THE ALIMENTARY SYSTEM. 



37 



The movements of the eye are controlled by an elegant sot of muscles. A 

 pectcn is found extending into the vitreous Iiiuuour, and, in addition to an 

 upper and lower lid, the eye is further protected by a third shield called the 

 nictitatmg m enibrane. 



THE ALIMKNTARY SYSTEM. 



In their Alimentary Systoii birds present us with many structures of in- 

 terest. Throughout the class we find the jaws ensheathed by hard, horny in- 

 tegument, but teeth are absent in all existing forms in the adult. Hints of 

 them have been found in some embryo parrots, and in the extinct OdontornitJies, 

 while in other extinct bird-types and forms they were also developed. 



Three salivary glands appear in birds, the sublingual pair being enor- 

 mously developed in the Plci. Great variation is seen in the tongue, and it is 

 sometimes practically absent. It is specially contrived, and extremely long, 

 in all true Woodpeckers, while it is very small in the Gannets and their 

 steganopodous allies, and in Balmniceps rex. Birds lack the velum palati. 



The length of the muscular oesophagus depends upon the length of the 

 neck of its possessor, and it may have several dilatations in its continuity 

 above the intestinal tract. In Parrots, Pigeons and Fowls, for example, a crop 



Fig-. 4. Part of intestinal tract, and the gizzard of the Jungle Fowl of India (Callus 

 bankiva). Drawn, natural size, by the author, and taken from specimens of the birds sent 

 him by Dr. Richard W. Burke, of Cawnpore, India. 



