436 DISSECTION OF THE PIGEON. 



the lungs and into the air-sacs, the lungs being thus filled 

 with fresh air. 



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 easily 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 they can no longer be inflated. 



The air-sacs are nine in number : a median interclavicu- 

 lar, a pair of cervical or pre-bronchial, two pairs of thoracic 

 or intermediate, and a pair of abdominal or posterior. 



Make a median ventral incision through the shin of the 

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

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

 the oesophagus, and is easily recognised by the close-set rings 

 by which it is surrounded. 



Separate the trachea from the surrounding tissues in the 

 hinder part of the neck, and pass a ligature 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 blowpipe 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 close up to the ventral surface when inflated. 

 Each communicates with the posterior border 

 of the corresponding lung, near its outer angle. 



