194 The Bird 
animals. There are so many resemblances between birds 
and reptiles that we naturally turn to the latter for com- 
parison, but even here we find a great unlikeness. We 
learned, when we reflected on the number of ribs of a 
bird, that the repetition of so many similar structures 
was merely the last remaining vestige of ancestral body 
segments, which reach their extreme development (in 
number and similarity) among the worms; but in regard 
to muscles birds show little or nothing of this. In liz- 
ards we may count dozens upon dozens of bands of muscles 
succeeding one another, all more or less alike, from head 
to tail, but it is only in the neck of a bird that we shall 
find anything like this. 
In order to give to muscles a firm anchorage, they 
must of course be attached to the bones. At these 
points of attachment deep furrows or cavities are often 
found in the surface of the bones, and in still other ways 
we are reminded, even in fossil bones, of the flesh and 
muscle which once moved them. These muscle impres- 
sions are often a valuable source of identification in 
naming the bones of creatures which, many thousands 
of years ago, disappeared from the earth. And indeed 
so great variety exists in the muscles of living birds that 
many of them, those of the upper arm for example, are 
of considerable value in classification. 
Nerves 
The last great system of internal organs which we 
shall consider, and perhaps the most mysterious of all, 
is that of the nerves. We have learned that the back- 
