NUTRITION 373 



3. The lining of the small intestine is thickly covered with 

 microscopic projections (villi). 



The villi are so fine and so numerous, that, under a lens, the 

 intestinal lining looks like a piece of velvet. By these means the 

 absorbing surface is increased five times, so that the total area of 

 the intestine is not less than twenty-five square feet, or about twice 

 as great as that of the skin. 



Muscular Action. The intestinal walls are provided with layers 

 of involuntary muscles which perform two functions by their con- 

 traction and expansion. 



1. They mix and separate the food, thus constantly exposing it 

 to digestive action. 



2. They keep the food moving slowly through the digestive 

 canal. 



The efficiency of digestion and absorption depends as much 

 upon these muscular movements as upon the chemical action of 

 the digestive fluids, themselves. To provide the fluids for intestinal 

 digestion there are three kinds of glands, (1) the intestinal glands, 

 (2) the liver, (3) the pancreas. 



Intestinal Glands. The intestinal glands are small, simple and 

 very numerous, being located in the lining among the villi. They 

 secrete a strongly alkaline fluid containing sodium carbonate and 

 also enzymes that act on starches and sugars. This sodium car- 

 bonate (and other alkalis from the pancreatic fluid) combine with 

 part of the fats, forming soaps, which are soluble and are thus 

 absorbed. 



The Liver. The liver is the largest gland in the body. It is 

 located between the diaphragm and stomach, thus being the upper- 

 most of the abdominal organs. The secretion of the liver is called 

 bile and is a thick brown liquid, of which about one quart is 

 produced daily. Bile has several important functions, as 

 follows: 



1. Bile is, itself, a waste substance, removed from the blood. 



2. It aids in digestion and absorption of fats. 



3. It stimulates the action of the intestine. 



4. It tends to prevent decay of intestinal contents. 



