104 THE ANATOMY OF VEETEBEATED ANIMALS. 



to the re-entering angle of tlie thyroid, and to the base of 

 the tongue. Folds of mucous membrane, extending from the 

 eijiglottis to the arytenoid cartilages, ai-e the anjepiglottic 

 ligaments. The inner surfaces of these end below in the/«7se 

 vocal cords, between which and the true chordcB voccdes, lie re- 

 cesses of the mucous membrane, the ventricles of the larjTix. 



The chief accessory cartilages are the cartilages of San- 

 torini, attached to the summits of the arytenoid cartilages, 

 and the cartilages of Wrisherg, which lie within the ary- 

 epiglottic ligaments. 



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

 another apparatus, the lower 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 Respiration. — The mechanism by which 

 the aerating medium is renewed in these different respira- 

 tory organs is very various. Among branchiated Vertebrata, 

 Amphioxas stands alone in having ciliated branchial organs, 

 which form a network very similar to the perforated pharyn- 

 geal wall of the Ascidians. Most Fishes breathe by taking 

 aerated water in at the mouth, and then shutting the oral 

 aperture, and forcing the water through the branchial clefts, 

 when it ilows over the branchial filaments. 



Pidmonated Vertebrata which have the thoracic skeleton 

 incomplete (as the Amphibia), breathe by distending their 

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

 being shut, pumping it, by the elevation of the hyoidean ap- 

 paratus and floor of the pharynx, into the lungs. A Frog, 

 therefore, cannot breathe properly if its mouth is kept wide 

 open. 



In most Beptilia, and in all Aves and Mammalia, the 

 sternum and ribs are capable of moving in such a way, as 

 alternately to increase and diminish the capacity of the 

 thoracico-abdominal cavity, and thereby to give rise to an 

 inspiratory and expiratory flow of air. 



In the Beptilia, the elastic lungs dilate with the inspira- 

 tory, and contract with the expiratory, act ; but in Aves, 

 the air rushes through the principal bronchial passages of 



