402 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. 



The air-sacs are nine in number ; a median interclavicular, 

 a pair of cervical or pre-bronohial, two pairs of thoracic or 

 intermediate, and a pair of abdominal or posterior. 



Make a median ventral incision through the skin of the 

 neck, a/nd pin out the flaps. Note the oesophagus, 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 by which it is surrounded. 



Separate the trachea from the surrounding tissues in the 

 hinder part of the neck, and pass a Ugature round it. SUt 

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

 pipe down the trachea through the sUt, and tie it flrmly in. 

 Inflate the lungs andavr-sacs through theblow-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 inpire the viscera beneath, and reflect the flaps 

 ■ outwards. 



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

 sacs, with very thin transparent membranous walls, 

 which lie 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. 



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

 smaller sacs, lying along the outer sides of the anterior 



