204 TEXT-BOOK OF PHYSIOLOGY 



also surrounded by bands of striated muscle-fibers known collectively as the 

 external sphincter. 



The walls of the large intestine consist of three coats: viz.: serous, 

 muscular, and mucous. 



The serous is a reflection of the general peritoneal membrane. 



The muscle is composed of both longitudinal and circular fibers. The 

 longitudinal fibers are collected into three narrow bands which are situated 

 at points equidistant from one another. At the rectum they spread out so 

 as to surround it completely. As the longitudinal bands are shorter than 

 the intestine itself, its surface becomes sacculated, each sac being partially 

 separated from adjoining sacs by narrow constrictions. The circular fibers 

 are arranged in the form of a thin layer over the entire intestine. Between 

 the sacculi, however, they are more closely arranged. The sacculi have been 

 termed haustra from their supposed function, that of absorbing or drawing 

 water from the intestinal contents thus imparting to them a certain degree 

 of consistency. In the rectum the circular fibers are well developed, and at 

 a' point two or three centimeters above the anus they form, as stated above, 

 the internal sphincter. 



The mucous membrane of the large intestine possesses neither villi nor 

 valvulae conniventes. It contains a large number of tubules consisting of a 

 basement membrane lined by columnar epithelium. They resemble the 

 follicles of Lieberkiihn. The secretion of these glands is thick and viscid 

 and contains a large quantity of mucin. 



The Contents of the Large Intestine. As a result of the actions of 

 saliva, of gastric, intestinal, and pancreatic juice, and of the bile, the food is 

 disintegrated and liquefied. The nutritive principles, protein, starches, sugars, 

 and fats, undergo chemic changes and are transformed into amino-acids and 

 peptids, dextrose, soap and glycerin, fat acids, under which forms they are 

 absorbed. After the more or less complete digestion and absorption of these 

 nutritive substances the residue of the food, comprising the indigestible and 

 undigested matter, passes out of the small intestine into the large intestine 

 and forms a portion of its contents. This residue consists of the hard parts 

 of the cereals, vegetable seeds, cellulose, etc., the quantity and variety of 

 which depend on the nature of the food. These substances, passing into 

 the large intestine along with the excrementitious matter of the bile, become 

 incorporated with the mucous secretions and assist in the formation of the 

 feces. 



The Movements of the Large Intestine. After the reception of the 

 foregoing indigestible residue, excrementitious matter, etc., the walls of 

 the large intestine begin to exhibit movements similar to those exhibited 

 by the walls of the small intestine. These movements may also be ob- 

 served by Roentgen ray appliances. A peristaltic wave, arising in the cecal 

 region gradually transfers the intestinal contents up the ascending colon 

 and as far as the middle of the transverse colon at least. At this point 

 a contraction arises which then passes in the reverse direction and returns 

 the intestinal contents to the cecal region. The effects of the peristaltic 

 waves are thus to some extent interfered with by anti- peristaltic or anastal- 

 tic waves. An antiperistaltic wave occurs in the cat about every fifteen 

 minutes and lasts for about five minutes. The effect is a still further 

 admixture of the intestinal contents with the secretions and a longer 



