'2-20 



STRUCTURE AND CLASSIFICATION OF BIRDS 



this condition in Bucorvus abyssinicus ; ' it is exactly the 

 same in one or two other species which I have subsequently 

 studied. 



The Syrinx, Aceros nipalensis. The last rings of the 

 trachea are fused together to form a solid box, at the sides of 

 which, however, the individual rings are recognisable. In front 

 the last three rings are thus fused, but behind two additional 

 rings fuse with the others to form a wide and deep bony 

 plate. The tracheal rings lying in front of these five show 

 the dovetailing arrangement which is so often found in the 

 tracheal rings. The pessulus is well developed and bony, 

 but, owing to the complete fusion of the tracheal rings both 



posteriorly and anteriorly, 

 it is impossible to say 

 from which rings it is de- 

 veloped. 



The intrinsic muscles 

 of the syrinx are attached 

 near to the boundary line 

 between the last and the 

 penultimate tracheal 



rings. 



The bronchial semi- 

 rings are cartilaginous, and 

 there is a considerable in- 

 terval between the first of 

 these and the last tracheal 



FIG. 102. SYRINX OF Aceros nipalensis. 

 I-'KONT VIEW. (AFTEK BEDDARD.) 



ring. 



Bucorvus abyssinicus. The syrinx of this hornbill (fig. 

 103) differs in many particulars from the last. The tracheal 

 rings are not ossified, and there is no box formed by their 

 fusion. Only posteriorly are the penultimate ring and 

 the two in front of this fused just at the origin of the 

 pessulus ; anteriorly the pessulus is fused with the ante- 

 penultimate tracheal ring, which forms with it a three-way 

 piece ; the last two tracheal rings do not meet in front. The 



1 ' Notes on the Visceral Anatomy of Birds. I. On the so-called Omentum,' 

 P. Z. S. 1885, p. 842 ; and above, p. 44, fig. 29. 



