108 AVES. 



and above the two posterior thyroid pieces, there is situated a small 

 rounded, cordiform or quadrangular plate, which must be viewed as 

 the cricoid cartilage. In the Cassowary and Parrot, the posterior 

 surface of the cricoid cartilage lies perfectly free, but is partly con- 

 cealed in many Birds, as the Gallinazo, and in others can not be seen 

 upon an external view of the larynx. The narrow elongate-triangular 

 arytenoid cartilages are somewhat similar to those of the Mammalia, 

 and bound the fissure of the glottis. 



Three pairs of Laryngeal Muscles varying only in their size may 

 be distinguished in all Birds. Upon the anterior surface of the 

 thyroid cartilage there is situated, particularly in the Rapaces, a 

 strong muscular fasciculus, which arises from the lingual bone, de- 

 scends the trachea to a greater or less distance, and is attached to 

 the inferior margin of the thyroid cartilage ; it serves to draw up 

 the larynx as well as the trachea, and correspond thus in action with 

 the hyo-thyreoideus and crico-thyreoideus muscles of the higher ani- 

 mals. A thyrio arytanoideus posticus, the function of which is to 

 open the rima glottidis, is situated upon the uppermost part of the 

 dorsal surface of the larynx, takes a broad origin from the lower mar- 

 gin of the lateral posterior pieces of the thyroid cartilage, and passes 

 to the extreme apex of the arytenoids. Upon removing this pair a 

 broader thinner muscle will be brought into view, which constantly 

 arises from the superior margin of the arytenoid cartilage and unites 

 posteriorly and anteriorly with that of the opposite side. It acts as a 

 sphincter contracting the fissure of the glottis, and is called thyreoi- 

 deus lateralis s. compressor laryngis. There are no chordae vocales 

 in the superior larynx. 



The Trachea of Birds is usually of great length, and consists of a 

 large number of rings, the fewest 20 to 70 occurring in the Passeres, 

 while among the Grallae and Natatores the Ducks present above 

 100, the Heron, Pelican, and Ostrich, above 200, and the Crane and 

 Flamingo the surprising number of upward of 300 rings. These 

 rings are for the most part complete, approximated very closely to- 

 gether and remain often partly membranous as in Tetrao. In many 

 Birds the rings, at least at their commencement, are fissured, e. g. in 

 the Woodpecker. In the Scansores and Picariae, many Gallinae, 

 Rapaces and Grallae, the rings continue cartilaginous throughout 

 the whole of their existence, while in the majority of cases, as in 

 the Passeres, the Ostrich, many Grallae and Natatores, they are 

 completely ossified, so that the trachea becomes a hard cylindrical 

 tube, as in the Goose. The trachea generally descends straight 



