INTRODUCTORY 19 



a relatively enormous cavity, having extremely thin walls, lying 

 immediately under the skin of the neck, and opening im- 

 mediately under the tongue. Peculiar to the male, it must be 

 regarded as a variation of the pouch described just now in the 

 Adjutant. Only one other bird possesses a similar chamber, and 

 this is the Musk Duck {Biziura lobatd). But here the pouch is 

 very small, and is lodged in a sac depending from the under- 

 side of the lower jaw. 



The pouch of the Emu is formed by an evagination of the 

 lining membrane of the windpipe, and makes its escape along 

 the ventral aspect of the trachea, near its middle, where the 

 tracheal rings for some distance are cut away, as it were, to 

 permit of the exit of the pouch (p. 401). 



The pulmonary system of air-sacs make their first appear- 

 ance in the embryo at about the eleventh day, as small vesicles 

 from the surface of the lungs, formed by dilations of branches 

 of the bronchial tubes. As they develop they push the peri- 

 toneal membranes before them. Thus they acquire a two- 

 layered wall — the outer serous layer formed by the peritoneum, 

 the inner by the lining of the bronchial tubes from which they 

 take origin. 



The Digestive System 



The digestive system of birds betrays its reptilian origin 

 in many ways, but perhaps most markedly so in the arrange- 

 ment of the annular and longitudinal muscular layers of the 

 intestine, the longitudinal layer lying within the annular, while 

 in the mammals the reverse order obtains, the longitudinal 

 layer, with the serosa, forming the outer wall of the tube. 



In most birds the food is swallowed without any attempt at 

 mastication, and passed backwards into the oesophagus. As 

 a rule this tube, in the region of the furcula, dilates to form 

 a more or less globular or bi-lobed and thin-walled " crop ". 

 Here, mixed v/ith saliva and water, and warmed by the heat 

 of the body, the food is softened and passed on to the stomach. 

 This again is divisible into two distinct portions, an anterior, 

 the " Proventriculus," with thick glandular walls, and a posterior, 

 the " Ventriculus " or " Gizzard," whereof the walls are thick 

 and muscular, and furnished with a more or less rugous and 

 indurated inner surface. In grain-eating birds the inner walls 

 of the gizzard are furnished with a pair of apposed and greatly 



