GENERAL ZOOLOGY 



the large intestine through the ileocaecal valve. In man the first part of the 

 food mass enters the large intestine about 4 hours after having been eaten, and 

 the discharge continues for about 2 hours. After being retained in the lower 

 part of the large intestine for from 10 hours to 2 days, this undigested and 

 undigestible material, now known as feces, is egested or defecated. The 

 amount of material egested is about 10 per cent of the amount ingested. In 

 the large intestine of herbivorous animals there are many bacteria which di- 

 gest cellulose, a carbohydrate present in the walls of plant cells, and produce 

 simple sugars from it. Although some of this sugar is absorbed, it must be 

 kept in mind that the bacteria digest this material for their own use. It is 

 only incidentally that it affords nourishment for the animal harboring the 

 bacteria. Bacteria in the digestive tract of many animals produce substances 

 important for their hosts' growth and maintenance. For example, in man 

 intestinal bacteria produce vitamin K (p. 32) and vitamin Bj2 (p- 34). Some 

 idea of the great number of bacteria comprising the intestinal flora can be had 

 when it is stated that from one-fourth to one-half the dry weight of feces 

 consists of bacteria. 



The principal facts about digestion are summarized in Figure 3.20, which 

 should be carefully studied in tracing the breakdown of each of the classes of 

 foods as they pass along the digestive tract. 



Absorption 



The simple nutrients which are the end products of digestion must be 

 absorbed from the digestive tract into the circulating fluids and be dis- 

 tributed to all the cells of the body before they can take part in cellular 

 metabolism. Absorption may be defined as the passage of simple food com- 

 pounds through the cells lining the digestive tract into the blood or lymph. 

 Between the mucous membrane and the muscular coats of the tract is the sub- 

 mucosa, a region of loosely arranged cells with interlacing fibers (Fig. 3.19). 

 It is in this region, separated from the digestive cavity by the mucous mem- 

 brane, that the delicate lymphatics and the thin-walled capillaries which 

 connect arteries and veins are found (Fig. 3.21). In being absorbed, sub- 

 stances pass through the cells of the mucous membrane and endothelium of 

 the capillaries, as well as the small amount of lymph between the two. 



Two mechanisms are responsible for the phenomenon of absorption — 

 diffusion, or movement from a region of high concentration (lumen of in- 

 testine) to a region of lower concentration (epithelial cell of intestinal lining), 

 and active transport. Diffusion obeys certain physicochemical laws in the 

 living organism as in non-living systems. Active transport requires expendi- 

 ture of cellular energy to move substances from the lumen of the intestine into 

 the intestinal epithelium. 



Although certain foods, such as glucose, vitamins, water, and the inorganic 

 salts, require no change before they are ready for absorption, they are not 



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