granules in the protoplasm next the free side of the cell. During secretion these granules are pushed out of the cell, and seem in some way to form the secretion.

It will be seen immediately that one of the most important functions of the pancreatic juice is the formation of peptone from proteid, which operation is carried out by a special ferment called trypsin. It has been found that this ferment can only be obtained from the active pancreas, and that the wider the inner granular zone of the cells is, the richer in ferment is the glycerine extract made from the gland. But it has also been found that if a glycerine extract be at once made from an actively secreting, absolutely fresh gland, i. e., removed from the dead animal while still warm, the extract is found to be quite inert toward proteids, while an extract made from a portion of the same pancreas which has been kept some hours after death is very active; and a portion of the fresh pancreas pounded in a mortar with a little weak acid so as to develop the trypsin acts in an alkaline solution and forms peptone energetically.

We must therefore conclude that the special proteolytic ferment of the pancreas does not exist prior to the period at which the secretion is poured out from the gland cells.

Although a definite relation seems to exist between the amount of granules in the active cells and the degree of efficacy of the secretion, the ferment does not appear in full force for some time after the height of the gland activity has been established, and it is likely that the presence of an acid helps in the birth of the ferment.

It has therefore been assumed that the granules of the gland cells give rise, not to the proteolytic ferment, but to a fermentproducing substance which is called Zymogen.

So that if we trace the history of the pancreatic proteolytic ferment, we shall find that, so far as this trypsin is concerned, there can be no question as to whether it preexists in the blood and is removed thence by the gland or not, because by studying the process the final elaboration of the secretion is seen to take place after it has got into the ducts or into the intestinal cavity. Thus the blood gives nutriment to the protoplasm of the gland

cells. The protoplasm of the cells, by its intrinsic chemical processes, manufactures peculiar granules. These granules give rise, among other things, to zymogen, which in the presence of an acid begets trypsin.

PANCREATIC DIGESTION.

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 40° 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 XXIII), are but a step in the direction 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. (1) Saponification.-By the action of a special ferment (steapsin) 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; (c), 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 (Amylopsin). 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.

CHAPTER X.

BILE.

The liver has two chief functions,* which are so distinct in their ultimate object that they may be conveniently described separately. One of these, namely, the secretion of bile, is mainly excrementitious.

Bile is one of the fluids connected with digestion, being poured into the intestine, and therefore is treated under this

FIG. 71.

Section of the Liver of the Newt, in which the bile ducts
have been injected, and can be seen to form a network of
fine capillaries.

heading; but its influence upon digestion is not so great as was formerly supposed.

The other function of the liver is nutritive, consisting in the formation of glycogen. The glycogenic function of the liver belongs to the history of the nutrient materials after their absorption, and is of the first importance in attaining the elaboration of the blood, and will therefore be reserved for the chapter on that subject.

*The formation of urea may also be mentioned here, for there is no doubt, as will be seen later on in speaking of the excretions, that the liver has an important share in producing this substance.