46 ANIMAL BIOLOGY. [Parti. 



edge of the sternum to the cloaca. The contents of the peri- 

 toneal cavity or coelom will be partially hidden by the great 

 omentum, a sheet of membrane loaded with fat. By lifting it, 

 the coils of the intestine and the membranous air-sacs will be 

 displayed. Make transverse incisions along the posterior border 

 of the sternum for a short distance on either side of the median 

 line, being careful to injure as little as possible the air-sacs. It 

 will be well at once to describe the position of the chief air-sacs. 

 They are membranous bags with transparent walls, and are con- 

 nected with the respiratory system by tubes which pass through 

 the substance of the lungs. 



The air-sacs are nine in number, eight lateral in four pairs, 

 and one median, resulting from the confluence of a fifth pair. 

 There is one pair cervical, or prebronchial, lying above the lobes 

 of the crop. Then follows the median inter clavicular, resulting 

 from the union of a sub-bronchial pair, and sending out prolonga- 

 tions towards the arm-pits. It lies in the fork of the merry- 

 thought. The third and fourth pairs (anterior and posterior 

 thoracic, or intermediate) are covered over by the sternum. The 

 last pair (abdominal or posterior) are those seen on opening 

 the ccelom. The ends of the posterior thoracic are just 

 visible beneath (dorsal to) the sternum without further dis- 

 section. 



The sternum must now be removed, an operation that 

 requires some little care. The central keel or carina will be 

 seen in the mid-line of the thorax. Having freed the crop from 

 its anterior edge, cut into the muscles of the breast by a longi- 

 tudinal incision on each side of the carina. This incision should 

 be just deep enough to sever the great pectoral muscle without 

 cutting the subclavian muscle that lies beneath it (on its dorsal 

 side). With the handle of the scalpel these two muscles may be 

 separated in the mid-region of the sternum, and the edges of the 

 great pectoral may then be separated from their anterior attach- 

 ment to the merrythought (furcula), and their posterior attach- 

 ment to the hinder border of the sternum. In reflecting the 

 pectorals, the large pectoral veins will be brought into view, 

 probably gorged with blood. If they be cut the blood will gush 



