AIR-TUBES AND LARYNX 283 



gradually formed (secondary and tertiary bronchi). The ends of 

 these branches are swollen, forming vesicles known as infundibida, 

 which are made up of a number of alveoli, and are surrounded by 

 blood capillaries, through the thin walls of which the interchange 

 of respiratory gases takes place (Figs. 227 and 228). 



In the following account the air- tubes will be dealt with 

 separately from the lungs proper. 



Air-Tubes and Larynx. 



The walls of the air-tubes may consist, in addition to their 

 lining of ciliated epithelium, of connective-tissue and elastic and 

 muscular fibres only, but usually cartilaginous elements are also 

 formed, and these serve to keep the tubes permanently open. The 

 most anterior of these cartilages, which support the larynx, become 

 differentiated to form a frame on which the structures by means of 

 which the voice is produced the vocal cords, are stretched : these 

 cartilages are acted upon by muscles. The relative length of the 

 windpipe, as a rule, corresponds with that of the neck. 



Dipnoi. In these the glottis is supported by a fibre-cartilage, 

 and leads into a muscular vestibule communicating with the lung. 

 A larynx and trachea are not differentiated. 



Amphibia. The vestibule, or laryngo-tracheal chamber, com- 

 municates with the pharynx on the one hand and with the lungs on 

 the other, and is supported by cartilages : it is provided with 

 intrinsic (dilator and constrictor) and extrinsic muscles, the former 

 derived from pharyngeal muscles and the latter from trunk muscles. 

 A definite trachea is differentiated in Siren, Amphiuma, and the 

 Gymnophiona only ; it reaches a length of 4 to 5 or more centi- 

 metres, and its wall is strengthened by a series of small irregular 

 cartilages, which usually tend to unite into bands (Fig. 229) : 

 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. 



The phyletically oldest skeletal parts are a pair of arytenoid 

 cartilages, situated in the walls of the vestibule (Fig. 229) : these 

 appear to have arisen by a modification of part of the fifth bran- 

 chial arch (comp. Fig. 233). Distally to them there is, in the 

 Anura, another cartilage corresponding to the cricoid of higher 

 forms, and traces of this also occur amongst Urodeles (e.g., Siren). 



In Anura a highly differentiated larynx is present. 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 lies between the posterior cornua of 

 the hyoid (thyro-hyals) and is supported by a thin arytenoid cartilage 



