Xii PHYLUM CHORDATA 477 



At the junction of the trachea with the bronchi is found 

 the characteristic vocal organ, the syrinx (syr), occurring in 

 no other class. 



The lungs (Fig. 287, Ing) are very small in comparison 

 with the size of the bird, and are but slightly distensible, 

 being solid, spongy organs, not mere bags with sacculated 

 walls, as in Amphibia and many reptiles. Their dorsal sur- 

 faces fit closely into the spaces between the ribs and have 

 no peritoneal covering ; their ventral faces are covered by a 

 strong sheet of fibrous tissue, the pulmonary aponetirosis or 

 pleura, a special development of the peritoneum. Into this 

 membrane are inserted small, fan-like costo-pulmonary mus- 

 cles, which arise from the junction of the vertebral and sternal 

 ribs. 



Each main bronchus gives off secondary bronchi, and 

 these branch again, sending off tubes which end blindly near 

 the surface of the lung and give off blind dilations commonly 

 known as alveoli. In addition to these, each main bronchus 

 also gives off branches which end in a series of thin-walled 

 air- sacs, which lie in the body-cavity, and are in communi- 

 cation with the pneumatic cavities of the bones. 



The heart (Fig. 288) is of great proportional size, and like 

 that of the crocodile consists of four chambers, i.e., the right 

 and left auricles, and right and Jeft ventricles. There is no 

 sinus venosus, that chamber being, as it were, absorbed into 

 the right auricle (Fig. 288, A, r. aii). The right ventricle 

 (Fig. 288, B) partly encircles the left, the former having a 

 crescentic, the latter a circular cavity in transverse sections. 

 The left auriculo-ventricular valve has the usual membranous 

 structure consisting of two flaps connected with the wall of 

 the ventricle by tendons, but the corresponding valve of the 

 right side (R. V) is a large muscular fold, very characteristic 

 of the class. 



