DIGESTION 205 



in the portion known as the descending colon are more or less solid, it is 

 apparent that an absorption of liquid must take place during the transit 

 from the cecum to the rectum. This is made possible by the retardation of 

 the intestinal contents caused by the antiperistaltic or the anastaltic wave, 

 together with the haustral contractions. Between the two modes of activity 

 different portions of the intestinal contents are exposed to the mucosa and 

 by which the liquids are in large measure absorbed. In the distal portion 

 of the large intestine, the characteristic movement is peristaltic by which the 

 more solid contents are carried to the sigmoid flexure where they accumulate 

 until the desire to evacuate the mass is experienced. By the contraction of 

 the pelvic portion of the descending colon the mass is forced into the rectum 

 from which it is discharged from the body. The function of the large 

 intestine is therefore to receive, to reduce to a proper consistency, to tempor- 

 arily store and subsequently discharge its contents, consisting of the indi- 

 gestible residue of the food, together with excretions of intestinal glands 

 which have descended from the small intestine and which constitute in part 

 the feces. 



The Nerve Mechanism of the Large Intestine. The nerve mechan- 

 ism of the large intestine includes both motor and inhibitor nerves. The 

 motor nerves comprise both pre- and post-ganglionic fibers; the former 

 have their origin in the spinal cord, from which they emerge in the third and 

 fourth sacral nerves and pass by way of the pelvic nerve to the pelvic 

 ganglia around the cells of which their fibers arborize; the latter (post- 

 ganglionic) fibers emerge from the cells of these ganglia and are distributed 

 to circular and longitudinal muscle-fibers of the intestinal wall. 



The inhibitor fibers also comprise both pre- and post-ganglionic fibers; 

 the former have their origin in the lumbar region of the spinal cord, from 

 which they emerge in the second to the fifth lumbar nerves; they then pass into 

 and through the sympathetic chain and the inferior splanchnic nerves to the 

 inferior mesenteric ganglion around the cells of which they arborize; the 

 post-ganglionic fibers pass directly to the muscle-fibers of the intestinal wall. 

 Stimulation of the pelvic nerve with induced electric currents causes con- 

 traction of the muscle-fibers; stimulation of the hypogastric nerves causes 

 an inhibition of the contraction. 



Intestinal Fermentation. Owing to the favorable conditions in both 

 the small and large intestine for fermentative and putrefactive processes 

 e.g., heat, moisture, oxygen, and the presence of various microorganisms- 

 some of the food, when consumed in excessive quantity or when acted on by 

 defective secretions, undergoes a series of decomposition changes which are 

 attended by the production of gases and various chemic compounds.^ Dex- 

 trose and maltose are partially reduced to lactic acid; this to butyric acid, 

 carbon dioxid, and hydrogen. Fats are reduced to glycerol and fat acids, the 

 glycerol, according to the organisms present, yields succinic acid, carbon 

 dioxid, and hydrogen. Some of the protein derivatives the amino-acids 

 may be reduced to still simpler compounds under the action of bacteria. 

 Lysin, through the loss of a molecule of CO 2 , gives rise to cadaverin; orni- 

 thin to putrescin; tryptophan yields indol, skatol; tyrosin yields phenol and 

 probably cresol. 



Indol. This compound is of especial interest as it is the antecedent of 

 indican, found in the urine. Though it is in part discharged in the feces, it 



