ANATOMY OF THM PIGEON. 527 



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

 verse depressions, corresponding to as many ribs. The 

 bronchi 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 

 thin-walled bags, nine in number : three near the clavicle, 

 four 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 ofEshoots, which foresliadow 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 (esophagus 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 {Or), 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 shown in 

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

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

 reaches 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 



