The pancreatic juice is, of all digestive fluids, the most general solvent. It acts upon the three great classes of food stuffs which require modification to enable them to pass through the barrier that intervenes between the intestinal cavity and the blood current. It changes proteids into peptones, emulsifies fatty substances, and converts starch into soluble sugar. The ferments to which its activity is due may be separately described.

I. Action Of Pancreatic Juice On Proteids

The ferment which produces peptone is trypsin. Some of the conditions required for its perfect operation are the same as those necessary for the action of the gastric ferment, pepsin; namely, a certain degree of dilution, and a temperature of about 400 C. But it differs from pepsin in the most important characteristic of its action. While the presence of an acid is absolutely necessary for peptic proteolysis, we find that an alkaline reaction is required for this action of the pancreatic ferment, and as the peptic peptone has to pass through preliminary stages in which it closely resembles acid albumin, so the tryptic peptone is first produced from alkali albumin, which has been formed as a preliminary step by the alkali of the pancreatic juice. The addition of the sodium carbonate aids the action, and indeed seems to play a part which closely corresponds to that taken by the hydrochloric acid in gastric digestion.

The change to alkali albumin and peptone as accomplished by the trypsin, is not accompanied by any swelling of the albumin such as occurs in the formation of the acid albumin in the stomach, but the proteid is gradually eroded from the surface and thus diminished in size.

Moreover, the alkali albumin is not made directly into peptone, but passes through a stage in which it resembles globulin, and is soluble in solutions of sodium chloride.

Besides these differences between the mode of action of pepsin and trypsin in producing peptones, trypsin has a peculiar power upon proteids, which has no analogue in the peptic action. While the pancreatic peptone is being produced, a further change occurs, which gives rise to the formation of two crystallizable nitrogenous bodies known as leucin and tyrosin, the former belonging to the fatty acid, and the latter to the aromatic acid group. These substances, which are commonly found together as a result of the decomposition of peptones, seem inseparable from pancreatic digestion, and increase in amount toward the later stages of the process.

The amount of peptone produced reaches a maximum in about four hours, after which the proportion of the different unknown decomposition products appears to increase at the expense of the peptone. Among these substances must be named indol and skatol, the materials from which the process of pancreatic digestion derives its peculiarly disagreeable odor.

This breaking up of the surplus proteid food into bodies which cannot be of much utility in the economy, and which, as will appear hereafter (compare Chapter xxm), are but a step in the direstion of their elimination, is probably an important part of the pancreatic function, as it relieves the economy of a surcharge of albuminous substances.

Small quantities of phenol are also found in conjunction with the above.

II. Action On Fat

The action of the pancreatic juice on fats is of two kinds, (i) Saponification. - By the action of a special ferment (steapsiri) a small proportion of the neutral fats is split up into glycerine and the corresponding fatty acids. The acids thus produced readily unite with the alkali present, to form a little soap. The chemistry of the change will be found at p. 79, and may be shortly stated, taking olein as an example. Olein is a compound of oleic acid and glycerine. Olein in presence of this ferment and soda gives glycerine and oleic acid, and the latter combines with soda to form soap. This process materially aids in the next. (2) Emulsification. - Which means that the fat is reduced to a state of very fine subdivision, as it exists in milk. The production of this condition is facilitated by (a), the albumin in solution; (b), the alkalinity of the fluid; (V), the presence of soap alluded to above; and (d), the motion of the intestines. This process of emulsification may be imitated by adding about one-quarter volume of rancid linseed oil to a solution of sodic carbonate and shaking in a test tube. It will be found that the addition of a little soap and albumin will make the emulsion more perfect and more permanent.

III. Action On Starch

The amylolitic power of the pancreatic juice depends on a separate ferment (Amy/opsin). Its action seems to be identical with that of the saliva, with the exception that it is more rapid and energetic, and is said to affect raw as well as boiled starch. This power is found to exist in the extract of the gland, whether it has been removed from a fasting or from a recently fed animal, and therefore does not depend upon the gland being engaged in active secretion.