36 PRACTICAL ORNITHOLOGY. 



has thus passed within No. 2 on the left side, and over it on the 

 right ; and in this manner has been produced the appearance 

 exhibited by Fig. 111. Of course without the contraction or 

 double notch in each ring this effect could not be produced, for 

 it is there that the rings cross each other. Thus then, the 

 trachea in its extreme state of contraction, when it is reduced 

 from one-half to one-third of its greatest length, undergoes but 

 a very slight diminution in its diameter. The elastic membrane 

 passes from the edge of one ring over the next, to be inserted 

 into the edge of the third. Tlie intermediate ring then slips in 

 behind those on each side of it ; while its other lateral half slips 

 before those on each side of it. This mechanism I have observed 

 in almost all the tracheae which I have examined, although the 

 rings vary much in breadth and thickness in different species. 

 Surely while we are pleased with having so satisfactorily 

 traced it, we may be permitted to admire its extreme beauty, 

 and to exclaim with the Psalmist, " How wonderful are thy 

 works, O Lord ! in wisdom hast thou made them all !" 



Many curious modifications of structure are observed in the 

 trachea, the peculiarities of which tend, along with those of the 

 alimentary canal and other organs, to indicate the affinities of 

 the different groups of birds. Thus the syrinx is destitute of 

 muscles in the Rasores, and in the Swans, Geese, and Ducks, 

 which among the aquatic birds are analogous to the Pheasants 

 and Partridges among the land birds. But the general infe- 

 rences that may be deduced from these circumstances must be 

 deferred until we have obtained a view of the whole series. I 

 shall therefore conclude with a few remarks. 



In some birds the rings of the trachea are very narrow, and 

 their intervals proportionally wide ; and sometimes the rings 

 are cartilaginous, or but partially ossified. All these circum- 

 stances are observed in the tracheae of the Golden Eagle, the 

 White-tailed Sea Eagle, and the Gulls. Often, on the other 

 hand, as in Cranes, Herons, Swans, Ducks, and Mergansers, 

 the rings are very broad, with very small intervals. Sometimes 

 the bronchi are very short, as in most of the Waders ; and 

 sometimes they are very long, and of great diameter, as in Cor- 

 morants and Bitterns. Although generally membranous along 



