LESSON VII PANCREATIC DIGESTION 1. A 1-per-cent. solution of sodium carbonate, to which a little glycerine extract of pancreas has been added, forms a good artificial pancreatic fluid. 2. Half fill three test-tubes with this solution. A. To this add half its bulk of diluted egg-white (1 in 10). B. To this add a piece of fibrin. C. Boil this; cool; then add fibrin. 3. Put all into the water-bath at 40° C. After half an hour, test A and B for alkali-albumin by neutralisation, for albumoses by nitric acid, and for albumoses and peptone by the biuret reaction. 4. Note in B that the fibrin does not swell up and dissolve, as in gastric digestion, but that it is eaten away from the edges to the interior. 5. In C no digestion occurs, as the ferments have been destroyed by boiling. 6. Take a solution of starch, equal quantities in three test-tubes. D. To this add a few drops of pancreatic extract. E. To this add a few drops of bile. F. To this add both bile and pancreatic extract. 7. Put these into the water-bath, and test small portions of each every half-minute by the iodine reaction. It disappears first in F; then in D; while E undergoes no change. Test D and F for maltose by Trommer's test. 8. Shake up a few drops of olive oil with artificial pancreatic juice. A milky fluid (emulsion) is formed, from which the oil does not readily separate on standing. 9. The foregoing experiments illustrate the action that pancreatic juice has on all three classes of organic food. i. On Proteids.-Fibrin, albumin, &c. are converted into proteoses and peptone by the ferment trypsin in an alkaline medium. ii. On Carbohydrates.-Starch is converted into sugar (maltose) by the ferment amylopsin, especially in presence of bile. iii. On Fats.-These are emulsified. In the body they are also partly saponified by the ferment steapsin; but this cannot be shown with the artificial juice, as steapsin is not soluble in glycerine. The pancreas is a compound racemose gland, like the parotid. There are, however, histological points of difference between them. Microscopic examination of the gland-cells in different stages of activity reveals changes comparable to those already described in the case of salivary and gastric cells. Granules indicating the presence of a zymogen which is called trypsinogen (that is, the precursor of trypsin, E MEDICA the most important ferment of the pancreatic juice) crowd the cells before secretion: these are discharged during secretion, so that in an animal whose pancreas has been powerfully stimulated to secrete, as by the administration of pilocarpine, the granules are seen only at the free border of the cells (see fig. 21). As in gastric juice, experiments on the pancreatic secretion are usually performed with an artificial juice, made by mixing a weak FIG. 21.-Part of an alveolus of the rabbit's pancreas: A, before discharge; B, after. alkaline solution (1 per cent. sodium carbonate) with glycerine extract of pancreas. Quantitative experiments on human pancreatic juice gave the following results : The organic substances in pancreatic juice are (a) Ferments. These are the most important both quantitatively and functionally. They are four in number: i. Trypsin, a proteolytic ferment. ii. Amylopsin or pancreatic diastase, an amylolytic ferment. iii. Steapsin, a fat-splitting ferment. iv. A milk-curdling ferment. (b) A small amount of proteid matter, coagulable by heat. (c) Traces of leucine, tyrosine, xanthine, and soaps. The inorganic substances in pancreatic juice are Sodium chloride, which is the most abundant, and smaller quantities of potassium chloride, and phosphates of sodium, calcium, and magnesium. The alkalinity of the juice is due to phosphates and carbonates, especially of sodium. ACTION OF PANCREATIC JUICE The action of pancreatic juice, which is the most powerful and important of all the digestive juices, may be described under the headings of its four ferments. 1. Action of Trypsin.-Trypsin acts like pepsin, but with certain differences, which are as follows: (a) It acts in an alkaline, pepsin in an acid medium. (b) It acts more rapidly than pepsin, but the same series of proteoses can be detected as intermediate products in the formation of peptone. (c) An albuminate of the nature of alkali-albumin is formed in place of the acid-albumin of gastric digestion. (d) It acts more powerfully on certain albuminoids (such as elastin) which are difficult of digestion in gastric juice. (e) Acting on solid proteids like fibrin, it eats them away from the surface to the interior; there is no preliminary swelling as in gastric digestion. (f) Trypsin acts further than pepsin, on prolonged action partly decomposing the hemipeptone which has left the stomach into simpler products, of which the most important are leucine and tyrosine. It leaves the antipeptone unaffected. 2. Action of Amylopsin.-The conversion of starch into maltose is the most powerful and rapid of all the actions of the pancreatic juice. It is much more powerful than saliva, and will act even on unboiled starch. The absence of this ferment in the pancreatic juice of infants is an indication that milk, and not starch, is their natural diet. 3. Action on Fats.-The action of pancreatic juice on fats is a double one it forms an emulsion, and it decomposes the fats into fatty acids and glycerin by means of its fat-splitting ferment steapsin. The fatty acids unite with the alkaline bases to form soaps (saponification). The chemistry of this is described on p. 17. The fat-splitting power of pancreatic juice cannot be studied with a glycerine extract, as steapsin is not soluble in glycerine: either the fresh juice or a watery extract of pancreas must be used. The formation of an emulsion may be studied in this way. Shake up olive oil and water together, and allow the mixture to stand; the finelydivided oil globules soon separate from and float on the surface of the water; but if a colloid matter like albumin or gum is first mixed with the water, the oil separates more slowly. A more permanent emulsion is formed by an alkaline fluid, and especially when a small amount of free fatty acid is being continually liberated; the acid combines with the alkali to form a soap, which is stated to form a thin layer on the outside of each oil globule, and so prevents them running together again. Pancreatic fluid possesses all the necessary qualifications for forming an emulsion: i. It is alkaline. ii. It is viscous from the presence of proteid. iii. It has the power of liberating free acids. 4. Milk-curdling Ferment.-The addition of pancreatic juice to milk causes clotting; but this action can hardly ever be called into play, as the milk upon which the juice has to act has been already curdled by the rennin of the stomach. INTESTINAL DIGESTION The pancreatic juice does not act alone on the food in the intestines. There are, in addition, the bile, the succus entericus (secreted by the crypts of Lieberkühn), and bacterial action to be considered. The bile, as we shall find, has little or no digestive action by itself, but combined with pancreatic juice it assists the latter in all its actions. In our practical exercise we have already seen this is true for the digestion of starch. It is also true for the digestion of proteid, and very markedly so for the digestion of fat. Occlusion of the bileduct by a gall-stone or by inflammation prevents bile entering the duodenum. Under these circumstances the fæces contain a large amount of undigested fat. The succus entericus appears to have to some extent the power of converting starch into sugar; whether it acts on proteids or fats is very doubtful; its most important action is due to a ferment it contains called invertin, which inverts saccharoses that is, it converts. cane sugar and maltose into glucose. 1 Bacterial Action.-The gastric juice is an antiseptic; the pancreatic juice is not. A feebly-alkaline fluid like pancreatic juice is just the most suitable medium for bacteria to flourish in. Even in an artificial digestion the fluid is very soon putrid, unless special precautions to exclude or kill bacteria are taken. It is often difficult to say where pancreatic action ends and bacterial action begins, as many of the bacteria that grow in the intestinal contents, having reached that situation in spite of the gastric juice, act in the same way as the juice itself. Some 1 The original use of the term 'inversion ' has been explained on p. 11. It may be extended to include the similar hydrolysis of other saccharoses, although there. may be no formation of levorotatory substances. form sugar from starch, others peptone, leucine, and tyrosine from proteids, while others, again, break up fats. There are, however, certain actions that are entirely due to these putrefactive organisms. i. On carbohydrates. The most frequent fermentation they set up is the lactic acid fermentation: this may go further and result in the formation of carbonic acid, hydrogen, and butyric acid (see p. 13). Cellulose is broken up into carbonic acid and methane. This is the chief cause of the gases in the intestine, the amount of which is increased by vegetable food. ii. On fats. In addition to acting like steapsin, lower acids (valeric, butyric, &c.) are produced. The formation of acid products from fats and carbohydrates gives to the intestinal contents in the lower part of the intestine an acid reaction. iii. On proteids. Fatty acids and amido-acids, especially leucine and tyrosine, are produced; but these putrefactive organisms have a special action in addition, producing substances having an evil odour, like indole, skatole, and phenol. There are also gaseous products in some cases. If excessive, putrefactive processes are harmful; if within normal limits, they are useful, helping the pancreatic juice and, further, preventing the entrance into the body of poisonous products. It is possible that, in digestion, poisonous alkaloids are formed. Certainly this is so in one well-known case. Lecithin, a material contained in small quantities in many foods, and in large quantities in egg-yolk and brain, is broken up by the pancreatic juice into glycerine, phosphoric acid, stearic acid, and an alkaloid called choline. We are, however, protected from the poisonous action of choline by the bacteria, which break it up into carbonic acid, methane, and ammonia. LEUCINE AND TYROSINE These two substances have been frequently mentioned in the preceding pages. As types of the decomposition products of proteids they are important, though probably only small quantities are normally formed during digestion. They belong to the group of amido-acids. On p. 16 we shall find a list of the fatty acids; if we replace one of the hydrogen atoms in a fatty acid by amidogen (NH2), we obtain what is called an amido-acid. Take acetic acid: its formula is C,H,O2; replace one H by NH2, and we get C2H3(NH2)O2, which is amido-acetic acid or glycocine. If we take caproic acid-a term a little higher in the series-its formula is C6H12O2; amido-caproic acid is C6H11(NH2)O2, which is also called |