136 The Vertebrate Organ Systems 



numerous villi which give the surface a velvety appearance. Each 

 villus is approximately 1 mm. in length, and there are from ten to 

 forty per square millimeter. The folds and the villi increase the sur- . 

 face of the small intestine enormously. Some of the epithelial cells 

 are enlarged into goblet cells which secrete a lubricating mucus. Be- 

 tween the villi are tubular depressions, the intestinal glands, which are 

 lined with epithelial cells. The mucosa of the large intestine is very 

 smooth, lacking the villi and folds of the small intestine. On the sur- 

 face are the numerous openings of the tubular glands which secrete 

 a lubricating mucus. 



-ij|^{ fftirrl GLANDULAR 



^^W C?^ EPITHELIUM 



GOBLET CELL 



^ IMTFITIMAI 



26%iS; GLAND 



LONGITUDINAL. 



MUSCLES 

 SEROUS LAYER 



Fig. 42. — Portion of cross section of the small 

 intestine (somewhat diagrammatic). 



The submucosa and serous coats are very similar throughout the 

 digestive tract. The arrangements of the muscles forming the muscu- 

 lar layer is also quite similar in all parts. The only exception to this 

 is found in the stomach where there is a third inner, oblique layer, 

 which is confined to the cardiac end. 



The Mesenteries.- — Supporting the entire digestive tract are the 

 mesenteries (Fig. 39) which are continuous with both the visceral 

 and parietal peritoneum. Due to the enormous length of the digestive 

 tract, these membranes are much folded. The blood vessels supply- 

 ing the digestive tract are located in them. 



