The copious secretions of two of the largest glands of the body - the pancreas and the liver - are poured into the duodenum. This is the widest part of the small intestine, and the extent of the surface of its lining membrane is increased by crescentic, shelf-like projections called valvulce conniventes, so that its secreting follicles are numerous. In its walls are also small racemose glands not found in other parts of the alimentary tract.
The pancreas is a large compound sacculated or acinous gland, composed of numerous irregular packets of gland tissue attached by its lateral branchlets to the main central duct. The saccules are elongated, and have the same general construction as those of the serous salivary glands already described, but they are less closely held together by the intervening connective tissue, and thus the pancreatic tissue does not show such a regular and compact arrangement on section as the salivary glands. A single layer of irregular or slightly conical secreting cells in the saccule, shows a difference of structure in its central or peripheral sides, so that an external or homogeneous zone, and an internal granular zone, may be distinguished. Each zone corresponds to one-half of the cells, the clear half being next the boundary, and the granular half next the lumen of the saccule. The relative width of these zones varies with the digestive process, so that the nuclei which are situated between them sometimes appear to be in the outer clear zone, and sometimes in the inner granular zone. The outer zone colors readily with carmine, while the inner zone remains unstained.
The large duct which passes down the axis of the gland, receiving tributaries on all sides, is surrounded with a layer of loose connective tissue which forms its outer coat. The proper coat of the duct is composed of elastic tissue, lined by a single layer of cylindrical epithelium.
From a temporary fistula the secretion of the pancreas can be obtained in sufficient quantity to determine its character and properties. It is difficult to establish a satisfactory permanent fistula: the secretion soon alters its characters, becoming thin, and losing its efficacy, probably owing to an altered or abnormal state of the gland.
An artificial pancreatic juice may be extracted by water from the gland taken a few hours after death from an animal killed during active digestion (a couple of hours after eating) and carefully minced. This extract, used with proper precautions, will have the same effect as the secretion itself.
A glycerine solution containing the active principles of the pancreatic secretion may be made from the pancreas by treating the minced gland for a couple of days with absolute alcohol, removing the alcohol, and allowing it to soak for a week in sufficient glycerine to cover it. This glycerine extract, filtered, contains but little else than pancreatic ferments.
The pancreatic juice is a very thick, transparent, colorless, strongly alkaline fluid, which turns to a jelly if cooled to o° C. It often contains about ten per cent, of solids when obtained from a temporary fistula, but it may have as little as two per cent.
Of these a considerable proportion are organic, namely: -
1. Albumin, which is coagulated by boiling.
2. Alkali albumin, precipitated by acetic acid or by adding magnesium sulphate to saturation.
3. Leucin and tyrosin.
4. Fats and soaps.
5. Salts, particularly sodium carbonate, to which it owes its alkalinity.
6. Three ferments, to which it owes its specific action on the food stuffs.
The pancreas does not continue in a state of activity during the interval between the periods of active digestion. When the gland is at rest it is of a pale yellow color, and is flaccid, but during active digestion it becomes more turgid, and assumes a pinkish color, from the increased flow of blood. The secretion commences immediately after taking food, and rises rapidly for a couple of hours, then falls and rises again in the later hours of digestion, five to seven hours after a meal: then it gradually falls for eight to ten hours, and ceases completely when digestion is at an end. The first rise which accompanies the introduction of food into the stomach is certainly brought about by nervous agencies of a similar nature to those of the stomach, the secretion of which follows closely upon mastication. The second accompanies the passage of the undigested food through the small intestines, and may be most conveniently explained as the result of reflex nervous stimulation of the gland cells.
The great complexity of the nervous mechanism of the glands of the intestinal tract makes it difficult to ascertain the exact channels traversed by the afferent and efferent impulses. The following observations, if accurate, would tend to prove that certain inhibitory impulses pass from the stomach along the vagus to the medulla, and are thence reflected to the gland by its vasomotor nerves. During vomiting, or when the central end of the divided vagus is stimulated, the secretion of the pancreas ceases. Section of the nerves which surround the blood vessels distributed to the pancreas causes considerable (paralytic) flow of secretion which stimulation of the vagus cannot check.
No nerve channels have been demonstrated to carry exciting impulses direct to the glands, as the chorda tympani does to the submaxillary; but the direct stimulation of the gland itself, or of the medulla oblongata, is said to induce activity of the gland.