DIGESTIVE TISSUES 



289 



This organ consists, unlike 

 the other gizzards, of an en- 

 larged, muscular portion of the 

 oesophagus, whose inner surface 

 is-beset with tooth- like structures 

 and invaginated between these 

 teeth into the glands. The epi- 

 thelium is primarily a stratified 

 one and continues to possess this 

 characteristic where it is in- 

 volved in the tooth formation. 

 In its relation to the glands it is 

 carried down into them, its more 

 distal cells showing a marked 

 secretory activity, probably of 

 mucin. In the fundus the outer 

 cells have become so markedly 

 columnar and so active as se- 

 cretory cells that one has diffi- 

 culty in deciding that it is not a 

 true, simple, columnar epithe- 

 lium. . 



The presence of a certain 

 amount of stratification at the 

 base, however, together with its 

 origin, furnish ample evidence 

 that it is in reality a pseudo- 

 stratified form. 



The teeth are mesodermal in 

 formation, the epithelium taking 

 no share in their formation. 

 They arise from a common base 

 which is a stiff basket work of 

 hard tough fibers that arise in 

 the subepithelial connective tis- 

 sue through the activities of 

 some of its cells. The shell 

 thus formed is elastic enough to 

 allow of the grinding move- 

 ments of the gizzard. In fact, 

 it is not complete in the median 

 line, thus forming two halves 

 which grind upon one another. 



FIG. 252. A single compound tooth from the 

 cesophageal gizzard of the harvest fish, Sese- 

 rinus paru. /., transverse and oblique section of 

 the deep and hard reticulum, from which the 

 tooth arises; r.b., two sections of the basal ring, 

 from which the sides of the skeletal core (s.c.) 

 arise to support the papilla; sp.t., spike-shaped 

 cusps which pass through the stratified epithe- 

 lium; conn.t., connective tissue between skeletal 

 core and epithelium; s.c., soft core of connec- 

 tive tissue with fat cells, blood vessels, and lym- 

 phatics; gl., glands at base of tooth, x 80. 



