142 THE STRUCTURE AND LIFE OF BIRDS  cuHap. 
as its activity is due to one or another set of nerves ; 
it moves with great rapidity, instantaneously enlarg- 
ing or reducing the size of the pupil... The muscles of 
the heart, too, are peculiar. Though involuntary, they 
are striated, and yet unlike other striated muscles. 
The amount of contraction possible to a muscle 
varies with its length; its strength depends upon its 
i 
il 
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i | 
i ai | 
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Fic. 36 (after Huxley). 
A, Striated muscle of frog ; B, of mammal, ‘teased out”; c, non-striated muscle. 
thickness. Thus a short thick muscle will have strength, 
but no‘great range ; a long thin muscle, great range, 
but little strength. It has been found that a muscle 
cannot contract more than one-third of its length. It 
will be important to bear in mind this principle, when 
we come to consider the varying lengths of the 
breastbone and, consequently, of the muscles arising 
from it, in birds with different methods of flight. 
1 See p. 127. 
