194 Tin: INTl RNAL HNVIRONMENT OF THE BODY Part III 



cells which are absorbing it. Between the villi arc the microscopic openings 

 of the intestinal glands. Each villus is covered with epithelial cells into which 

 the food is absorbed (Fig. 11.19). In the core of each one is an arteriole 

 and a venule with their connecting capillaries; the blood in these vessels 

 picks up the digested food (except the bulk of the fat) and transports it to 

 the liver. Also in the core is a minute lymph vessel, a lacteal, that ends 

 blindly at the tip, but at the other end is continuous with larger lymph ves- 

 sels. Lymph vessels from all over the body finally coalesce and form the 

 thoracic ducts which flow into large veins. Thus the fat, now a milky white 

 emulsion, ultimately reaches the blood. 



There is no adequate theory to account for the assimilation of the vital 

 proteins in the various tissues from the collection of amino acids that is 

 assembled in the blood stream. It is known, however, that there is a rigid 

 selection and that a given cell, perhaps a muscle or nerve cell, always as- 

 similates particular amino acids. The changes of proteins and amino acids 

 within the cells have been revealed by tracing the paths of compounds con- 

 taining "labeled" isotopes of hydrogen, carbon, or nitrogen during metabolism. 



Function of the Large Intestine (colon and rectum). Reabsorption of water 

 is the principal function of the large intestine in all mammals. It produces no 

 digestive enzymes and little or no food is absorbed in it, but it secretes a 

 large amount of mucus which acts as a lubricant. The indigestible matter is 

 gradually admitted from the small intestine with considerable water and 

 some unabsorbed secretions. An enormous population of bacteria is always 

 present. In no part of the alimentary canal are all bacteria destroyed. 



At the junction of the small and large intestine there is a sac or caecum, 

 large in birds and other herbivorous animals and small or missing in carnivores 

 (Fig. 11.6). In many mammals, especially in monkeys and man, there is a 

 blind sac at the end of the caecum, the vestigial appendix (Fig. 11.7). In 

 the primitive monotremes, e.g., the duckbill platypus of Australia, the rectum 

 opens into the cloaca as it does in the frog, but in all other mammals it has 

 a separate external opening. 



