinese 
Grouse functions as both the flexor and adductor 
of the second toe by inserting mainly on the 
proximolateral corner of phalanx I. The absence 
of M. adductor digiti II in Chinese Grouse is 
lunctionally compensated by M. flexor perforans 
digiti II. The adduction of the second toe is 
important tor Chinese Grouse to move in trees, 
especially on thin branches. It has been reported 
the middle toe is held on top of the curved surface 
°. ,he hriinch antl '*10 second and fourth toes fall 
slightly below and lightly support the foot from 
both sides when moving on a thin branch 
(Kuz’mina 1992). 
A fourth feature is the augmentation of the 
flexion and adduction of digit IV. M. flexor 
perforans digiti IV arises by three heads in 
galliform birds, the middle head being mainly 
tendinous in most forms (Hudson et al. 1959). The 
relatively better development of this muscle in 
Chinese Grouse, indicated by a more fleshy middle 
head, suggests a greater bending force of this toe 
than other gallifomts. The bipennatc M. extensor 
brevis digiti IV acts more as an adductor than as an 
extensor by inserting on the medial surface of the 
proximal phalanx. Furthermore, it is the only 
intrinsic foot muscle possessing an ossified tendon. 
Ossified tendons are denser and stiffer than 
unossified tendons, and experience less deforma¬ 
tion under a given load (Bennett and Stafford 1988, 
Bledsoe et al. 1993, Landis and Silver 2002). The 
functional role of ossified tendons in birds is 
obscure: possibly they serve in some way to 
reinforce the action of the muscle, as well as to 
accurately control the positions of the digits (Bock 
and von Wahlert 1965, Vanden Beige 1970. 
Bennett and Stafford 1988, Bledsoe et al. 1993). 
In this ease, it facilitates the adduction of the toe 
related to grasping thin branches tightly from the 
lateral side when moving in trees. 
ACKNOWLEDGMENTS 
We are exiremely grateful to R. L. Zusi for reading the 
manuscript and providing useful suggestions and comments: 
M. D. Spit/cr. H. F. James, and C. M. Milensky assisted with 
many aspects. We also thank C. E. Braun and an anonymous 
reviewer for their constructive comments and useful edits 
This work was supported by the National Science Foundation 
of China (30870263, 31071931), and Funding Project for 
Academic Human Resources Development in Institutions nl 
Higher Learning Under the Jurisdiction of Beijing Munic¬ 
ipality (Grant No. PHR201107120). 
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Baumel j. J.. A. S. King, J. e. Breazile. H. E. Evans, 
and J. C. Vanden Berge. 1993. Handbook of avian 
anatomy: Nomina Anatomiea Avium. Second Edition 
Number 23. Publications of the Nutlall Ornithological 
Club. Cambridge, Massachusetts. USA. 
Ben-nett, M. B. and J. A. Stafford. 1988. Tensile 
properties of calcified and uncalcified avian tendon' 
Journal of Zoology 214:343-351. 
Berger, a. j. 1956. The appendicular myology of iht 
Sandhill Crane, with comparative remarks on the 
Whooping Crane. Wilson Bulletin 68:282-304. 
Bledsoe. A. H.. R. J. Raikow, and A. G. Glasgow. 1995. 
Evolution and functional significance of tendon 
