DIGESTIVE TISSUES 



295 



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, alco- FIG. 260. Part of the secreting epithelium from 

 hoi, Or acids it appears as a a mucous gland in the base of 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 oesophagus of the 

 cat has also a thick wall 

 composed of two muscu- 

 lar layers, a connective- 

 tissue submucosa, and an 



_ * 



epithelium. It differs 



f rom that Q f the squ ^ J n 

 . ,. 



having the longitudinal 

 muscles outside the circular muscles. The more significant difference, 

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



FIG. 261. Transverse section of oesophagus of squid, 

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

 connective tissue; l.mus., longitudinal muscle fibers; 

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



