572 



COMPARATIVE ANATOMY. 



a straight course. This is the case in the Penelopidas, various Swans, 

 and in the Crane. In the last named a loop of the trachea is even 

 enclosed in the sternum. 



The most peculiar character is the formation of a lower larynx 

 (Syrinx) in the CarinataD; as a rule this is formed by the end of the 

 trachen, and by the commencement of the bronchi. The variations 



in the form of this portion are due to 

 lateral compression, or to the fusion of 

 several rings at the end of the trachea; 

 this latter is divided by a bony ridge 

 (pessulus), which rises up at the angle 

 of furcation of the bronchi and forms 

 the tympanum. A membrane (mem- 

 brana tympaniformis interna), is 

 stretched, as if on a frame, from the 

 median surface of either bronchus to 

 the pessulus. The membrana tympa- 

 niformis externa is stretched between 

 the last tracheal and the first bronchial 

 ring, or between a pair of modified 

 bronchial rings. In the male Anatida3 

 we find various kinds of vesicular, 

 and asymmetrical enlargements of the 

 tympanum. In Singing Birds, a fold, 

 the membrana semilunaris, projects 

 inwards. As there arc vocal membranes, which are elastic folds of 

 the mucous membrane, on either bronchus, a double glottis is 

 marked off. The action of a special supply of muscles varies the 

 tension of the vocal chords, and narrows or widens the glottides. 

 Several pairs of muscles pass into the trachea, which depress it, and 

 relax the vocal chords. In addition to these, the Singing Birds are 

 distinguished by the possession of a muscular apparatus, formed of 

 five or six pairs of muscles (Fig. 323, a — -/). 



Fig. 323. Lower larynx. Muscular 

 organ of voice of the Crow. 

 A From the side, B From in 

 front, a — / Muscles which move 

 the lower larynx. g Membrana 

 tympaniformis interna. 



§ 



427. 



In, and above the Amphibia the lungs, which commence at the 

 ends of the air-passages, form the respiratory organs of the higher 

 Vertebrata. They do not form the sole organ quite at once, for in 

 all Amphibia there are branchia? either during the larval stage, or 

 throughout life (Perennibranchiata). There is a scries of similar 

 differentiations in the anatomical characters of the lungs, as in the 

 air-passages which lead to them. Simple sacs arc gradually re- 

 placed by complicated organs, the respiratory surface of which is 

 continually increased by the formation of smaller internal cavities. 



Among the Amphibia the lungs arc exactly like those of the 

 Dipnoi ; in the Perennibranchiata their internal cavity has its 

 surface only slightly increased. In Proteus and Menobranchus 

 they are formed of simple, but very long tubes, which are slightly 



