DIGESTIVE TISSUES 



295 



r. c* 



about 9 by 25 microns. Their cytoplasm has been inflated by a secre- 

 tion mass so as to form a capsule with but a film of cytoplasm for 

 a wall. The small, rounded nu- 

 cleus, more or less distorted by 

 the pressure of the secretion sub- 

 stance, lies at the base of the 

 cell in the thickest part of the 

 cytoplasm. The secretion mass 

 is mucus. Fresh mucus is a vis- 

 cid, cloudy substance. In all 



Specimens fixed in formalin, ako- FIG. 260. Part of the secreting epithelium from 

 hoi, Or acids it appears as a a mucous gland in the base A the bat's tongue. 

 r r.c., resting cell (young cell?). X 700. 



dense, floccular mass which does 



not stain readily in acid stains (Fig. 260). This cell type is character- 

 istic of all mucous tissues, except that where the mucous cells are 

 scattered they are enlarged distally and are called goblet cells. 



Conductive tissues. The oesophagus of the squid is a tube with a 

 thick wall. The outer coat is composed of connective tissue covering 

 a heavy layer of circular smooth muscle fibers inside of it. Beneath 

 this is a longitudinal layer of smooth muscle. A connective-tissue sub- 



mucosa supports the in- 

 ner layer, which is com- 

 posed of very low, stout, 

 columnar, epithelial cells. 

 Each cell has a finely 

 granular, homogeneous 

 cytoplasm and an oval 

 nucleus. These cells elab- 

 orate a tough cuticle (Fig. 

 261) which is probably 

 elastic. When not in use, 

 the whole membrane lies 





in longitudinal folds. 



The (Esophagus of the 

 cat has also a thick wall 

 composed of two muscu- 

 lar layers, a connective- 

 tissue submucosa, and an 

 epithelium. It differs 

 from that of the squid in 

 having the longitudinal 

 muscles outside the circular muscles. The more significant difference, 

 however, is in the epithelium. A cuticle is not developed; instead the 



FlG. 261. Transverse section of cesophagus of squid, 

 Loligo Pealii. CM., cuticle; ep., lining epithelium; conn.t., 

 connective tissue; Lmus., longitudinal muscle fibers; 

 c.mus., circular muscle layer. X 200. 



