xii.] THE EXCRETORY ORGANS. 469. 



toed Sloth (Bradypus\ in which it is bent abruptly on itself in 

 a single fold ; and with Reptiles, except some Crocodiles and 

 Chelonians in which it is similarly bent It may be still 

 more bent, even coiled (as in many Birds), and the coils may 

 lie beneath the skin, external to the pectoral muscles, as in 

 Anas senii-pahnata, or sheltered in an excavation in the 

 keel of the sternum, as in the common Crane and male wild 

 Swan. 



In that there are two bronchi and no more, man agrees 

 with the great majority of Vertebrates that possess a trachea. 

 One bronchus, however, may abort with its appended lung (as 

 in many Serpents), or there may be a third bronchus, as in 

 Ruminants, the Hog, and Cetaceans. 



The rings of the trachea may be ossified, as in most Birds. 

 They may be as few as three, as in the Dugong ; or as mary 

 as no, as in the Camel ; or even 300, as in the Flamingo and 

 the Boa. They may completely encircle the tube, as in Birds 

 generally, and in the Horse (though in most Mammals they 

 are incomplete behind, especially in Rodents). Some of 

 the rings of the trachea may be incomplete in front, as in the 

 Emeu. 



The trachea may increase in size downwards, as in the 

 Turkey ; or it may present a more sudden enlargement before 

 bifurcating to form the bronchi, as in the Merganser ; or two 

 such enlargements, as in the Goosander. It may be divided 

 by a septum extending from above downwards, as in the 

 Penguin and Sphargis. 



A special structure may be developed (as in most Birds) 

 at the junction of the trachea and bronchi. This is the syrinx, 

 and is the organ of song in singing birds. It is also called 

 the lower larynx, and is quite a different part from the true 

 or upper larynx, shortly to be described. 



The syrinx is generally formed by the coalescence or modi-, 

 fied shape of the lower rings of the trachea and the upper 

 ones of the bronchi, the latter being incomplete internally, so 

 that the sides of the bronchi which look towards each other 

 are at their upper part merely membranous, forming the 

 membrana tympaniformis. Internally the syrinx is generally 

 divided by a bar of bone, from the upper margin of which a 

 delicate membrane ascends into the trachea, ending in a free 

 concave margin, whence it is termed the membrana semi- 

 litnaris. This highly vibratile membrane, together with two 

 elastic folds of mucous membrane, placed one on each outer 

 side of the commencement of each bronchus, are the special 



