RESPIRATORY SYSTEM OF BIRDS. 219 



just described, from the situation of the superior larynx with rela- 

 tion to the rictus or gape of the bill, and from the absence of lips 

 by which this might be partially or entirely closed, it is plain that 

 it cannot be considered as influencing the voice, otherwise than by 

 dividing or articulating the notes after they are formed by the 

 lower larynx. The superior larynx presents, indeed, but few 

 varieties in the different species of Birds; and these relate chiefly 

 to certain tubercles in its anterior, which vary in number, and do 

 not exist at all in some species, as the Singing Birds ; being chiefly 

 present in those birds which have a rough unmusical voice. In the 

 Pelican, the Gigantic Crane, and most of the Rasores, a process 

 extends backward into the cavity of the upper larynx from the 

 middle of the posterior surface of the thyroid cartilage, and seems 

 destined to give additional protection to the air-passage. 



The trachea, figs. 93, 94, G, is proportionally longer, in conse- 

 quence of the length of the neck in Birds, than in any other class 

 of animals, its length being further increased in many species by 

 convolutions varying in extent and complexity. A species of 

 Sloth (Bradypus tridacti/his) among Mammals, and a species of 

 Crocodile ( Crocodilus acutus) among Keptiles, present an analo- 

 gous folding of the trachea. 



The trachea is composed in Birds of a series of bony, and some- 

 times, as in the Ostrich, of cartilaginous rings, included between 

 two membranes. In those cases in which they are of a bony 

 structure, the ossification is observed to commence at the anterior 

 part of each ring, and gradually to extend on both sides to the 

 oj^posite part. 



The tracheal rings, w^hether bony or cartilaginous, are, with 

 the exception of the two uppermost, always complete, and not, as 

 in most quadrupeds, where the windpipe bears a different relation 

 to the organ of voice, deficient posteriorly. They differ in shape, 

 being sometimes more or less compressed. They are generally of 

 uniform breadth, but in some species are alternately narrower at 

 certain parts of their circumference and broader at others, and in 

 these cases the rings are generally closely approximated together, 

 and, as it were, locked into one another. This structure is most 

 common in the Grallatorcs, where the rings are broadest alternately 

 on the right and left sides. 



With respect to the diameter of the traclical rings, tliis may 

 sometimes be pretty uniform throughout, and the trachea will con- 

 sequently be cylindrical, as in the Cantores, the Grallatorcs which 

 have a shrill voice, the females of the Natatorci<,w[ii\. most Raptor cs 

 and Rasores : or the rings may gradually decrease in diameter, 



