CHAPTEE IV. 



The othee Systems or Oekans, and the Development 



AND MiGBATION OF BlEDS. 



The MuscuiAE Sxstem. 



'T'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 fulcra. 



The muscles of birds are very compact and red, especially 

 those which are the most exercised. They ai-e packed where 

 they can best be carried with respect to the centre of gravity. 

 They are thus very voluminous on the breast, vhile 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 flexors or extensors or rotators or tensors or 

 elevators or depressors or ahduetors or adductors, according to 

 the sort of motion their contraction results in. 



Only those njuscles 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, 



