624 AVES [CH. 



which stiffen it form complete rings, so that it is not easily com- 

 pressed (Fig. 312). Like most other land vertebrates, birds have a 

 larynx or organ of voice at the top of the trachea formed in the 

 usual manner by the enlargement of some of these rings of cartilage, 

 and the stretching of a thin membrane between them and two 

 special cartilages, the arytenoids, which lie at the opening of the 

 windpipe into the gullet. The larynx however appears to be 

 functionless, and the effective organ of voice in Birds, the syrinx, 

 is found much deeper down, at the spot, namely, where the wind- 

 pipe splits into two tubes, the bronchi, which lead to the lungs. 

 The last rings surrounding the trachea just before it bifurcates are 

 more or less fused with their successors and predecessors so as to 

 form a box with stiff walls called the tympanum. The inner walls 

 of the bronchi, just where they join one another, are thin and mem- 

 branous, and constitute a membrana tympaniformis interna. 

 From the fork a flexible valve, termed the membrana semi- 

 lunaris, projects up into the tympanum, and as here the cartilage 

 rings have the form of half-hoops, which are drawn together by 

 special muscles, the width of the opening of the bronchus into the 

 windpipe is small. When air is forcibly expelled the valve above 

 mentioned is set vibrating like the reed in an organ-pipe, and by 

 this mechanism the song is produced. The muscles which connect 

 the half-rings together (intrinsic muscles) and two which connect 

 the syrinx with the sternum (extrinsic muscles) by altering the 

 tension of the sides of the trachea, and consequently the rate at 

 which it vibrates, change the pitch of the note produced. A syrinx 

 such as we have described is found in the vast majority of birds. 

 It is termed a broncho-tracheal syrinx because both bronchi and 

 trachea are concerned in its formation. In a few North American 

 birds a tracheal syrinx is found in which the organ of voice is 

 constituted by a portion of the trachea where the rings are thin and 

 delicate, so that the sides are flexible. In a few birds allied to the 

 Cuckoo there is a bronchial syrinx, a thin flexible membrane being 

 formed about the middle of each bronchus by the incompleteness of 

 some of the rings. 



The alimentary canal commences with the buccal cavity or 

 stomodaeum, partially divided by the palatal flaps into an upper 



air-passage, and a lower food-passage. The flaps as 

 Systfm. tlve we nave seen are stiffened by the maxillopalatines. 



The tongue, which is pointed and horny, ensheaths 

 the glossohyal bone; it is protruded by the action of muscles 



