438 ANIMAL BIOCHEMISTRY 



of ruminants, although it differs ironi the rumen in that digestive 

 changes take place in the caecum, owing to the presence of the en- 

 zymes originating in the pancreas and small intestine. In a general 

 way, the size of the caecum is inversely proportional to that of the 

 stomach. In the horse the caecum is very large and holds about 16 

 per cent of the total contents of the digestive tract. This organ allows 

 the food to remain for long periods of time in contact with the diges- 

 tive juices from the small intestine, and, as a result of bacterial action, 

 the horse is able to digest and utilize crude fiber to a very appreciable 

 extent. The caecum in ruminants is relatively small, with a capacity 

 of about 3 per cent of the tract. That of hogs is not much higher, 

 being about 5 per cent. 



Located near the intestinal inlet to the caecum is another tube, 

 somewhat larger, called the colon, forming one end of the large in- 

 testine. An enlarged section near the terminus or anus is called the 

 rectum. In the horse the large intestine is also highly developed and 

 may constitute as much as 45 per cent of the total capacity of the 

 digestive tract; like the caecum, it serves as a storage organ in which 

 the hydrolysis of difficultly digestible foods continues. In the domestic 

 fowl two caeca connect with the intestine at the same point, forming 

 a Y-shaped structure (see Figure 17-3). 



Digestion products are not assimilated to appreciable extents until 

 the small intestine is reached. Some exceptions include alcohol, salts 

 if the food is very high in minerals, and components of certain con- 

 diments like pepper and mustard. The small intestine is lined with 

 folds of tissue containing a multitude of tiny projections known as 

 villi. Although the duodenum is the section most densely populated 

 with villi, the jejunum and ileum contain many, and since the last 

 two sections are very long they probably make the major contributions 

 to absorption. 



The villi are supplied with two circulatory systems, a blood system 

 consisting of arterial and venous networks, and a lacteal or single 

 lymphatic capillary which communicates with the lymphatic system. 

 From the villi, absorbed nutrients are carried via the mesenteric veins 

 to the portal vein and thence to the liver and body tissues. A major 

 portion of the fats, however, is absorbed in the lacteals and reaches 

 the tissues via the lymphatic system. 



The role of bile in fat digestion has already been mentioned. Glyc- 

 erol, a normal product of fat hydrolysis, is water soluble and readily 

 absorbed. Fatty acids, however, are not water soluble and cannot be 

 absorbed as free acids. Consequently they form soluble complexes 

 with bile salts, in which form they diffuse through the intestinal mem- 



