182 PHYSIOLOGY AT THE FAKM. 



varies much in different birds. There is also much difference 

 in different birds in the size of the nasal and palatine openings. 



In most birds the upper jaw or mandible has a greater or 

 less mobility. In birds in general the upper jaw is united 

 into one piece with the cranium by means of elastic bony 

 plates; but in some, as in the parrot tribe, it constitutes a 

 particular bone articulated with the cranium. In a few the 

 upper jaw is immovable ; for example, in the cock of the 

 wood (Tetrao urogallus). 



There is in birds a bone named os quadratum, by which the 

 lower jaw is articulated with the cranium on both sides. 

 Connected with the os quadratum is another small bone rest- 

 ing, by its opposite end, against the palate in such a manner 

 that it elevates the upper jaw when the os quadratum is 

 carried forward by its muscles. The os quadratum is usually 

 termed the tympanic bone by recent comparative anatomists. 



The horny substance investing the two mandibles acts the 

 part of teeth, and it is even sometimes serrated so as to imitate 

 the form of teeth. 



According to Cuvier, the muscles concerned in the mastica- 

 tion of birds belong to the lower jaw and to the tympanic 

 bone (os quadratum). For example, in the domestic duck 

 there are six tympanic muscles, three external, one of which 

 represents the masseter, and three internal. Two muscles be- 

 sides the masseter raise the lower jaw, one of which corresponds 

 to the temporal, the other to the pterygoid ; there is besides a 

 large broad muscle which depresses the lower jaw.* 



In poultry the upper mandible is vaulted, and the nostrils 

 are pierced in a large membranous space at the base of the 

 beak, and covered by a cartilaginous scale. 



Salivary Glands. There are salivary glands in birds, 



* For the muscles of the jaws in birds, see Meckel, ' Anatomic Com- 

 paree,' tome viii. p. 188 et seq. Paris, 1836. 



