THE DIGESTIVE SYSTEM 



461 



Fig. 17-7. The length of the digestive tracts of animals varies with the diet, as shown in these figures. 

 The carnivore (dog) has the shortest track (with respect to trunk length), the herbivore (rabbit) has 

 the longest, while the omnivore (man) has a gut of intermediate length. 



tion of the end products of digestion occurs. 

 The small intestine is divided into three 

 parts that differ slightly from one another 

 in their anatomy. The first 10 inches is the 

 duodenum, into which the ducts from the 

 liver and pancreas empty. This is followed 

 by about 10 feet of a region known as the 

 jejunum, which is particularly rich in intes- 

 tinal glands. The remaining portion is the 

 ileum, and is characterized by the vast 

 number of tiny finger-like projections of its 

 lining, the villi (Fig. 17-10). These struc- 

 tures increase the surface area of the intes- 

 tine tremendously, thereby making possible 

 the adequate absorption of digested food 

 substances. The small intestine is thus 

 adapted to retain food for a considerable 

 period of time in order that digestion may 

 be completed and absorption take place. In 

 this respect, it should be recalled that car- 

 nivores possess shorter intestines than herbi- 

 vores because meat digests more rapidly 

 than do plant materials (Fig. 17-7). 



The entire gut is supported by a dorsal 

 mesentery which holds such parts as the 

 stomach and duodenum in place but at the 

 same time allows the rest of the small intes- 

 tine considerable freedom of movement. In 

 lower vertebrates there is evidence that a 

 ventral mesentery was once present which 

 held the entire digestive tract in a line from 

 mouth to anus. As the gut increased in 

 length this ventral support was lost. Actu- 



ally, it no longer performed any particular 

 function, since the animal lived in a hori- 

 zontal position where its gut hung like 

 clothes from a line. However, when man 

 decided to walk on his hind legs, the dorsal 

 mesentery could not continue to support 

 the gut as well as previously, and so the gut 

 had a tendency to slide posteriorly into the 

 pelvis. In youth, the abdominal muscles 

 are adequate to compensate for this defi- 

 ciency but as age advances the gut tends 

 to sag more and more, resulting in the pot- 

 belliedness of advancing years. It requires 

 a great amount of physical effort to keep it 

 in place, a price that a tired body and 

 a genial philosophy will not condone. 



The liver 



This, the largest gland in the body, is 

 located on the right side just under the dia- 

 phragm (Fig. 17-2). It is tunneled with 

 spaces and with vessels which are filled 

 with large quantities of blood, and because 

 of its construction will not tolerate injury. 

 Fatal internal injuries suffered in car acci- 

 dents frequently involve the liver. Unfortu- 

 nately, the steering wheel of the automobile 

 is located over this area of the body, and 

 since any sudden stopping of the car throws 

 the victim against the wheel, the liver, be- 

 ing the most vulnerable organ in that area, 

 is most apt to be damaged. 



Bile, which is drained from all parts of 



