268 THE ANATOMY OF VERTEBKATED ANIMALS. 



elongated, membranous fontanelles. In the Chameleons, tlie 

 mucous membrane of the larynx between the circular cartilage 

 and the first ring of the trachea protrudes in the form of an 

 air-sac. 



In the AmpMsboenoidea, and in the Ophidla, the skeleton 

 of the larynx consists of two lateral longitudinal bands of 

 cartilage, united by from four to sixteen transverse bands. In 

 other words, the structure which answers to the circular carti- 

 lage is greatly elongated, and has many transversely-elon- 

 gated fontanelles. There is a single arytenoid cartilage, 

 which is sometimes represented by a process of the anterior 

 dorsal margin of the circular cartilage. An epiglottis is rarely 

 present. 



In Birds there are distinct thyroid, cricoid, and arytenoid 

 cartilages, which may be more or less completely ossified. 

 Sometimes an epiglottis is added. 



The voice of Birds, however, is not formed in the larynx, 

 but in the syrinx, or lower larynx, which may be developed 

 in three positions : 1. At the bottom of the trachea, from tlie 

 trachea alone. 2. At the junction of the trachea and bionchi, 

 and out of both. 3. In the bronchi alone. The syrinx may 

 be altogether absent, as in the Ratitm and the GathartidoB, or 

 American vultures. 



The commonest form of syrinx is the second mentioned 

 above, or tlie hroncho-tracheal syrinx. It is to be met with in 

 all our common song-birds, but is also completely developed in 

 many birds, such as the crows, which have no song. In its 

 commonest condition this form of syrinx presents the following 

 characters : The hindermost rings of the trachea coalesce, and 

 form a peculiarly-shaped chamber, the tympanum. Immedi- 

 ately beyond this, the bronchi diverge, and from their posterior 

 wall, where one bronchus passes into the other, a vertical fold 

 of the lining membrane rises, in the middle line, toward the 

 tympanum, and forms a vertical septum between the anterior 

 apertures of the two bronchi. The anterior edge of this 

 septum is a free and thin membrana semilunaris, but in its 

 Ulterior a cartilaginous or osseous frame is developed, and 

 becomes united with the tympanum. The base of the frame 

 is broad, and sends out two cornua, one along the ventral, and 

 the other along the dorsal, edge of the inner wall of the bron- 

 chus of its side ; which, in this part of its extent, is membra- 

 nous and elastic, and receives the name of the membrana tym- 

 ipaniformis interna. 



The bronchial " rings " opposite this are necessarily in 



