THE DIGESTIVE AND RESPIRATORY SYSTEM 313 



pieces into an anterior pair of arytanoids, upon either side 

 of the glottis, and a posterior pair of tracked pieces. Within 

 the Class of Amphibia there are no new pieces formed beyond 

 these, but they exhibit a great variety of forms, and become 

 especially complex in the Anura, where they are employed in 

 the production of various sorts of notes used as sexual calls 

 (Fig. 87, c-e). The muscles associated with these skeletal 

 elements consist originally of a pair of dilatators, which are 

 attached to the outer edges of the cartilages and serve to 

 draw them apart, and a double pair of adductors, the laryngei, 

 which stretch across from one to another and serve to approxi- 

 mate them. These give rise in many of the more complicated 



A 



a b c d e 



FIG. 87. Laryngeal cartilages of various Amphibians. 



(a) Necturus (mud-puppy), (b) Proteus. (c) Amphiuma. (d) Triton (Newt), 

 (e) Rana (frog). 



cases to an entire system of muscles, mainly connected with 

 the arytaenoid cartilages, which form the essential skeletal or- 

 gan of the larynx, and to which the vocal cords in the form 

 of mucous folds become attached. 



In the Sauropsida there are two conspicuous points of ad- 

 vance ; the one concerns the larynx, the other the trachea. The 

 first consists of the addition of the 4th pair of branchial car- 

 tilages, which become reduced in size, unite in the middle and 

 form a triangular flap, the epiglottis; this, during passive 

 breathing, stands erect above the glottis but shuts down over 

 the latter during the act of swallowing, thus preventing the 

 entrance of solid food into the trachea. The second advance 

 consists of the presence of a series of rings of approximately 



