THE ORGANS OF VOICE. 103 



Stomacli. But such air-sacs — even when tliey remain per- 

 manently connected with the exterior by an open passage or 

 pneumatic duct — are air-bladders, and not lungs, because they 

 receive their blood from the adjacent arteries of the body, 

 and not direct from the heart, whUe their efferent vessels 

 ai'e connected only with the veins of the general circulation. 



The wall of each pulmonic air-sac is at first quite simple, 

 but it soon becomes cellular by the sacculation of its 

 parietes. In the lower pulmonated Vertebrata,the sacculation 

 is more marked near the entrance of the bronchus ; and when 

 the lung-sac is long, as in many Amphibia and in Snakes, 

 the walls of the posterior end may retain the smooth con- 

 dition of the embryonic lung. In Chelonia and Crocodilia. 

 the lung is completely cellular throughout, but the bronchi 

 do not give off branches in the lungs. In Birds, branches 

 are given off at right angles ; and, from these, secondary 

 branches, which lie parallel with one another, and eventually 

 anastomose. In Mammalia, the bronchi di\dde dichoto- 

 mously into finer and finer bronchial tubes, which end in 

 sacculated air-cells. 



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

 in the Chamceleonidce, and the principal bronchial tubes 

 terminate in large air-sacs in Aves. 



The. Larynx and the Syrinx. — The trachea is commonly 

 kept open by complete, or incomplete, rings of cartUage, 

 and the uppei-most of these undergo special modifications, 

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

 certain cii-cumstances, becomes an instrument of voice. 



When completely developed, the larynx presents a ring- 

 like cartilage called cricoid, which lies at the summit of the 

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

 arytenoid cartilages are moveably articulated, and a thyroid 

 cartilage of a Y-shape, open behind, is articulated move- 

 ably with its sides. Folds of the mucous membrane, con- 

 taining elastic tissue, termed the vocal cords, stretch from the 

 arytenoid cai'tilages to the re-entering angle of the thyi-oid 

 cartilage, and between them lies a slit-like passage, the 

 glottis. This is covered by a cartilage, the epiglottis, attached 



