RAVEN. 81 



muscles in close contact. Its direction is obliquely down- 

 wards and forwards, its substance in part hid by the muscle 

 marked e, and it is attached by a broad base to the last 

 bony ring of the tube, to the cartilaginous projection im- 

 mediately below, and sends one portion to be inserted upon 

 the extreme end of the first bronchial bone. Figure 4 

 represents these five muscles, three of them being partly 

 detached to render them more obvious by separation. I 

 have called these four muscles the long and short, anterior 

 and posterior tensors : the muscle marked d, from its in- 

 sertion upon the sternum, may still retain the name of 

 sterno-tracheal. Thus, it will be seen, the lungs govern 

 the quantity of air, as well as the force with which it is 

 sent through the trachea, while the muscles influence the 

 diameter, and the length of the bronchial tubes. The 

 principle upon which the organs of voice in birds is founded 

 is that which prevails in wind instruments generally; 

 the notes in the ascending scale being produced by a cor- 

 responding contraction of the diameter, or the length of the 

 tube, and vice versa. It may, perhaps, be objected, that 

 the utmost extent of motion which birds appear to have the 

 power of exercising over the different parts of their organ 

 of voice, seems insufficient to account for the effects pro- 

 duced; but it may in answer be urged, that the closest 

 examination, or most scientific demonstration of the chordae 

 vocales and muscles in man, with all the auxiliary appen- 

 dages, afford but an imperfect illustration of the varied 

 and extraordinary powers of the human voice. 



VOL. II. 



