Stomachic digestion, we have seen, is mainly that of plastic food, and the resulting fluid, the chyme, chiefly consists of fat, perhaps sugar, any starch unchanged by saliva, and indigestible residue.

Intestinal Digestion, which is chiefly of calorific food, begins in the duodenum, two fluids, bile and pancreatic juice, usually through the same opening, being poured on the food. The liver and bile will be found described in another chapter, but we shall here mention its uses in digestion, which are much less important than formerly supposed. We have seen that it arrests chemical change in the stomach, and, in the same way, it checks decomposition of the albuminoids, and thus prevents greater foetor in the fæces. It may neutralize, by its alkali (which, however, is very scanty) the acidity of the chyme, and is thought to stimulate the intestines, being often termed "nature's purge." Fotor and constipation are certainly symptoms of jaundice. Its main use is to promote osmose by rendering the walls of the vessels more permeable to fat, for if a capillary tube be wetted with bile, it will be found that oil will rise six or eight times higher in it than if dry or wetted with water. The detergent and saponifiable properties of gall are well known to scourers. Although it has then no direct digestive action it is not merely excrementitious, as, if excluded from the intestines of dogs, and yet perfectly excreted by a fistula, they die emaciated, no fat being absorbed. All the constituents of the bile are not found in the fæces, as their amount of sulphur is not increased, but, like gastric juice, it is being secreted, absorbed, and re-secreted by a kind of circulation.

The Pancreas is a flat, pinkish-white gland placed across the second lumbar vertebra, about 7 inches long, 3 broad, 1 or 2 thick, and about 3 oz. in weight. It resembles the salivary glands so much as to be named "the abdominal salivary gland," as it is lobulated, incapsulated merely by areolar tissue, and has no special

or single artery. It differs in being single, in having its duct all through it, and in its secretion. Its structure, relations, and abundant supply of blood, as well as that of some other digestive viscera, is shown in the annexed figure. Mercury injected shows that the gland is composed of lobules, which are cœcal terminations of the minute branches of the common duct connected by

The solid organs exposed by turning upwards the stomach. 1. Right Lobe of Liver. 2. Gall Bladder. 3. Left Lobe of Liver. 4, 5. Stomach. 6. Esophagus and Cardiac Orifice. 7. Spigelian Lobe of Liver. 8, 8. Cruræ. 9. Pyloric Orifice. 10, 10. Superior transverse and inferior transverse portions of Duodenum. 11, 11, 11. Head, body, and tail of Pancreas.

areolar tissue, and all lined by the peculiar secreting cell. The duct, which was discovered by Wirsungius, passes from tail to head, one-third from the posterior surface, receiving its tributaries at right angles, like the legs of a centipede, and, joining the bile duct, opens into the lower part of the vertical division of the duode

num. The gland exists in all vertebrates, save perhaps some fishes-the shark, ray, and some osseous fishes having it, and the pyloric appendages of others being analogous.

The secretion may be obtained by rapidly inserting a tube into the duct of a dog after it has been fully fed. It is a colourless, viscid, or syrupy fluid, frothing on being shaken; has a sp. gr. of 1.010, and is always alkaline. Every drop of it is changed into a solid mass by heat, nitric acid, alcohol, and some salts, becoming fluid again by adding an alkali. If the mass, solidified by heat or alcohol, be dried, it will dissolve in water, thus differing from albumen-which, however, Tiedemann and Gmelin said was present to the amount of 4 per cent. This animal matter is termed pancreatin, or phymatin. It rapidly putrifies, but does not ferment sugar nor change urea into carbonate of ammonia. On mixing pancreatic juice with oil, fat, or butter, and agitating for some minutes, a perfect emulsion, like milk, is obtained. The odour of butyric acid is evolved if butter be used, which shows that some separation of fatty acids and the base glycerin occurs. If the duct be tied, the fat is passed unabsorbed, and the chyle is found colourless. This explains those cases of "diarrhoea adiposa" reported by Dr. Bright, and produced by diseased pancreas. The same action is shown in the rabbit, where the pancreatic opens 17 or 18 inches lower than the bile duct; if the animal be fed on fatty food, the upper lacteals will contain a colourless, the lower a white chyle. This emulsifying power is owing to its albuminoid principle enveloping each particle of fat with a cell-wall, so that they are prevented from running together, and being thus kept in a finely divided state, are more easily taken up by the lacteals. No other animal fluid can emulsify, save, perhaps, that of Brunner's duodenal glands. It is thought, in common with saliva and many solid tissues, as that of the gland itself,

to saccharize starch. That the pancreatic juice, and not the saliva, is the agent for the conversion of starch into sugar is an opinion advocated by Pavy, who says that the saliva of dogs has no saccharizing power: however this may not be true of vegetable-feeders. Corvisart, Brinton, and Harley believe pancreatic juice may produce peptone from any albuminous matter which has escaped the action of gastric juice. This is probable if it become acid-for the main difference between these digestive fluids is their reaction; if acid, acting on nitrogenized matter; if alkaline, on unnitrogenized.

The Intestines measure about 25 feet, from the pylorus to the anus, the small intestine being about 20 feet, and the great intestine, from the cœcum to the anus, about 5. From this tube there is always a diverticulum, the vermiform appendix very large in rodents, but of unknown use. Other diverticula occasionally exist, and have been admirably described by Prof. Struthers. The coats of the intestines are arranged in four planes :—1. Serous, which is almost complete in the jejunum and the ileum, partial in the duodenum and the great intestine, and absent for the last 3 or 4 inches of the rectum. is indistensible, so a loose areolar layer ties it to (2) the muscular coat. This consists of two layers-a longitudinal, best marked along the attachment of mesentery, and a circular, far more abundant, surrounding, in complete circles, the (3) fibrous coat, in which vessels and nerves break up into very minute and numerous branches, the latter presenting frequent ganglioform dilatations. 4. The mucous coat, which is of the compound variety, bending inwards to form the follicles, or outwards, for about the same distance over the villi.

It

The Follicles (with which Lieberkuhn's name is often, without reason, connected) are cœcal tubes dipping through the whole depth of the mucous membrane, lined and almost blocked up by columnar epithelium, which produces the intestinal juice. They multiply the mucous

surface enormously, but decrease in number and depth gradually from the jejunum, as do also the valvulæ conniventes. These, which are so called because they flap or

Epithelial Cells from

wink like the eye-lids, are permanent folds of the fibrous coat, doubly covered by mucous membrane, whose surface they thus much increase. They pass somewhat spirally round the gut for about three-fourths of its calibre, and project into its cavity a quarter or half an inch. The Villi are like everted follicles, giving the membrane its velvety character, and increasing gradually from the duodenum to the jejunum, where 10,000 have been counted on the square inch. Thence they gradually get fewer, and cease on the surface of the

jejunum.

cœcal valves towards the ileum. They are slightly tapering, and about long. There is an artery on one side, a vein on the other, joined by a capillary flexus of astonishing minuteness surrounding a branching lactael.These vessels are supported by a fine areolar tissue filled with granules, which gradually develop into cells towards the free end of the villus. When active, a villus becomes opaque and turgid with blood, and under prolonged fasting, shrivels up. Kölliker has discovered in them a layer of muscular tissue continued from the intestine. It may force onward the chyle. In the duodenum alone are Brunner's glands, which are racemose, like The Artery, Vein, and Capil salivary or pancreatic glands,