134 The Vertebrate Organ Systems 



The Small Intestine. — Most of the lower and central part of 

 the abdominal cavity is occupied by the coiled small intestine. This 

 is the longest part of the digestive tract and is some twenty-three feet 

 in length in human beings. It varies in length in other vertebrates, 

 but, in general, is longer in herbivores than in carnivores. In an animal 

 such as the frog that is herbivorous when a larva and carnivorous 

 when an adult, the intestine actually shortens and broadens at the time 

 of metamorphosis. The name "small intestine" is derived from the 

 fact that the diameter of this portion of the digestive tract is much 

 less than that of the large intestine. In man it is but one and one-half 

 inches in the anterior part and less than one inch in the lower end. 



Three regions are recognized in the small intestine : the duode- 

 num, the jejunum, and the ileum. The duodenum is a short straight 

 portion (ten inches in human beings) leading from the pyloric end 

 of the stomach to the jejunum. At first the duodenum extends up- 

 ward, then it makes a sharp bend and passes downward. The coiled 

 jejunum in human beings is seven and one-half feet long; it has some- 

 what thicker walls and a wider lumen than does the following portion, 

 the ileum. The ileum constitutes the remaining length of the small 

 intestine. At its terminal end, it joins with the large intestine. 



The inner lining of all three sections has numerous small finger- 

 like projections, the villi. These increase the surface of the small in- 

 testines enormously, and are estimated to number at least 4 to 5 

 million in man. 



The digestion which was started in the mouth and stomach is com- 

 pleted here in the small intestine. The soluble food particles then 

 are absorbed by the villi and passed into the circulatory system. 



The Large Intestine. — In contrast to the small intestine, the 

 central cavity or lumen of the large intestine is about two and one-half 

 inches wide in man. Its length is about five feet ; thus it is considerably 

 shorter than the small intestine. Where the ileum and large intestine 

 join are two flaps of tissues which form the ileo-caecal valve. This 

 allows the passage of undigested materials and blocks their return to 

 the small intestine. The ileum joins the large intestine about two 

 inches above the latter's termination. That portion of the large in- 

 testine below this juncture is known as the caecum. In man, civets, 

 some rodents, and monkeys, the small wormlike vennifonn appendix is 

 at the end of the caecum. When infected, this small process causes 

 trouble quite out of proportion to its size. Among some herbivorous 



