570 COMPARATIVE ANATOMY. 



seen to be entrusted witli the function of a vocal organ, and is 

 known as the larynx. Part, therefore, is thus differentiated from 

 the rest of the air-passage. The rest has very much the same 

 characters in its unpaired portion, or trachea, and its paired or 

 bronchial portion : the lai-ynx varies much more considerably. In 

 the Amphibia the two cartilages above mentioned, and known as 

 the arytfenoid cartilages {a), form a support for the two folds which 

 bound the entrance into the larynx. The change in the position of 

 the cartilages, which is effected by muscles, opens and closes the 

 entrance into the larynx. Functionally, therefore, they are of more 

 importance than the more indifferent parts which have the character 

 of supporting organs. These aryteenoid cartilages rest on the 

 anterior ends of the two longitudinal ridges of cartilage, which 

 in other forms are connected on the ventral surface by transverse 

 processes directed towards one another, and which thereby gave 

 rise in many Amphibia to an unpaired portion of the supporting 

 framework of the larynx {G c). 



In the Reptilia the two longitudinal ridges are more perfectly 

 connected transversely, but a low condition is implied by their 

 continuous connection with the arytaenoid cartilages ; and this is 

 especially the case in some of the Ophidii. In others, these cartilages 

 are more completely separated {D a). This character is even found in 

 the Saurii, but in them the portion which carries the arytfenoid carti- 

 lages is converted into a circular, and generally a closed piece. A 

 second portion of the larynx may then be distinguished, which forms 

 the circular cartilage, and is to be found in course of formation as 

 early as the Amphibia {G c). In the Chelonii and Crocodilini this 

 is more completely separated from the tracheal skeleton, and its 

 anterior portion is considerably widened out. Not unfrequently it is 

 possible to make out in it indications of its connection with several 

 rings of cartilage. In Birds, this circular piece is made up of one 

 anterior and broader, and two posterior and more delicate portions ; 

 a small piece is, further, placed on the latter and carries the 

 arytsenoid cartilages. 



Lastly, in the Mammalia the large circular piece of the Reptilia 

 is divided into two portions, for the anterior upright plate forms the 

 thyroid cartilage, while there is a second piece, which is circular 

 and generally very massive posteriorly (cricoid cartilage), and this 

 cai'ries the arytsenoid cartilages on its posterior and more elevated 

 portion. 



§ 426. 



Other parts, which serve more or less for the production of the 

 voice, are connected with this laryngeal skeleton. Of these, the folds 

 of mucous membrane which are placed at the sides of the entrance 

 into the larynx are worthy of note, for they are converted by tension, 

 and by the development of elastic tissue into vocal chords. They 

 enclose a fissure — the glottis, which can be altered iu width owing 

 to the attachment of the vocal chords to the movable arytaenoid 



