ANATOMY OF THE 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 
ramifieations, 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 offshoots, which foreshadow the air-sacs of 
birds. 
The alimentary canal consists of seven parts: the cs- 
ophagus, crop, glandular and muscular stomachs, large and 
small intestines, and cloaca. The cesophagus 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 (Cr), 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 (Cr) 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 
