LUNGS OF VERTEBEATA. 571 



cartilages. These cliords are found in most of tlie Anura^ in the 

 Greckos and Cham^eleons among the Saurii, and in the Orocodilini. 

 They are not found in the Ophidii. 



In BirdSj the vocal organ is placed in the lower portion of the 

 trachea, and forms the so-called lower larynx. This arrangement 

 compensates for the absence of vocal chords in the true larynx. 

 Among Mammals the chords are atrophied in the Cetacea only ; in all 

 essential points they present just the same arrangements as in Mau. 



As the various cartilaginous pieces are differentiated from the 

 primitive laryngo-tracheal musclCj muscles are separated off to move 

 the portions which have become free. In the Reptilia these are 

 replaced by a constrictor and a dilatator muscle, which are also 

 present in a modified form in Birds. In the Mammalia, the 

 musculature is complicated in comparison with the simpler arrange- 

 ment found in Reptiles ; this is seen partly in the number, and partly 

 in the arrangement of the muscles. They correspond in all essential 

 points to what obtains in Man. 



A pi'ocess which hangs over the entrance to the laiynx from in 

 front — the epiglottis — is merely indicated in the Reptilia by a process 

 of the supporting cartilage ; this is sometimes of no small size. It 

 is well developed in Birds. Many Birds, however, are provided with 

 a special epiglottis, the cartilage of which is only suturally connected 

 with the supporting cartilage. But these arrangements never 

 succeed in completely covering the entrance into the larynx. The 

 cartilage of the epiglottis is more completely separated in the 

 Mammalia, where it forms a protective apparatus during the passage 

 of the food over the entrance to the larynx. In the Sirenia it is 

 atrophied, while in the Cetacea it is converted into a long spout-like 

 piece, which unites with the similarly elongated arytsenoid cartilages 

 to form a cone, which projects into the internal nares, through 

 which the air passes in and out. 



In some of the Amphibia the portion of the air-passage which 

 commences at the larynx is more distinctly divided into the trachea 

 and its two branches, the bronchi; these pass directly into the 

 walls of the pulmonary sacs. The ends of the laryngo-tracheal 

 cartilages extend into these, either in the form of finer processes 

 (Menobranchus, Menopoma), or as broader pieces which give off 

 lateral processes (Bafo). As the transverse processes grow towards 

 one another at the anterior end of these ridges (cf. Fig. 322, G b), 

 they represent the earliest cartilaginous rings. These rings are 

 developed on the trachea of the Reptilia, which is generally long, 

 and either are, or are not complete. In the Ophidii and Saurii a 

 remnant of the primitive arrangement is indicated by the connection 

 of the rings with one another by means of longitudinal ridges. 



The trachea of Birds is always distinguished by its great length, 

 and the rings, which are generally completely closed, are always more 

 largely differentiated. The two bronchi have the same structure. 

 We frequently meet with enlargements' at various points in the 

 trachea (Natatores), while in many Birds this tube does not keep to 



