ANATOMY OF THE PIGEON. 527 



Jlacli lung has upon its outer and dorsal surface five trans- 

 Terse depressions, corresponding to as many ribs. The 

 l)ronchi and pulmonary blood-vessels enter together the 

 :anterior third of the lungs, and follow one another in their 

 ramifications, but the bronchus traverses the lungs, giving 

 off numerous branches, and opens into the abdominal air- 

 sac, while upon the surface of the lungs there are small 

 openings communicating with the remaining air-sacs. 

 These structures the student had best tear through and 

 altogether neglect in his first dissection. The air-sacs are 

 i;hin-walled bags, nine in number : three near the clavicle, 

 ^our in the thorax, and two in the abdomen ; their ramifi- 

 •cations extend even into the bones, most of which are ac- 

 cordingly found to be hollow. This striking organization 

 is one of the most characteristic peculiarities of birds, and 

 ^serves to lighten the body by filling very large spaces with 

 air, besides fulfilling certain other less obvious functions. 

 In many chameleons and some Geckos the lungs have di- 

 verticula or offshoots, which foreshadow the air-sacs of 

 birds. 



The alimentary canal consists of seven parts : the oes- 

 ophagus, crop, glandular and muscular stomachs, large and 

 -small intestines, and cloaca. The oesophagus extends about 

 "three fifths of the way down the right side of the neck, and 

 is approximately of the same diameter as the trachea, with 

 regard to which, as before mentioned, it lies symmetrically. 

 It opens into the crop {Gr), a thin- walled sac, which fills 

 "the triangular space between the base of the neck and the 

 keel of the sternum, and forms a large part of the curved 

 outline of the breast. In the specimen figured, the left half 

 •of the crop has been removed to show the irregular folds 

 upon the inner surface, the deep lateral pouch and the 

 three posterior longitudinal folds of one side, which serve 

 to guide the food onward to the stomach. As showr in 

 Fig. 459, D, the crop {Cr) ends just to the right of and 

 •above the trachea, in a dorsally-placed, narrow tube, that 

 Teaches to the origin of the bronchi, and there gradually ex- 

 pands into the glandular stomach, which cannot, however, 

 ■be seen in a general dissection, while the heart, lungs, and 



