122 NATURAL HISTORY. 
enables the Eagle, and other birds also, to look directly 
at the sun. ‘The sense of smell is very acute in all birds 
in which it can be of service in searching for food, as, 
for example, in those that live on carrion. While all 
birds have ears, there is only one kind, the Owl tribe, 
that has any external ear. In all others there is merely 
an opening to the passage leading to the internal appa- 
ratus of hearing, and even this is concealed among the 
feathers of the head. 
208. Birds are digitigrade, § 92. You can see this to 
be true in the case of the Ostrich, Fig. 4, if, comparing 
the bones of the leg with the same bones in man, Fig. 1, 
you begin at the thigh-bone and go downward. In Fig. 
101 you have the bones of a bird’s leg, a being the thigh- 
Fig. 101. Fig. 102. Fig. 103. 
bone, 6 the bones of the leg proper, c the heel-bone, long 
and extending upward, and d the bones of the foot. In 
Fig. 102 is the outline of the leg of a man, with letters 
to correspond with those of Fig. 101, that you may read- 
ily make the comparison. In Fig. 103 you have the 
perching apparatus of birds represented, and you can 
see how it is that they can sleep on their perches with- 
out falling off. There is, you observe, a large muscle in 
front of the thigh-bone; from this along tendon or cord, 
A, extends down the leg, and in the foot it divides into 
branches, which go to all the toes. When the muscle 
pulls on this the toes will all be bent, as every body 
