DIGESTION AND RESPIRATION 59} 



are pale and the skin assumes the yellowish tinge characteristic of 

 jaundice. 



All of the blood returning from the intestine, where it has absorbed 

 a variety of materials, passes through the liver before entering the 

 general circulation of the body. In the minute capillary-like spaces of 

 the liver the blood comes into intimate contact with the hepatic cells, 

 which take up, store, interconvert, and alter in many ways the absorbed 

 food molecules. The liver cells also detoxify certain poisonous substances 

 and excrete some of them in the bile. 



The pancreas is an important digestive gland, producing quantities 

 of enzymes that act upon carbohydrates, proteins and fats. These enzymes 

 enter the intestine by way of a pancreatic duct that joins the common 

 bile duct. An accessory pancreatic duct may be present and empty 

 directly into the intestine. The pancreas contains patches of endocrine 

 tissue, the islets of Langerhans, which will be considered in Chapter 30. 



221. The Intestine 



Most digestion, and virtually all of the absorption of the usual end 

 products of digestion, occur in the intestine. Most of the digestive 

 enzymes foiuid in tlie intestine of vertebrates come from the pancreas, 

 but distinct intestinal glands are also present in the wall of the intes- 

 tine of birds and mannnals. Adequate siuface area tor absorption is 

 made available by the length of the intestine, and by outgrowths and 

 internal foldings of various sorts. 



The structural details of the intestine vary considerably among 

 vertebrates. Primitive fishes have a short, straight valvular intestine 

 extending from the stomach to the cloaca. Its internal surface is in- 

 creased by a spiral valve. Tetrapods have lost the spiral valve and make 

 up for this by an increase in the length of the intestine, which becomes 

 more or less coiled. The tetrapod intestine has become further differ- 

 entiated into an anterior small intestine and a posterior large intestine. 

 The first part of the small intestine is known as the duodenum, and, in 

 mammals, the two succeeding parts are the jejunum and ileum. Most 

 of the large intestine is known as the colon, but in mammals the caudal 

 end, which has evolved from part of the cloaca of more primitive 

 vertebrates, constitutes the rectum. The rectum opens on the body 

 surface through the anus. A blind pouch called the caecum is present 

 at the junction of small and large intestines. This is very long in such 

 herbivores as the rabbit and horse and contains a colony of bacteria 

 that digest cellulose. Man has a small caecum with a vestigial vermi- 

 form appendix on its end. An ileocaecal valve is located at the end of 

 the small intestine and prevents bacteria in the colon from backing up 

 into this region. 



A transverse section of the small intestine of a mammal illustrates 

 the microscopic structure of the digestive tract (Fig. 26.4). As in the 

 frog's stomach (section 185), there is an outer covering of visceral 

 peritoneum, a layer of smooth muscle, a layer of vascular connective 

 tissue, the submucosa, and finally the innermost layer, the mucosa. 



