ORGANS OF VOICE AND RESPIRATION. 109 



down the neck from the superior to the inferior larnyx, and then 

 divides at the commencement of the thoracic cavity into two bron- 

 chi. It is very rare for the trachea to divide higher up, namely, in 

 the neck, as in the Humming-bird. The bronchial tubes are mostly 

 short, and usually narrower than the trachea, though occasionally 

 they are wider and dilated at their origin, as in the Pelican and 

 Merganser. They are usually composed of a series of half-rings 

 completed by an elastic membrane ; more rarely, as in the Stork, of 

 perfect rings which are occasionally continued some distance into 

 the lungs. 



Two pairs of muscles are generally found which draw down the 

 trachea, one of which is frequently arrested in its development, or 

 wanting altogether. The superficial pair, which is particularly 

 well-developed in the Natatores, as the Merganser and Duck, but ab- 

 sent in the smaller birds, e. g. the Passeres, Scansores, and Picariae, 

 lie upon the sides of the whole length of the trachea, and arise from 

 the upper part of the inferior larynx, and the internal surface of 

 the furcula, whence their name of m. cleido or ypsilo-tracheales. 

 The second pair, which are generally present, yet appear to be 

 wanting along with the first in the Parrots, are called the m. sterno- 

 tracheales, arise from the external part of the anterior margin of the 

 sternum, are attached likewise to the lateral surfaces of the trachea 

 and the upper part of the inferior larynx, and ascend more or less 

 upward covered by the preceding pair of muscles. In the Pigeons 

 the last two muscles or sterno-tracheales arise as usual from the ster- 

 num, but pass both asymmetrically to the right side of the trachea. 



In addition to the normal peculiarities of structure already 

 noticed, the trachea exhibits singular dilatations and convolutions, 

 which appear however to occur only in some orders, as the Nata- 

 tores, Grallae, and Gallinae, and what is remarkable, are frequently 

 absent in closely allied genera, or even species of the same genus, 

 and often constitute characteristic sexual distinctions, occurring in 

 many cases only in the male, and being either entirely absent, or 

 exhibited under a lesser degree and with particular modifications 

 in the female, or else being met with in the same conditions in both 

 sexes. 



Claiming our attention in the first place are those elongated dila- 

 tations which are situated commonly about the middle of the trachea, 

 and provided with cartilaginous and bony rings ; thjey are usually 

 single, but are double in the Drake and male Merganser. Anas 

 leucocephala and Mergus serrator have a single dilatation of the 



