Soluble Globulin Acid Metaprotein Primary Proteoses Secondary Proteoses Peptones.
Some proteins which occur in food do not undergo these changes. Thus elastin is unaf fected by peptic digestion in the time available in the stomach. The collagen of connective tissue is probably converted first into gelatine and then into gelatoses and gelatine peptones. The protein constituent of the conjugated pro teins is usually converted into proteose and pep tone, the prosthetic group being set free. Thus in the digestion of nucleo-protein by gastric juice an insoluble residue of nuclein is formed; in the digestion of mucin (gluco-protein), glucosamine is found in the products.
The effect of gastric juice upon caseinogen the phospho-protein of milk is peculiar in that there is a conversion of the caseinogen into a comparatively insoluble substance, casein. This action of gastric juice has been for many years ascribed to a separate ferment called rennin, but latterly evidence has been brought forward which suggests that the formation of casein from caseinogen is due to pepsin itself. The matter has not been conclusively settled, and it will be convenient to retain the term rennin in the meantime when describing the effect of gastric juice on caseinogen.
Adding rennin to milk and allowing it to stand at a temperature of 37° C. the milk soon becomes clotted, and after a time the clot shrinks, squeezing out a clear fluid, whey, which contains all the constituents of milk except caseinogen and fat.
Three factors are necessary for the forma tion of the clot, namely, caseinogen, rennin and lime salts. If rennin is added to a solution of caseinogen and the mixture is kept for a short time at a temperature of 37° C. and then boiled to kill the enzyme, the addition of calcium chloride will bring about the formation of casein. Obviously the enzyme has produced some change in the caseinogen, and the only factor required to complete the conversion into casein is the addition of lime salts. The process may be represented thus :— Caseinogen+ Rennin (or pepsin) Soluble casein + Calcium salts Casein.
Gastric juice also acts upon neutral fats, but only on those which are in the form of a fine emulsion, such as yolk of egg and milk. The fats are split into glycerol and fatty acids by the agency of an enzyme, lipase. The fat-splitting function of gastric juice, however, is limited in extent, and is of relatively small importance as compared with that which takes place in the small intestine. Pepsin indirectly assists the di
gestion of fat by dissolving the cell envelopes of the fats' cells of adipose tissue contained in the food. In this way fat is set free and prepared for the subsequent digestive action of pancreatic juice.
Nervous Control of the Secretion of Gas tric Juice.— When a dog that has been pro vided with a Pawlow accessory stomach sees food or simply smells food gastric juice begins to drop from the canula in the accessory stom ach. Also, if the oesophagus of the dog is divided and the two ends sewed to the surface at the lower part of the neck, food may be masti cated and swallowed by the animal, but none of it will reach the stomach because it falls out of the end of the oesophageal fistula. In such an animal when food is eaten a secretion of gastric juice from the accessory stomach takes place; as no food has reached the stomach this indicates that the first secretion is due to a. nervous reflex. The secretion of gastric juice will not occur if both Vagi nerves are cut.
The efferent nerves to gastric secretion are therefore the two Vagi. When juices produce purely nervous influence in the absence of any food reaching the stomach it is called a psychic secretion or appetite juice.
The Chemical Mechanism of Gastric Se cretion.—Gastric juice may be secreted even after the two Vagi are divided. This type of gastric juice, therefore, would not be due to influence of nerves. The introduction of meat into the main stomach of a Pawlow dog is followed in from 20 to 30 minutes by a secretion of gastric juice. The substances which stimulate gastric secretion act by directly excit ing the cells of the gastric membrane, which, in turn, produce a hormone. The word hormone comes from the word (I stimulate). There is a hormone formed in the pyloric end of the stomach, according to Edkins. It is called gastric secretin or Gastrin. Hormones are readily diffusible. They are of low molecular weight and each exercises a specific function in stimulating the activity of one particular organ or tissue, and when this function is performed it is rapidly destroyed in the body. It does not act as an antigen, that is does not excite the production of an anti body which would interfere with the perform ance of its function.