Zhang el al. • CHINESE GROUSE MYOLOGY 
335 
medial surface of the hypotarsus. The belly is 
narrow distally and extends one-third the way 
down die bone. The tendon of insertion en- 
sheathes that of M. flexor hallucis longus at the 
base of the hallux, and inserts on the ventral 
surface of the proximal end of phalanx I. 
M. abductor digili 11 (Fig. 6: ABD2) is the 
weakest of the intrinsic foot muscles. It arises 
by fleshy fibers from the medial surface of the 
tarsometaiarsus. The weak belly extends slightly 
less than the distal third of the bone and inserts by 
a short tendon on the ventromedial surface of the 
base of phalanx I of digit 11. 
M. extensor proprius digiti 111 (Fig. 6; EFD3) 
lies between M. extensor brevis digiti IV and M. 
abductor digiti 11. It originates by lleshy fibers 
from the cranial surface of the distal third of the 
larsometatarsus. The short, flat tendon inserts on 
the dorsal surface of the base of the proximal 
phalanx of digit III. 
M. extensor brevis digiti IV (Fig. 6: EBD4) is a 
stout muscle that arises directly from the cranio- 
lateral surface of a little more than the proximal 
half of the tarsometaiarsus. The belly is nearly 
bipennate and narrows distally. The slender 
tendon passes through the foramen vasculare 
distale between the troehleae for digits III and 
IV. and inserts on the medial surface of the base 
°t phalanx I, digit IV. Most of the tendon, 
including the part enclosed by the distal belly, is 
ossified. 
M. abductor digiti IV (Fig. 6: ABD4) is 
unipennate and the strongest among the intrinsic 
loot muscles. It originates directly from the 
caudolateral surface of the larsometatarsus with 
the belly extending almost to the base of trochlea 
IV. The short tendon inserts on the ventrolateral 
comer of the base of phalanx I, digit IV. 
DISCUSSION 
The following muscles are absent in Chinese 
Grouse: M. adductor digili II. M. adductor digiti 
IV. M. extensor brevis digiti 111 and M. lumhri- 
calis. Data for the Chinese Grouse agrees with 
t'lher tetraonids in the absence of M. adductor 
digiti II. the same number of sesamoids, and the 
relatively weak development of the intrinsic foot 
muscles which originate from the larsometatarsus 
and insert on the digits (Hudson et al. 1959). Four 
intrinsic foot muscles are absent in Chinese 
Grouse. The loss of intrinsic fool muscles is 
commonly found in species of small size, such as 
hummingbirds (only M. extensor hallucis longus. 
M. flexor hallucis brevis, and M. adductor digiti II 
are present) (Zusi and Bentz 1984) and more 
highly derived taxa. c.g., Passeriformes, in which 
the intrinsic foot muscles It) the forward toes are 
almost entirely lost (Raikow 1982. 1985). The 
bellies of M. extensor proprius digiti 111 and M. 
abductor digiti II in Chinese Grouse are limited to 
the distal third of the tarsometatarsus. M. extensor 
hallucis longus and M. flexor hallucis brevis are 
confined to the proximal half and proximal third 
of the larsometatarsus. respectively. Similar to 
other tetraonids, the tendency for the short toot 
muscles to be weak and attenuated may be an 
adaptation to the extreme cold climate occurring 
in northern high latitude regions (Hudson et al. 
1959). 
Forest tetraonids, including genera Bonasa and 
Tetrastes, habitually forage in trees and shrubs 
during winter and early spring, and are more 
arboreal than tundra and grassland species. The 
branch to relatively shorter limbs and the broad, 
low sacrum ensures better equilibrium while 
hopping from or moving along branches (Kuz'¬ 
mina 1992). 
The pelvic myology of Chinese Grouse exhibits 
four features related to an arboreal mode of life. 
One feature is the modification of the vinculum 
between the two superficial flexors supplying 
digit III. The third toe has three sets of flexor 
tendons in birds. The deepest, M. tlexor digitorum 
longus inserts mainly on the angular phalanx and 
is important in flexing distal phalanges. The other 
two superficial flexors. M. flexor perforans digiti 
111 and M. flexor perforans et perforates digiti III, 
function as the flexors of subdistul phalanges. 
During perching, when the weight of the bird 
does not necessarily rest uniformly upon all of the 
toes or toe surfaces, independent flexion of 
various subdistal phalanges must be of impor¬ 
tance (Owre 1967). The tendons of M. flexor 
perforans digiti III and M. flexor perforans ct 
perforatus digili III. in their course down the 
posterior side of the larsometatarsus, are connect¬ 
ed by a strong tendinous band (vinculum) which 
suggests simultaneous action of the two toe 
flexors in some birds, such as Anatidae. Catli- 
artidae. Grits. Lams, most Galliformes and ratites 
(Hudson 1937, 1948; Fisher 1946; Berger 1956; 
Hudson el al. 1959; George and Berger 1966; 
Patak and Baldwin 1998). The vinculum is 
typically broad and short, and divided into a 
group of parallel, more or less separate strands. 
This vinculum is absent in Accipitridae, Pandion , 
