266 THE ORGANS OF DIGESTION 



sists of a serous, a muscular, a submucous, and a 

 mucous coat. 



The Serous Coat. This is derived from the peri- 

 toneum and surrounds the bowel completely, except 

 in the duodenum, where only the first portion is com- 

 pletely covered. Along its mesenteric border, where 

 the mesentery (a fold of peritoneum) is attached, is 

 an uncovered interval for the entrance and exit of 

 arteries, veins, nerves, and lymphatics which pass 

 between the layers of mesentery. 



The Muscular Coat. This consists of an outer 

 longitudinal and an inner circular set of muscle 

 fibers. These muscular layers propel the food along 

 the intestines, as well as assist by their action in mixing 

 it with the intestinal juices during active digestion. 



The Submucous Coat. This is composed of areolar 

 tissue and holds the mucous and muscular coats 

 together. It contains the branches of the nutrient 

 arteries to the bowel, previous to their distribution to 

 the mucous coat, also the lymph channels and nerves. 

 The lymph nodules are lodged in this layer; they are 

 pear-shaped with their apex lying in the mucous 

 membrane. These are called solitary follicles and 

 Peyer's patches. The submucous coat in the small 

 intestines extends up into the valvulse conniventes. 

 In the duodenum the duodenal glands are lodged in 

 the submucous coat. 



The Mucous Membrane. This is lined with columnar 

 epithelium. It is soft and velvety in appearance. 

 The membrane is highly vascular near the beginning 

 of the duodenum, and gradually becomes paler as the 

 lower portion of the bowel is reached. The membrane 

 is thrown into folds called valvulse conniventes. Each 

 fold is simply two layers of membrane folded upon 

 itself and held together by fibrous tissue. They 

 increase the absorbing surface of the intestinal canal 

 and retard the progress of the food, according to some 

 authors. They measure J to J an inch in width, and 



