RESPIRATORY ORGANS. 251 
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 (Protopierus) or a little 
to one side (Lepidosiren, Ceratodus) and the air duct passes up on the right side 
of the cesophagus 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 
Fic. 258.—Trachea, etc., of 
Amphiuma, after Wilder. a, 
arytenoid cartilages; b*, fourth bran- 
chial arch; dir, dilatator trachee, 
muscle; hp, hyopharyngeus mus- 
ae trachea with cartilages in its 
walls. 
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 
gi 
