466 



ORGAN SYSTEMS OF MAN 



the bacteria then attack the extensive hning 

 of the visceral organs, resulting in a general 

 infection that can be fatal. However, with 

 the use of antibiotics and a surgeon's skill 

 there is usually little danger today of such 

 an infection proving fatal. 



From the region of the caecum the colon 

 extends anteriorly along the right side as 

 the ascending colon, crosses to the left side 

 as the transverse colon, passes down the 

 left side as the descending colon, then 

 bends in an S-shape (sigmoid flexure) to 

 become the rectum. The digestive tract 

 terminates in the anus which is closed by 

 two powerful muscles, the anal sphincters. 



Colon movements 



The chyme, which is now referred to as 

 feces, is gradually moved through the colon 

 by mild peristaltic waves as well as peri- 

 odic, over-all massive contractions. When 

 feces reach the rectum, which is normally 

 empty, a desire to defecate becomes very 

 pronounced. During defecation a large 

 peristaltic wave moves over the entire colon, 

 aided by voluntary contractions of the ab- 

 dominal muscles. 



Failure of regular bowel movements, 

 commonly referred to as constipation, is the 

 subject of much discussion and more misin- 

 formation. If feces are not regularly elimi- 

 nated from the lower bowel, they tend to 

 accumulate and the mere pressure caused 

 by their presence may produce headaches 

 and the usual symptoms associated with 

 constipation. Similar symptoms can be elic- 

 ited by simply packing inert material, such 

 as cotton, in the lower bowel. Therefore, no 

 toxins are produced during constipation, al- 

 though this is usually thought to be the 

 case. The various cathartics that are so well 

 advertised to relieve this condition act in 

 various ways. Some, such as cascara, stimu- 

 late the reflex centers in the brain which, 

 in turn, initiate peristaltic contractions; 

 others such as epsom salts (magnesium sul- 

 fate) are not readily absorbable, and since 

 they are taken in high concentrations there 



is a tendency for water to remain in the 

 digestive tract. Castor oil as well as magne- 

 sium sulfate strongly irritate the nerve end- 

 ings in the lining of the gut which bring 

 about violent peristaltic activity reflexively. 

 Usually a controlled diet will relieve con- 

 stipation without resorting to these artifi- 

 cial means. Much of the catharsis to which 

 people subject themselves is not only un- 

 necessary but harmful. 



Absorption 



Once the food is digested it is absorbed 

 into the blood stream and carried to all 

 parts of the body to provide the material for 

 energy release and for growth and repair. 

 With the exception of alcohol, which is ab- 

 sorbed in the stomach, nearly all organic 

 and inorganic compounds are absorbed in 

 the small intestine. Elaborate provisions 

 have been made to facilitate the process of 

 absorption. In the first place, the small in- 

 testine is 23 feet long and, secondly, the 

 villi, previously referred to, multiply the 

 surface area of the lining at least 10 times. 

 Consequently, there is a great expanse of 

 surface area into which the chyme comes 

 in close contact, giving every absorbable 

 particle an opportunity to find its way into 

 the blood stream. 



The important anatomical unit that facil- 

 itates absorption is the villus (Fig. 17-10). 

 It is a thin-walled projection lined with 

 capillaries and with a central space, the 

 lymph vessel or lacteal. As the watery 

 chyme bathes the villi, an osmotic equilib- 

 rium is constantly strived for between the 

 blood and the intestinal contents, but since 

 the blood is moving rapidly through the 

 capillaries, the flow of absorbable materials 

 is toward the circulating blood. Therefore, 

 the end products of protein and carbohy- 

 drate digestion, together with inorganic 

 salts and water, are absorbed directlv into 

 the blood stream and are transported via 

 the hepatic portal system directly to the 

 liver. The products from the breakdown of 



