142 THE STRUCTURE AND LIFE OF BIRDS chap. 



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. 1 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 



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mm 4j 

 iliiiiiiii 



B 



* fiiii i 

 iS 



Fig. 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. 



See p. 127. 



