The term “small intestine” is somewhat misleading, for in an adult it is all of twenty feet long. “Small” refers only to its diameter which is considerably smaller than that of the large intestine. It is in reality one continuous tube, but is usually medically considered as being divided into three sections. The first part or that part connected to the stomach is almost a foot long and for this reason is called the duodenum, which means “twelve finger breadths”. The section following the duodenum is about eight times as long and is known as the jejunum. This word means “hungry”, “barren” or “scanty” and refers to the fact that the jejunum is almost always empty in a post mortem examination. The rest of the intestine is called the ileum which means “twisted”, “rolled up together” or “kinked”. This word might well be applied to all the divisions, for the small intestines are turned or twisted, or folded upon themselves so that they occupy a surprisingly small space when one stops to think of their length more than three times the height of a tall man.
The small intestines do not simply merge into the large intestine which, as we have said, has a much larger diameter. They end at the side of the first section of the large intestine a few inches from where it properly begins. Back of this point is a small part of the large intestine known as the cecum which means “blind”. The cecum is, in reality, a blind pouch or sack. At its lower end is a small tail like appendage known as the vermiform (worm-shaped) appendix. Its length may range from one-half to nine inches, but the average is usually three to four inches. At one time it must have had a particular function to perform, but in the course of evolution it seems to have lost its function in man and now causes serious trouble.
But to get back to the food which we left in the process of being digested. Each time the pylorus opens, the liquidy food mass or chyme passes into the upper part of the small intestine or duodenum. Here it usually remains until several of the small masses of food have been sent into it from the stomach, and here the process of digestion continues. The chyme is acted upon by three different digestive juices the intestinal juice which is secreted by glands called Lieberkuhns follicles in the walls of the intestines, the bile and pancreatic juice.
As soon as the acid chyme from the stomach passes through the pylorus, the pancreatic juice begins to flow into the intestine and start its work. It is a clear, alkaline liquid containing three digestive enzymes.
One acts on protein foods, another splits fats, into simpler compounds that can be absorbed, and a third, like the ptyalin of the saliva, breaks down complex carbohydrates into simpler forms.
Bile is a slightly alkaline solution secreted by the liver. It enters the intestines through the same duct as the pancreatic juice. It considerably increases the solubility of the fatty acids and decreases the surface tension between watery and oily fluids, thereby making a more perfect emulsion. All this helps to speed up the action of the enzyme of the pancreatic juice which splits up the fats.
The intestinal juice is markedly alkaline and contains five different enzymes which act upon the partially broken down proteins and carbohydrates.
While all these juices are acting upon the food mass, the muscular walls of the intestines are not idle. After a certain amount of food mixed with the juices has accumulated in the duodenum, it becomes divided into small sections by constrictions of the walls. The segmentation continues for several minutes so that the individual masses are carried back and forth with a rhythmical movement which greatly aids the emulsification of fat. The constrictions also bring about a further mixing of the food with the digestive juices and brings the digested food into contact with the absorbing membranes of the intestinal walls. After a certain variable length of time, the constrictions cease and the segments are carried singly by peristalsis or wavelike movements, or are joined together and move on in a single mass.
Absorption takes place very readily through the blood vessels and lymph vessels of the small intestines so that the greater part of the digested food nutrients are absorbed before the chyme reaches the ileocecal valve which separates the ileum from the cecum. However, it has been observed that frequently the ileum is full four or five hours after the stomach has been emptied. Therefore there may be an accumulation of partially digested food in the lower part of the ileum. Here it remains and undergoes further digestion by the digestive juices with which it has been mixed, aided by segmentation of the intestine.