102 HUMAN PHYSIOLOGY. 



Intestinal Movements. During intestinal digestion the walls of the 

 intestine exhibit two kinds of movement, viz., a rhythmic segmentation and a 

 peristalsis. By the former the food is divided into segments and by the 

 latter, it is carried down the intestine. Shortly after the entrance of the food 

 into the intestine, segmentation begins by a contraction of bands of circular 

 muscle fibers. So soon as a mass of food is divided into segments each seg- 

 ment is in turn again divided by similar contractions. The lower half of each 

 segment then unites with the upper half of the segment below to commingle with 

 it and to expose new surfaces of the food mass to contact with the intestinal 

 juices and to the mucous membrane. A continual repetition of this process 

 results in a thorough mixing of the food with the digestive juices. Subsequent 

 peristaltic waves slowly carry the food down the intestine. 



The large intestine extends from the ileo-cecal valve to the anus, and is 

 about five feet in length. Like the stomach it consists of three coats: the 

 serous, the muscular, and mucous. The mucous membrane contains a 

 number of mucous glands, the secretion from which lubricates the surface 

 of the canal. The ascending portion of the large intestine possesses the 

 power of absorption, and hence its contents become less liquid and more 

 consistent. As the residue passes toward the sigmoid flfexure it acquires 

 the characteristics of fecal matter. This residue consists of the undigested 

 portions of the food, decomposition products, mucus, and inorganic salts. 



Defecation is the voluntary act of extruding the feces from the rectum, 

 and is accomplished by a relaxation of the sphincter ani muscle and by the 

 contraction of the muscular walls of the rectum, aided by the contraction of 

 the abdominal muscles. 



ABSORPTION. 



The term absorption is applied to the passage or transference of material 

 into the blood from the tissues, from the serous cavities, and from the mucous 

 surfaces of the body. The most important of these surfaces, especially in its 

 relation to the formation of blood, is the mucous surface of the alimentary 

 canal; for it is from this organ that new materials are derived which maintain 

 the quality and quantity of the blood. The absorption of materials from the 

 interstices of the tissues is to be regarded rather as a return to the blood of 

 liquid nutritive material which has escaped from the blood-vessels for nutri- 

 tive purposes, and which, if not returned, would lead to an accumulation 

 of such fluid and the development of dropsical conditions. 



The anatomic mechanisms involved in the absorptive processes are, 



