THE DIGESTIVE SYSTEM 



465 



mon practice in determining abnormalities 

 or diseases of the digestive system. 



There are two types of mixing move- 

 ments in the small intestine: segmental and 

 pendular. Both are confined to the limits of 

 a single loop of the gut. The segmental 

 movements start by constricting at regular 

 intervals throughout the loop (Fig. 17-9). 

 Relaxation appears in 10 minutes or so and 

 another series of constrictions occur which 

 are between the first series. This results in 

 the breaking up and mixing of the masses 

 of chyme. It is supplemented by the pen- 

 dular movements, which consist of mild 

 peristaltic waves that move the chyme back 

 and forth within the intestinal loop but do 

 not go beyond it. The chyme is moved pro- 

 gressively along tlie gut by means of peri- 

 staltic waves that occur both gently and in 

 sudden rushes. The action is a slow progres- 

 sive movement with a sudden rush, at times, 

 of the entire contents from stomach to 

 colon. The combined movements result in 

 a continuous progression of the chyme 

 through the small intestine until it reaches 

 the colon, or large intestine. 



THE LARGE INTESTINE 



The small intestine joins the colon in the 

 lower right region of the abdominal cavity 

 (Fig. 17-2). The opening is guarded by the 

 ileocaecal sphincter. Chyme that passes 

 into the colon is still in a very fluid state, 

 but as it passes through this region of the 

 digestive tract much of the water is reab- 

 sorbed. The reabsorption of water is a con- 

 servation measure, for if it were retained in 

 the gut the intake of water by mouth would 

 necessarily be much greater. Such demands 

 would impose insurmountable problems for 

 an animal that needed to go any distance 

 for its source of water. Since the chyme 

 moves very slowly through the colon ( 12- 

 14 hours ) there is ample time for the water 

 to be reabsorbed. During this time bacteria 

 grow so abundantly that tliey constitute 

 about half of the weight of feces. They 



cause no harm and probably do very little 

 good in the gut of man. In some herbivo- 

 rous vertebrates, however, they are thought 

 to be beneficial, aiding the digestion of cel- 

 lulose. 



The sac-like caecum at the beginning of 

 the colon terminates in a finger-like evagi- 

 nation called tlie vermiform appendix be- 

 cause it is worm-like in appearance (Fig. 



wov« of 

 constriction 



wave of 

 relaxation 



Fig. 17-9. Food moves along the digestive tract by peri- 

 stalsis. The motion is rapid in the esophagus, v/here 

 its purpose is to get the food to the stomach as soon 

 as possible. In the stomach and remainder of the gut 

 there are two kinds of movements. One that moves 

 the food along, as shown in the duodenum above, 

 and the other which brings about a mixing of the 

 food (segmental and pendular) as shown in the 

 stomach and lower intestine. 



17-2). This is a remnant of a once func- 

 tional portion of the caecum. Herbivorous 

 animals possess a very large caecum in 

 which a certain amount of bacterial diges- 

 tion takes place. The appendix in man some- 

 times becomes infected, and since it does 

 not drain well, the infection may reach such 

 proportions as to burst the peritoneal lining, 

 liberating the contents of the appendix into 

 the coelomic cavity. This is serious because 



