When food is passed into the stomach, se cretion occurs. In all cases the stomach, pale before, becomes suffused with blood, and the gastric juice is poured out The flow of the intestinal juice, the pancreatic juice and bile all follow the stimulation of the mucous mem brane of the duodenum, and in all cases the blood vessels enlarge so as to give the cells a good supply of blood, as we have seen, they themselves actually pass into a condition of activity as a result of the influence of special secreting nerves. Foods belong to four classes: (1) Proteids —albumens, globulins, etc.; for example, the white of egg, the chief constituent of meat, the gluten of bread. (2) Carbohy drates — starches, sugars, gums; for example, potato-starch, cane- or grape-sugar.. (3) Fats and oils; for example, suet, marrow, olive oil (4) Inorganic substances; for example, water, table salt, iron, phosphates.
A few substances are absorbed without being digested at all; for example, water and the salts, though even many of these undergo. some physical change. Grape-sugar is absorbed and probably proteids too are often absorbed in very small amount. Fat is profoundly modi fied during digestion. The saliva, of which about 30 ounces are secreted during the 24 hours, contains a ferment termed ptyalin, which is capable of turning boiled starch into a solu ble sugar called grape-sugar, or, according to other observers, into another soluble sugar termed maltose. The ptyalin may be extracted from the saliva or from the salivary glands themselves. It does not appear to be much exhausted during its activity. The saliva is alkaline, and the starchy food is no doubt partly converted into sugar during its sojourn in the mouth by its action. Starch digestion may continue in the stomach for 45 minutes to 1 hour for the hydrochloric acid of the gastric juice does not destroy the ptyalin until the concentration of free HCl has reached 0.2 per cent. Any nondigested starch is subsequently converted into sugar when the food reaches the small intestine by the pancreatic juice.
The Nervous Control of the Gall Bladder and the Relation of the Secretion of Bile to the Peristalsis and Secretion of the Stomach.
As an example of the highly complex neuro chemic reciprocal relations existing between the digestion in the stomach and digestion in the first part of the intestines below the stomach called the duodenum, we will describe the reciprocal innervation between the gall bladder and its outlet at the papilla of Vater. The
orifice of the common gall duct on the papilla of Vater is at the same time the orifice of the pancreatic duct.
The vagus is the constrictor nerve to sphincter at papilla of Vater and inhibitory to the gall bladder. The splanchnic is the motor or constrictor nerve to the gall bladder and inhibitory to sphincter at papilla of Vater.
Stimulation of central end of splanchnic causes dilatation by reflex stimulation of vagus (inhibitory).
Stimulation of central end of vagus causes contraction of the bladder and a dilatation of sphincter at papilla of Vater.
Now, when there is no food in the stomach the sphincter of papilla of Vater is tightly closed, but as soon as food is taken into the stomach and digested, the solid chyme is spurted out through the pyloric sphincter, by the pyloric reflect (Hirsch — von Meting), into duodenum, and thereby stimulating the afferent fibres of the vagus. This stimulates the splanchnics reflexly, thereby causing inhibition of sphincter at papilla of Vater, and at the same time causing contraction of the gall bladder which shoots out the alkaline bile into the duodenum through the common bile duct. This then mixes with the gastric acid chyme, previously expelled through the pylorus, thereby making it alkaline, and this alkaline mass will cause the sphincter of the papilla of Vater to contract tightly through its stimulating effect upon the fibres of the splanchnics. Then fol lows a reflex stimulation of the vagus which results in a contraction of the sphincter of the papilla of Vater and relaxation of the gall bladder. Or in other words, the alkaline chyme stimulates this sphincter surrounding the orifice of the gall duct and pancreatic duct to contract reflexly, just as HCI, when present on the duo denal side, causes the pyloric sphincter to con tract reflexly.
But the admirable nuerochemic adjustment is such that whilst acid HCl on the duodenal side of the pylorus causes the pyloric sphincter to close, this same add reaction of duodenal chyme causes the sphincter of the papilla of Vater to relax, that the alkaline bile and pan creatic juice may neutralize this acid, for the enzymes. of these juices require an 'alkaline medium for their best effect. When the duodenal chyme is again neutralized the pyloric sphincter opens, letting through four to ten cubic centimetres acid gastric chyme, but at the same time as long as the duodenal chyme is alkaline the sphincter of the papilla of Vater is dosed.