34 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



c) The muscular coat (tunica muscularis), the remainder of the intestinal 

 wall, consisting of two strata: 



(1) The circular muscle layer (stratum circular e) the inner layer, in which the 

 muscles run in a circular direction. 



(2) The longitudinal muscle layer (stratum longitudinals) , the outer layer, 

 in which the muscles run in a longitudinal direction. This layer also contains 

 a considerable quantity of connective tissue and hence takes more of the blue 

 stain than the preceding layer. 



d) The serous coat (tunica serosa, or visceral peritoneum], a very thin mem- 

 brane covering the outside of the intestine, invisible under the low power. It is 

 not really a part of the intestinal wall but the peritoneum investing the intestine. 



2. Detailed structure of the intestinal wall. Select a favorable place where 

 the mucous membrane is cut parallel to the long axes of its cells. The best 

 places are generally at the bottom of the folds. Study with the high power. 



a) The mucous membrane consists of the lining epithelium of the intestine 

 and subjacent parts. This lining epithelium is composed of a single layer of 

 tall columnar cells, each extending from the cavity of the intestine to the under- 

 lying connective tissue (stained blue). These cells were studied in Section III, 

 B, i, b. The cell walls are frequently difficult to see. Each cell possesses 

 near its base a large oval granular nucleus, stained red or orange. Why do the 

 nuclei appear so numerous (A)? Are they all in the same plane (determine this 

 by changing the focus)? The free ends (ends next the cavity) of many of the 

 epithelial cells contain flask-shaped depressions, called goblets, which are filled 

 with mucus. As mucus is chemically related to connective tissue, the goblets 

 are dyed blue by Mallory's stain. There are thus two kinds of cells in 

 the epithelium, the ordinary columnar cells and the goblet cells. Understand the 

 different appearance of various parts of the mucosa *by considering that 

 the plane of the section may not necessarily be parallel to the long axes of 

 the epithelial cells. 



The tissue immediately in contact with the epithelium and accompanying 

 it in all its foldings is considered to be a part, of the mucous membrane. It is 

 designated as the tunica propria or stroma, and consists of loosely woven white 

 connective tissue fibrils, which run in general parallel to the epithelium. The 

 connective tissue cells (of which only the nuclei are distinctly visible) are more 

 numerous here than in the next layer. However, no sharp distinction can be 

 drawn between the tunica propria and the submucous coat. The tunica propria 

 is abundantly perforated by blood and lymph spaces, and contains little nests 

 of cells whose function is not clear. 



b) The submucous coat is composed of white fibrous connective tissue con- 

 tinuous with the tunica propria, but its fibers do not run parallel to the cavity 

 of the intestine. Note in it the scattered connective tissue cells and the many 

 large lymph and blood vessels. 



