VI FORM AND FUNCTION 169 
There is connected with this subject another strange 
phenomenon. In many birds a thin muscle, called 
Ambiens, arises from the Pelvis just under the thigh- 
joint and passes forward on the inner side of the leg 
to the knee, before reaching which it becomes a tendon : 
it curves round the knec in a little tendinous tunnel 
occupied by itself alone, then doubles back on the 
outside of the leg and passes into one of the muscles 
which bend the toes as described above. It is very 
characteristic of birds that a muscle should, by means 
Fic. 45.—Leg of chicken, the side next the body. 
A, ambiens muscle ; kK, knee-joint. 
of a long tendon, do its work at such a distance: but, 
curiously, it is not found in by any means all the 
‘perching birds. And, besides, this the same muscle is 
to be found in crocodiles. This must not, how- 
ever, be taken to prove any very close relationship with 
birds. The fact that it is found in two families of birds 
may help to prove that they are closely allied, but such 
evidence is less dependable when we are dealing, not 
1 Refer to Appareil Locomoteur des Otseaux (M. Edmond 
Alix), p. 443. 
