THE AIE-SACS. .431 



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 interclavicular, 

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

 intermediate, and a pair of abdominal or posterior. 



Make a median ventral incision through the skin of the 

 neck, and 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 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-saos 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. Garry 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 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 



