Sauropsida: Class Aves 527 



projecting forward into the cavity of the syrinx. This semilunar 

 membrane is supported by a delicate rod of cartilage or bone, the 

 pessulus. Small muscles situated in the walls of the syrinx (Fig. 

 411, m) regulate the tension in the several vibratory membranes, thus 

 changing the pitch and quality of the sound. The location of the semi- 

 lunar membrane indicates that it is especially important in production 

 of sound. The syrinx varies greatly in details of structure, being more 

 or less complex according to the vocal abilities of the species of bird. 

 In some birds (e.g., ducks) it is asymmetric, the enlargement to form 

 the resonance chamber being mostly or entirely on one side. 



Circulatory System 



The circulatory organs of birds differ from those of reptiles in one 

 highly important respect. In reptiles the heart and arteries are so 

 arranged that the blood from the lungs, received in the left chambers 

 of the heart, is mixed with "impure" blood from the right side of the 

 heart before it passes into the general arterial circulation. For a "cold- 

 blooded" reptile with relatively slow metabolism, it is not a serious 

 matter if the blood is not at its possible best. For the bird such an 

 arrangement, counteracting the high efficiency of the lungs, would be 

 most unfortunate. The result would be equivalent to trying to get the 

 best performance out of an airplane engine with insufficient air mixed 

 with the fuel. 



In reptiles the connection between the heart and the dorsal 

 aorta is a complete aortic arch (the fourth of the embryonic series) 

 consisting of a right and a left arterial trunk. Curiously, however, the 

 trunk which ascends from heart to aorta on the right side of the body 

 connects with the left ventricle and therefore carries oxygenated 

 blood, while the trunk passing up on the left side of the body carries 

 "impure" blood from the right ventricle (Fig. 1ZE). The blood dis- 

 tributed to the body by the dorsal aorta is therefore mixed — even in 

 the crocodilians whose ventricle is completely divided into right and 

 left chambers. 



In birds the heart is completely divided into four chambers, right 

 auricle and ventricle and left auricle and ventricle. At an early stage in 

 the development of the bird, the heart is connected with the dorsal 

 aorta by the complete fourth aortic arch, consisting of right and left 

 trunks. In rare cases the two trunks cross just in front of the heart and 

 connect with the ventricles in reverse right-and-left order, as in adult 

 reptiles. Usually, however, the left trunk begins to degenerate before 

 the internal subdivisions of the heart and its excurrent trunks have 

 been completed, so that the full equivalent of the reptilian arrange- 

 ment is not attained. In all cases the left aortic trunk eventually dis- 



