CHAPTER TY; 
THE OTHER SYSTEMS OF ORGANS, AND THE DEVELOPMENT 
AND MIGRATION OF BIRDs. 
Tue Muscunar SYSTEM. 
HE flesh which invests the skeleton consists of a multitude 
of most delicate threads called “ muscular fibres,” which 
are variously aggregated in masses to form ‘“ muscles.” These 
are the organs of movement. They are generally attached at 
one or both ends to different bones, sometimes by the muscular 
fibres themselves, often by the intervention of a very strong 
and dense band of fibrous membrane called a tendon—the muscu- 
lar fibres being implanted into the tendon, and the tendon into 
the bone the muscle acts on. Muscles act by contracting ; that 
is, the fibres which compose it—and therefore the whole muscle 
they compose—temporarily change their shape, becoming shorter 
and stouter, and so causing those bones to approximate, to which 
the ends of the muscle are directly or indirectly attached. They 
act on the bones by making use of the latter as levers or fulera. 
The muscles of birds are very compact and red, especially 
those which are the most exercised. They are packed where 
they can best be carried with respect to the centre of gravity. 
They are thus very voluminous on the breast, while many 
muscles have very long tendons, so that they can act on parts 
remote from the centre of gravity, while their heavy fleshy 
substance is placed in proximity to it. This is especially the 
case with the muscles of the limbs. 
Muscles are called flewors or extensors or rotators or tensors or 
elevators or depressors or abductors or adductors, according to 
the sort of motion their contraction results in. 
Only those muscles will be here noted a knowledge of which 
may be useful to the Ornithologist for purposes of classification. 
As flight is the most essential and important action for a Bird, 
