390 DISSECTION OF THE PIGEON 



A complete dissection of the air-sacs is not easy to per- 

 form, and involves a good deal of damage to other structures. 

 The main features in their distribution and relations can, 

 however, be readily determined, as described below. The 

 dissection of the air-sacs must precede that of the other 

 systems, as if they are once cut into, the walls of the sacs 

 collapse, and can no longer be inflated. 



Make a median ventral incision through the shin of the 

 neck, and pin out the flaps. Note the asophagus, a wide 

 fleshy median tube ; and the trachea, which lies to the left of 

 the oesophagus, and is readily recognised by the close-set 

 cartilaginous rings with which it is surrounded. 



Separate the trachea from the surrounding tissues in the 

 hinder part of the neck, and pass a ligatu/re round it. Slit 

 open the trachea about the middle of its length : pass a blow- 

 pipe down the trachea through the slit, and tie it firmly in. 

 Inflate the lungs and air-sacs through the blow-pipe, and tighten 

 the ligature round the trachea to prevent the air escaping. 



Make a mid-ventral incision through the skin from the 

 posterior edge of the sternum to the cloaca, and a transverse 

 incision along the posterior border of the sternum. Carry the 

 incisions through the muscles of the abdominal walls, taking 

 care not to injure the viscera beneath, and reflect the flaps 

 outwards. 



1. The posterior or abdominal air-sacs are a pair of large 



sacs, with very thin transparent membranous walls, 

 which He along the dorsal wall of the pelvic cavity, 

 ventral to the kidneys but .dorsal to the intestine. 

 At the hinder end of the abdomen they come up close 

 to the ventral surface when inflated. 



Each communicates with the posterior border of 

 the corresponding lung, near its outer angle. 



2. The posterior thoracic air-sacs are a pair of rather 



smaller sacs, lying along the outer sides of the anterior 

 ends of the abdominal sacs. They lie mainly behind 

 the lungs, but overlap the ventral surfaces of their 

 hmder ends. 



