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EVOLUTION AND DISEASE. 



One of the most remarkable diverticula known to me 

 exists in the neck of the Emu (Droinceusnovcz-hollandice), 

 which demands consideration, as it serves as an admir- 

 able physiological type of diverticula in general. 



FIG. 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 



