ANATOMY OF BIHDS 



303 



principal bony balf-rings, as one or tte other end may be pulled, 

 being made to perform a slight rotatory motion, are incalculable ; 

 but their effects are delightfully appreciable by the rapt listener to 

 the singularly varied kind and quality of notes trilled forth in the 

 stillness of gloom by the nightingale." 



I should be able to make the plan of the syrinx clear to the 

 student with the assistance of Macgillivray's beautiful figures. 

 These are drawn from the rook, — a corvine croaker, indeed, but 

 one whose syrinx is in good order, though he has never learned to 

 play. As the modifications afi'ect principally the soft parts covering 



Fig. 100. — Coiling of the windpipe in the sternum of Grus canadensis ; reduced. 



and moving the music-box, one description of the latter is applicable 

 to most birds. The last lower ring, or piece composed of several 

 fused rings, of the trachea, at its bifurcation into bronchi, is enlarged 

 or otherwise modified (Fig. 101, ^^, aba), and crossed below from 

 front to back by a bony bar, the ^essulus (i*, at b ; ^^, a), or bolt-bar, 

 which, dividing it into lateral halves (as at ^% forms thus two 

 lateral openings instead of one median tube, — the beginnings of 

 each bronchial tube. A membranous plate, strengthened by 

 cartilage, rises vertically into the tracheal tube, forming a septwm, or 

 median partition, between the orifices of each bronchus. The free 

 curved upper margin of this septum, extending from front to back 



