110 PRACTICAL ZOOLOGY. 



through the skin along the line of the ridge, or keel, 

 of this bone, and loosen the skin on each side, con- 

 tinuing forward over the crop, being careful not to 

 tear the crop ; again inflate the crop, and examine it 

 more fully. Observe the fine lines running crosswise 

 and lengthwise in the walls of the crop ; these are the 

 muscle-fibers, transverse and longitudinal. Glands in 

 the lining of the crop secrete a milky liquid, in the 

 breeding season, to act on the food and soften it ; this 

 softened food is brought up from the crop and put 

 into the mouths of the young pigeons. 



3. Loosen the crop from the front of the breast and from 

 the neck. 



4. Find the windpipe, or trachea, with its white rings of 

 cartilage. 



5. On each side of the neck is a vein and a white cord, 

 the pneumogastric nerve j the vein is the jugular 

 vein. If it does not show distinctly, let the bird's 

 head and neck hang over the edge of the table, and 

 the vein will soon fill with blood. 



6. Insert the tube into the glottis, and inflate ; observe 

 the swelling of the whole body, and the inflation of 

 the thin- walled air- sacs in the hollow in front of 

 the breastbone. 



7. Break the bone of the upper-arm, the humerus, 

 cut through the skin and muscles, and push out 

 through this opening the end of the bone next to 

 the body ; note that it is hollow ; slip one end 

 of a rubber tube over the end of the bone, and 

 inflate ; what is the result of this experiment ? 

 Keeping another tube connected with the windpipe, 

 determine whether air can be sent in through 





