DIGESTION IN THE INTESTINES 



331 



uvula, the intestine, etc. Vomiting may occur from stimulation of sensory 

 nerves from many organs, e.g., kidney, testicle, etc., or by impulses arising 

 in the organs of special sense, the eye, olfactory membrane, etc. The sensory 

 impulses are coordinated by a nerve center located in the medulla. The 

 center may also be stimulated by impressions from the cerebrum and cere- 

 bellum or by changes arising in the center itself, the so-called central vomiting 

 occurring in disease of those parts. The efferent impulses are carried by the 

 phrenics and other spinal nerves and by the vagus. 



DIGESTION IN THE INTESTINES. 



The food that enters the small intestine has already been subjected to two 

 digestive enzymes. The ptyalin of the saliva and the pepsin of the gastric 

 juice together with the mechanical processes involved have reduced the food 

 to a pulpy mass, the chyme. This peptonized food contains most of the total 

 quantity of food eaten, little having been absorbed as we shall see later, but 

 much of the starch has been changed to soluble maltose and dextrose and 

 the proteid to albumoses and peptones. The discharge from the stomach 

 through the pyloric valve to the duodenum has been going on through three 



FIG. 264. 



FIG. 265. 



FIG. 264. Piece of Small Intestine (previously distended and hardened by alcohol), Laid 

 open to Show the Normal Position of the Valvulae Conniventes. 



FIG. 265. Section of the Pancreas of a Dog During Digestion, a, Alveoli lined with cells, 

 the outer zone of which is well stained with hematoxylin; d, intermediary duct lined with squa- 

 mous epithelium. X 350. (Klein and Noble Smith.) 



or four hours on an average for each full meal. This stream of food passing 

 down the small intestine, slowly because of the valvulae conniventes, meets 

 a number of secretions which contain enzymes which act on each of the three 

 great food principles, proteicte, fats, and carbohydrates. These secretions are 

 the pancreatic fluid, the succus entericus, and the bile. 



