DIGESTIVE TISSUES 



287 



lura the digestive cells are most numerous in the region of the opening 

 of the enteron. At no place, however, are these digestive cells as- 

 sembled to form a tissue. Food vacuoles yet function to a certain 

 degree. In certain absorbing cells such vacuoles are occasionally found 

 (see Fig. 248). 



EXAMPLES OF ACCESSORY DIGESTIVE STRUCTURES 



Masticating Structures. Gizzard. 

 a region of the alimentary canal in 

 which the muscular layer is most 

 highly developed. The layer of cir- 

 cular muscles lies next to the sub- 

 mucosa, and is much thicker than 

 the outer longitudinal layer. The 

 elements of this muscular tissue are 

 smooth, non-striated muscle cells 

 (see Fig. 96). The epithelium is 

 composed of columnar cells. The 

 cytoplasm of these cells is finely 

 granular and homogeneous. The 

 oval nuclei lie at the middle of the 

 cell. The basement membrane is 

 clearly defined. At their distal ends 

 the cells elaborate a heavy cuticle 

 which is constantly being formed as 

 it is worn down by abrasion in 

 grinding the food. Many lympho- 

 cytes find their way through the 

 basement membrane into the epithe- 

 lium (Fig. 250). 



In the gizzard of vertebrates, as 

 represented by the bird's gizzard, 

 we find that a short portion of the 

 digestive tube is enlarged and pro- 

 vided with unusually thick muscu- 

 lar walls in order that there may 

 be grinding power to triturate the 

 food. This is what has taken place 

 in the worm's gizzard, and the 

 similarity is further made apparent 

 layer of substance is placed on the 



The gizzard of an earthworm is 



\ 



m./. 



FIG. 250. Transverse section of part of the 

 gizzard wall of an earthworm, Lumbricus. 

 cu., cuticle; en.c., endoderm cells; cir.m.f., 

 circular muscle fibers; l.m.f., longitudinal 

 muscle fibers; conn.t., connective tissue. 

 X 400. 



by the fact that a heavy cuticular 

 internal surface to protect the soft 



