COLUMBA. 363 



lumerus. By this arrangement a contraction of the sub- 

 lavian results in raising the wing. The tendon runs along 

 )eside the large coracoid bone, and on the outer side of this 

 )one originates a small triangular muscle called the coraco- 

 rac/iialis, the tendon of which is inserted in the head of the 

 lumerus. It apparently helps in depressing the wing. 



The large keel of the sternum is developed in response to 

 he necessity for a large area of attachment for the " flight- 

 Quscles." In birds w^hich do not fly the keel is absent. 



The sternum may now be removed by cutting round its 

 dge posteriorly and laterally, and the abdominal cavity 

 aay be opened by a median ventral incision. 



The air-sacs should be noticed, large cavities with thin 

 i^alls. They are, nine in number, and communicate with 

 he lungs {v.i,). Some are also produced into the interior 

 •f the bones, such as the humerus. Three pairs of them lie 

 >ehind each other in a row on each side of the viscera, from 

 .^hich they are separated by an oblique septum. 



Down the neck may be noticed two long tubes, one 

 tiffened with bony rings, the trachea^ and the other soft, 

 ^. which is the oesophagus. The trachea can be 



traced down to the thorax where it passes 

 iorsal to the heart. The oesophagus expands into a large 

 h in- walled sac, the crop., from which it passes into the 

 lody-cavity dorsal to the heart and terminates in a 

 landular stomach. The stomach opens directly into a 

 irge round gizzard with very thick muscular walls. The 

 rst loop of the small intestine is, as in other types, termed 

 he duodenwn, and in its loop there rests a whitish pancreas. 

 rhe liver is bilobed and lies over the gizzard ; it has two 

 ile-ducts. The left opens into the proximal loop of the 

 iuodenum and the right into the distal; the left is thick 

 nd short but the right is longer and more delicate. The 

 lancreas has no less than three ducts which open into 

 be distal loop of the duodenum. The rest of the s^nall 

 ntestine is coiled and of considerable length. It ends 

 a a short rectum., and at the junction below the two 

 3 a pair of small pockets, the rectal cceca. The rectum 

 ipens into the cloaca. 



The alimentary system presents some peculiar characters. 

 Ul modern birds like the pigeon have no teeth, though 



