114 HUMAN PHYSIOLOGY. 



into segments and by the latter, it is carried down the intestine. 

 Shortly after the entrance of the food into the intestine, segmenta- 

 tion begins by a contraction of bands of circular muscle fibers. So 

 soon as a mass of food is divided into segments each segment 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 flexure 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 nutritive purposes, and which, if not returned, would 



