204 ELEMENTS OF OENITHOLOGT. 



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 loug and slender, and the arrangements of the 

 skeleton, as already stated *, are such that, though there are 

 muscles answering to those which rotate the hand in 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 patagii 

 which has an elastic tendon, and acts on the fold of skin on 

 the front of the wing between the shoulder and the wrist f. 

 It takes origin by muscle from the former, while its tendon 

 is inserted into the latter. It may be distinguished, as the 

 tensor patagii longus, from a tensor patagii 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 another tensor — sometimes called metapatagialis — 

 which acts on the fold of skin between the trunk and the 

 inner surface of the upper arm, and there is sometimes also a 

 muscle, called derma-tensor patagii, 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 patagii 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 fuUy 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 eSort, a firm grasp is maintained. Birds with an 

 ambiens are termed 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-aoetabular 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. 



