TUMOURS AND CANCERS. 233 



length ; the width varies from eight to twelve millimetres, 

 according to the retracted or distended condition of the 

 windpipe. The mucous membrane lining the trachea 

 passes through this slit, and forms a large bag or sac, 

 which lies between the trachea and the skin (fig. 118). 

 During the breeding season, the birds inflate the sac with 

 air and produce a peculiar drumming sound which 

 resembles the booming noise made by blowing forcibly 

 across the opening of a large but narrow-necked bottle 

 This cyst is not present in the emu chick, only the slit 

 in the trachea ; as the bird grows, so the mucous mem- 

 brane becomes slowly protruded through it. On one 

 occasion I have found this cyst filled with fluid mucus, 

 due to inflammation of its walls, and on attempting to 

 relieve it, the mucus passed into the trachea and literally 

 drowned the bird. 



Among diverticula, analogous to the tracheal pouch 

 of the emu, should be mentioned the singular pouch of 

 the Bustard (Otis tarda}. 



INFECTIVE TUMOURS. Tumours belonging to this 

 group are caused by micro-organisms. They demand 

 close attention, because they are the most generalized 

 of all tumours and occur in every kind of vertebrate 

 animal. 



Structurally, they consist of small round, or spindle- 

 shaped cells, intermixed with giant-cells in variable pro- 

 portions. Infective tumours are of two classes : (a) sar- 

 comata, (&) infective granulomata. A sarcoma usually 

 appears as a tumour, and later infects the system, pro- 

 ducing secondary nodules in different organs, such as 

 the lungs, liver, &c. The infective granulomata appear 



