204 ELEMENTS OF ORNITHOLOGY. 
The other muscles of the wing correspond in a more or less 
general way with the muscles of our own arm reduced; but 
their tendons are long and slender, and the arrangements of the 
skeleton, as already stated *, are such that, though there are 
muscles answering to those which rotate the handin us, in Birds 
they cannot rotate it. They can do nothing but open and shut 
the wing. There is, however, a muscle called the tensor patayit 
which has an elastic tendon, and acts on the fold of skin on 
the front of the wing between the shoulder and the wrist 7. 
It takes origin by muscle from the former, while its tendon 
is inserted into the latter. It may be distinguished, as the 
tensor patagr longus, from a tensor patagir brevis which arises 
in common with the former but is inserted into membrane 
within the bend of the elbow. The arrangement, however, of 
these tensor muscles—which sometimes receive the common 
name of propatagialis—differs in various ways in various birds. 
There is also ancther tensor—sometimes called metapatagialis— 
which acts on the fold of skin between the trunk and the 
inner surface of the upper arin, and there is sometimes also a 
muscle, called dermo-tensor patagv, which arises from the inside 
of the skin of the front of the neck, and passing thence over 
the shoulder is inserted, by a delicate tendon, in common with 
that of the tensor patagit longus. 
Amongst the muscles of the leg available for classification 
must be mentioned one called the ambiens, which exists in some 
birds and not in others. When fully developed it arises from 
the pelvis in the vicinity of the acetabulum, and ends in a 
tendon which passes over the outer side of the knee and ends 
by joining one of the flexor muscles which bend and contract 
the toes. When a bird is at roost and the knee bent by the 
weight of the body, such bending of the knee stretches the long 
tendon, and this (by the connection of the tendon with the 
flexor muscles) ipso facto causes the toes to contract, and so, 
without any effort, a firm grasp is maintained. Birds with an 
ambiens are permed homologonatous, those without it, anomalo- 
gonatous. A muscle called the biceps cruris varies as to the 
conditions it presents. Ordinarily it arises from the greater 
part of the dorsal margin of the post-acetabular part of the 
ilium. Its fibres end in a round tendon, which is inserted into 
about the middle of the fibula. 
* See ante, p. 194. t See ante, p. 154. 
