312 Mr. Yarbell on the Organs of Voice in Birds. 



The Velvet Duck is remarkable for a hollow bony enlarge- 

 ment in its trachea, situated about two-thirds down the tube, 

 made up of expanded tracheal rings firmly ossified together. 

 Upon each side of this enlargement a small muscle, marked e, 

 arises, which, passing downwards, is inserted upon the inner side 

 of the shaft of the os furcatorium, and the voice is probably in- 

 fluenced by the action of these muscles altering the relative 

 situation of this hollow bulb upon the tube. 



There is also another peculiarity, which, as far as I am aware, 

 has not been noticed. On making a longitudinal section of the 

 trachea of this rare British bird, (representations of the inner 

 surfaces of both halves of which are shown at Tab. XXI. Fig. 2, 

 & 3,) it will be seen, that the inner tube of the trachea at its upper 

 part has an aperture on each side, by which it communicates 

 freely with the cavity within the bony enlargement situated im- 

 mediately below the superior larynx, and brings to mind the 

 laryngeal cavities found in some of the higher animals. A slip 

 of paper is represented as passing through both apertures. 

 Letters d, d, represent the ordinary sterno-tracheal muscles. 



In the Golden-eye, the second pair of muscles is of large 

 size, and inserted also upon the shafts of the os furcatorium. 

 They arise in part below, and upon a portion of the surface of 

 the enlarged rings, which form the bulb on the tube of that bird. 

 When the voice is not exercised, these enlarged rings lie folded 

 flat upon each other, but by the contraction of these muscles, the 

 rings are set up at right angles to the axis of the tube, and the 

 air, forced through and vibrating in an enlarged hollow cavity, 

 acquires a degree of power which has obtained for this bird the 

 specific name Clangula. 



The trachea of the Red-breasted Merganser has also a perma- 

 nent enlargement of a portion of the rings, forming a bulb on the 

 tube. It has also a second pair of muscles (furculo-tracheal), 



which 



