288 



HISTOLOGY 



'&: 



m 



conn. t. 



parts themselves from the grinding action of the stones that are kept 



in the lumen to reduce the food material. 



As in the worm's gizzard, the cuticle is formed by the inner layer 



of simple epithelium against 

 which it lies (Fig. 251). There 

 is the difference here, however, 

 that the epithelial layer of the 

 bird does not secrete the lining 

 directly from its primary sur- 

 face as a solid layer but from 

 a close-set layer of simple, tu- 

 bular glands into which it is 

 thrown. The protective mate- 

 rial is produced from these 

 glands and issues from their 

 mouths as a series of liquid or 

 semiliquid strings, which spread 

 out and fuse with one another 

 to form the cuticle. They are 

 speedily hardened superficially 

 by the acid that comes down 

 with the food from the proven- 

 triculus, and as fast as the 

 inner, wearing surface is ground 

 away it is replaced from the 

 other surface by the glands 

 beneath. 



These glands dip into the 

 submucosa and form a consid- 

 erable layer. Their secreting 

 cells are said to produce no 

 digestive juice, although the 

 cuticular lining is thought to 

 contain such materials and is 

 used as a cure for some forms 

 of indigestion. 



Among the many very pe- 

 culiar masticating structures 

 found in animals are some of 

 the " gizzards " that may be 

 seen in the anterior part of the 



digestive tract of fishes. One such gizzard will be described in the 



harvest fish, Seserinus paru. 



FIG. 251. Part of a section of the wall of the giz- 

 zard in an English sparrow, cu., cuticular pro- 

 tective layer; gl., simple tubular glands in which 

 the cuticular layer is secreted as a fluid; conn.t., 

 connective tissue; mus., muscle. X 750. 



