310 - Multicellular Animals, Especially Man 



pyloric valve begins to open periodically 

 (p. 311), allowing small quantities of the 

 chyme to squeeze through. When this occurs, 

 a new sample of less thoroughly liquefied 

 food is passed on to the pylorus by the body 

 of the stomach, which thus acts as a reservoir 

 for the "pyloric mill." 



The movements of the stomach are co- 

 ordinated by nerve impulses from the auto- 

 nomic nervous system. Parasympathetic im- 

 pulses accelerate and strengthen the gastric 

 contractions, whereas sympathetic excitations 

 retard and weaken the motility. The latter 

 action delays digestion and prolongs the time 

 required for (he stomach to empty itself. 

 Consequently emotional stales, such as fear 

 and worry, which involve a generalized ex- 

 citation of the sympathetic system, are to be 

 avoided, particularly during and after a 

 heavy meal. For some reason, fat-rich meals 

 are emptied from the stomach very slowly, 

 whereas meals especially rich in carbohy- 

 drates are evacuated more rapidly than 

 usual. 



Factors Involved in the Liquefaction of the 

 Meal. A number of factors tend to liquefy 

 the food mass and convert it into chyme 

 while a meal remains in the stomach. Insolu- 

 ble compounds in the lood are transformed 

 to soluble compounds; the ptyalin of the 

 saliva continues to hydrolize starch into malt- 

 ose, especially in the interior of each bolus, 

 before the HO of the gastric juice permeates 

 the mass; and pepsin begins to convert the 

 proteins to freely soluble peptides as soon as 

 the food mass is thoroughly acidified. More- 

 over, considerable water, in the form of sa- 

 liva anil gastric juice, is added to the meal, 

 and this water increases the general fluidity, 

 in addition to exerting a solvent action upon 

 soluble compounds as they are produced 

 during digestion. Also the gastric contents 

 are gradually warmed by body heat, which 

 accelerates all chemical reactions and pro- 

 motes solution. Moreover, this heat melts the 

 fatty components of the food, permitting 

 them to be emulsified more thoroughly, as 

 the chyme is churned in the "pyloric mill." 



Emptying of the Stomach. When the strongly 

 acid gastric chyme is thoroughly liquid, it 

 is squirted, about a teaspoonful at a time, 

 through the pyloric valve into the first part 

 of the small intestine. In fact, the pyloric 

 valve begins to relax, allowing some of the 

 chyme to pass as soon as the content of the 

 mill reaches the proper consistency. The re- 

 laxing of the valve appears to be under re- 

 flex control, but the receptors and the 

 nervous pathway of this reflex have been dif- 

 ficult to determine precisely. 



The Small Intestine. The great length of 

 the small intestine (Fig. 16-24) is an index of 

 its importance; and in lact the small intes- 

 tine completes the digestion oi every type of 

 food, and is the site of the absorption of all 

 the end products of digestion. 



All types of glands pour their juices into 

 the small intestine. The intestinal mucosa is 

 dotted throughout with numerous unicellu- 

 lar mucous glands, and there is a prodigious 

 number of simple tubular glands, the in- 

 testinal glands, present in the wall of the 

 intestine. But even more important are two 

 large compound glands, the liver and the 

 pancreas, which send their juices into the 

 first part of the small intestine, near its 

 origin from the stomach. 



The great absorptive capacity of the small 

 intestine depends partly upon the fact that 

 it is more than 23 feet long, and partly upon 

 the fact that its internal surface is greatly 

 augmented by the presence of a large number 

 of hairlike structures, the intestinal villi. In 

 fact, the internal lining of some parts of the 

 intestine has the appearance of velvet, and 

 each hair, or villus, of this lining extends 

 inward, making contact with the chyme as 

 it passes along the tube (Fig. 16-25). 



On the basis of small differences of struc- 

 ture the small intestine is subdivided into 

 three parts. The first part, the duodenum, is 

 only about 10 inches long; but the duo- 

 denum is important because it receives the 

 bile from the liver, and the pancreatic juice 

 from the pancreas (Fig. 16-24). Unlike the 

 other coils of the small intestine, the duo- 



