AIR-PASSAGES. 253 



is, those lying round the glottis or entrance to the trachea (which 

 are phylogenetically the oldest cartilages of the whole 

 apparatus), become differentiated to form a special apparatus, the 

 larynx ; this is regulated by muscles, and has to do with the 

 production of the voice. 



A 15 (' I) 



FIG. 205. DIAGRAM ILLUSTRATING THE PHYLOGENETIC DEVELOPMENT OF THE 

 LUNGS ; A GRADUAL INCREASE OF THE BESPIRATORY SURFACE is SEEN IN 



PASSING FROM A TO D. 



The trachea, bronchi, and larynx thus constitute a kind of 

 hollow skeleton for the whole respiratory apparatus, and, as they 

 are formed secondarily, we should naturally expect them to be most 

 developed in the higher types. 



Air-Passages. 



Amphibia. The first traces of cartilaginous supports to the 

 glottis are seen in Urodeles, there being no skeletal elements in 

 this region in Dipnoans (Protopterus). At the same time, dilator 

 and constrictor muscles appear round the glottis. 



In Dipnoi, Salamandridse, and An ur a, there is no proper 

 trachea, but only a short laryngo-tracheal chamber leading 

 from the larynx to the lungs ; in the two last-mentioned groups 

 this is supported by cartilages. A definite trachea is, however, 

 present in Siren, Amphiuma, and the Gymnophiona ; it 

 reaches a length of 4 to 5 or more centimetres, and its wall is 

 strengthened by a series of small irregular cartilages, which usually 

 tend to unite into bands : only in the Gymnophiona, however, do 

 these bands begin to take on the form of half-rings, and to surround 

 the trachea more or less completely. 



In Anura a considerable advance is noticeable, as a highly 

 differentiated larynx is present in them. This is regulated by a 

 well-developed series of muscles, and is provided with vocal cords, 

 the sound produced by which is often intensified by the presence 

 of vocal sacs developed from the floor of the mouth. The laryngo- 

 tracheal chamber of Rana esculenta lies between the posterior 

 cornua of the hyoid (thyro-hyals), and is supported by a thin car- 

 tilage on either side of the glottis (Fig. 206, Co), as well as by a 

 ring-shaped cartilage, from which delicate processes pass backwards 

 to the roots of the lungs (Fig. 206, Cl l C7 4 ). The former correspond 



