THE METABOLIC MACHINERY OF ANIMALS 291 



the first part of the small intestine (duodenum). It is interesting to 

 remember that carbohydrates pass out of the stomach soon after 

 ingestion, remaining only about one half as long as proteins. Fats 

 hkewise remain a long time within 

 the stomach even when combined 

 with other foodstuffs. 



The Small Intestine. The 

 intestine is subdivided into a 

 region principally devoted to 

 absorption of digested foods, 



namely the small intestine and the (Y 



large intestine which to a lesser M 



extent is devoted to a continua- '^i^^cting' nioUon shoNvn in. 



tion of absorption, and to the ^■'^•^ ■ '^^/'^hmic.Segnrjenting- 



collection of waste products. ''T^overrjenLs. 



The entire small intestine of man, r\ f^^"--^ 



some twenty feet in length and e^ "^^^-^ 



about an inch in diameter, is cLiastalsis is Cannon's nanxe. 

 concerned with the digestion and ^°^ '^^'^ perisLxlLic wave xvl^ich 

 „. ,. c c 1 1.,- Tnoves olonS U^e intestine, 



absorption of foods and their prccecCad Joy -inhibition 



transfer to the blood stream. It t^. , .„ • , • 



, , ,. , , Diagram to illustrate peristalsis. 



IS also believed that some waste 



materials are actually excreted into the lumen of the gut. These 

 functions are accomphshed by a series of adaptations, one of which 

 is the extraordinary length of the small intestine, together with 

 numerous small circular ridges, -plicae circulares, which serve the 

 double function of giving an increased absorptive surface and of 

 retarding the rate of passage of foodstuffs. The other but by no 

 means the least important adaptation, is the presence of millions of 

 small knoblike projections, or villi. These tiny structures according 

 to Howell move actively either by lateral lashings or by extension 

 and retraction. It is believed that these movements are associated 

 with the act of absorption and probably play an important part in 

 emptying the lymph sac, or lacteal, lying in the center of each villus. 

 By means of the plicae circulares and the villi, the small intestine is 

 estimated to have an absorbing surface equal to twice that of the 

 surface of man's body. 



The internal structure of the villus is best seen in a longitudinal 

 section. The outer wall is composed of a thin layer of epithelial cells 

 in which the more complex fats are resynthesized before being 



