AVES BIRDS. 



605 



A. 



The trachea is of very great proportionate length in correspond- 

 ence with the elongated neck ; commencing at the root of the 

 tongue, and extending into the thoracic cavity, where it divides 

 into two bronchial tubes, one appropriated to each lung 

 (Jig. 274, /, /). The trachea of birds is composed of cartilaginous 

 rings, which are very generally ossified ; each ring, with the excep- 

 tion of two or three immediately beneath the upper larynx, forming a 

 complete circle (Jig. 275, A) surrounding the tracheal tube : these 

 rings are enclosed between the soft membranes of the trachea, and 

 thus keep the air-passages constantly permeable to the atmosphere. 



In many birds, especially among the web- footed tribes, the 

 trachea suddenly dilates into wide chambers, or cavities of different 

 forms and dimensions ; a circum- Fig. 275. 



stance the object of which has not 

 as yet been satisfactorily explained: 

 and, what is still more inexplica- 

 ble, in some genera, and those too 

 with the longest necks, as for ex- 

 ample the Wild Swan, and many 

 of the Wading Birds, the lower 

 part of the trachea is lengthened 

 out and variously contorted before 

 it terminates in the chest. This 

 long trachea is provided with mus- 

 cles whereby the rings may be approximated, and thus the 

 length of the tube is considerably modified : these muscles 

 (fig. 274, A, B, h) arise from the sternum, and sometimes also from 

 the furcula, and are continued along the sides of the windpipe 

 throughout its whole length. 



The upper larynx, or rima glottidis, is in birds but of secondary 

 importance in the production of vocal sounds ; it is a simple fissure 

 bounded by two osseous pieces (Jig. 275, A, B,/) corresponding with 

 the aryienoid cartilages of Mammalia : these, however, in the Bird 

 are not connected with chorda vacates ; but simply, as they are se- 

 parated or approximated, open or close the fissure of the glottis. 

 When, therefore, we compare the framework of this organ with the 

 cartilaginous pieces found in the larynx of Mammalia, considerable 

 difference is perceptible, insomuch that it is not easy positively to 

 recognise the analogous portions, more especially as in the Bird the 

 cartilages are more or less completely ossified. If the broad an- 

 terior plate (Jig. 275, b) be considered as the thyroid cartilage, we 



