THE ORGANS OF VOICE. 93 



Blind air-sacs are given off from the surfaces of the lungs 

 in the Ghammleonidm^ and the principal bronchial tubes termi- 

 nate in large air-sacs in Aves. 



The Larynx and the Syrinx. — The trachea is commonly 

 kept open by complete, or incomplete, rings of cartilage, and 

 the uppermost of these undergo special modifications, which 

 convert them into a Larynx^ an organ which, under certain 

 circumstances, becomes an instrument of voice. 



When completely developed, the larynx presents a ring- 

 like cartilasre called cricoid^ which lies at the summit of the 

 trachea. With the anterior and dorsal edge of this, two aryt- 

 enoid cartilages are movably articulated, and a thyroid car- 

 tilage of a V-shape, open behind, is articulated movably with 

 its sides. Folds of the mucous membrane, containing elastic 

 tissue, termed the vocal cords^ stretch from the arytenoid car- 

 tilages to the reentering angle of the thyroid cartilage, and 

 between them lies a slit-like passage, the glottis. This is cov- 

 ered by a cartilage, the epiglottis^ attached to the reentering 

 angle of the thyroid, and to the base of the tongue. Folds 

 of mucous membrane, extending from the epiglottis to the 

 arytenoid cartilages, are the aryepiglottic hgaments. The in- 

 ner surfaces of these end below in the false vocal cords, he- 

 tween which and the true chordcB vocales lie recesses of the 

 mucous membrane, the ventricles of the larynx. 



The chief accessory cartilages are the cartilages of JSan- 

 torini, attached to the summits of the arytenoid cartilages, 

 and the cartilages of Wrisberg^ which lie within the aryep^- 

 glottic ligaments. 



Birds possess a larynx in the ordinary position ; but it is 

 another apparatus, the loicer larynx or syrinx, developed 

 either at the end of the trachea, or at the commencement of 

 each bronchus, which is their great vocal organ. 



The Mechanism of JRespiration. — The mechanism by which 

 the aerating medium is renewed in these different respiratory 

 organs is very various. Among branchiated Vertebrata, Am- 

 phioxus stands alone in having ciliated branchial organs, which 

 form a net-work very similar to the perforated pharyngeal a "all 

 of the Asciclians. Most Fishes breathe by taking aerated wa- 

 ter in at the mouth, and then shutting the oral aperture, and 

 forcing the water through the branchial clefts, Avhen it flows 

 over the branchial filaments. 



Pulmonated Vertebrata, which have the thoracic skeleton 

 Incomplete (as the Amphibia), breathe by distending their 

 pharyngeal cavity with air; and then, the mouth and nostrils 



