THE DIGESTIVE SYSTEM. 333 



material from ileum to colon is the gastro-ileac reflex. The entrance 

 of the succeeding meal into the stomach excites the production of 

 peristaltic waves in the lower part of the ileum, each wave, as it 

 reaches the ileo-colic junction, being followed by relaxation of the 

 sphincter. In this way the contents of the ileum are propelled into 

 the colon in successive portions, and if they contain bismuth the level 

 which they attain in the ascending portion of the large intestine can 

 be observed by means of X-rays to rise towards the hepatic flexure in 

 an intermittent manner. 



The material which thus passes into the large bowel is in the form 

 of a jelly, coloured by the presence of bile pigment. It normally 

 contains hardly any nutritive substances, the derivatives of the diges- 

 tion of protein, fat, and carbohydrates having been almost completely 

 absorbed in the small intestine. Indigestible substances contained in 

 the food are present, especially cellulose, together with cast-off epithelial 

 cells and the unabsorbed portions of the various digestive juices. The 

 chief secretory waste products are the pigment of the bile, unabsorbed 

 glycocholate and taurocholate of sodium, and cholesterol. 



THE FUNCTIONS OF THE LARGE INTESTINE. 



In carnivorous animals the large intestine is short, and its function 

 is limited to the absorption of water and the consequent reduction in 

 bulk of the faeces. In herbivora, on the other hand, the large intestine 

 is of considerable length, and not only absorbs water but serves an 

 additional purpose. A large proportion of vegetable food-stuffs consists 

 of cellulose, which is not affected by the digestive enzymes. Cellulose 

 is decomposed in the large intestine of the herbivora by bacterial 

 action, being converted into fatty acids, which are absorbed and utilised 

 in the body. Further, in all the higher animals, the cells lining the 

 simple tubular glands of the large intestine are for the most part of 

 the mucus-secreting type, and are of service in producing mucin, which 

 acts as a lubricant and facilitates the passage of the feeces along the 

 bowel. 



In man the functions of the large intestine include secretion, 

 excretion, and absorption, and in addition some bacterial decomposition 

 takes place in its contents. (1) The secreted material, as in the higher 

 animals generally, is mucin, derived from the tubular glands of the 

 mucous membrane. (2) The substances excreted by the large 

 intestine are calcium, magnesium, and iron, chiefly in the form of 

 phosphates. The amount of calcium excreted by the bowel varies 

 with the amount contained in the urine. Acid urine, s such as occurs 



