232 EVOLUTION AND DISEASE. 
One of the most remarkable diverticula known to me 
exists in the neck of the Emu (Dromeus nove-hollandi), 
which demands consideration, as it serves as an admir- 
able physiological type of diverticula in general. 
Fic. 118.—Tracheal pouch of the Emu. (After Murie.) 
In the emu there is a natural defect in the trachea at 
a spot varying in position between the fiftieth and sixty- 
fifth rings. This spot has the form of an oblong slit, 
which in the adult bird is about seven centimetres in 
