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 (Odz¢s 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: (@) sar- 
comata, (4) 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 
