RESPIRATORY ORGANS. 



25 1 



THE AIR DUCTS. 



The opening from the pharynx into the air ducts is known as the 

 glottis, usually an elongate slit capable of being closed and opened 

 by appropriate muscles. This is immediately succeeded by the ducts, 

 which, except in the dipnoi, are more or less differentiated into regions 

 and have skeletal supports in their walls. 



In the dipnoi the glottis is either in the mid-ventral line (Protopterus) or a little 

 to one side (Lepidosiren, Ceratodus} and the air duct passes up on the right side 

 of the oesophagus to reach the lungs 'which are dorsal to the alimentary canal. The 

 tube is without skeletal supports and connects directly with both lungs without any 

 division into bronchi. 



Larynx. The beginnings of the larynx are seen in the amphibia, 

 where in the lower types (Necturus) a pair of cartilages are developed on 

 the sides of the glottis, in the position of 

 a reduced visceral arch, each cartilage 

 extending posteriorly a short distance 

 along the air ducts. In other genera of 

 urodeles the anterior end of each lateral 

 cartilage separates from the rest as an 

 arytenoid, the first of the laryngeal carti- 

 lages, imbedded in the walls of the glottis. 

 The rest of the lateral cartilages may 

 remain entire (fig. 258) or they may 

 separate into a number of pieces, extend- 

 ing along the lateral walls of the trachea 

 and bronchi. Usually the anterior pair 

 of these pieces fuse in the mid-ventral line, 

 thus forming the second (cricoid) ele- 

 ment of the pharyngeal framework. 

 These parts are moved by antagonistic 

 muscles. One set of these, extending to 

 the persistent branchial arches, serves as dilatators of the glottis; the 

 others, connected with the laryngeal cartilages themselves, constrict 

 the opening. In the anura the cricoid is converted into a ring, with 

 the arytenoid hinged within and anterior to it, the whole larynx moving 

 anteriorly to a position between the hinder processes of the hyoid plate. 

 Inside of the short larynx thus framed by these cartilages are a pair 

 of folds of the laryngeal lining, the vocal cords, extending parallel to 



FIG. 258. Trachea, etc., of 

 Amphiuma, after Wilder. a, 

 arytenoid cartilages; b*, fourth bran- 

 chial arch; dtr, dilatator tracheae, 

 muscle; hp, hyopharyngeus mus- 

 cle; tr, trachea with cartilages in its 

 walls. 



