The diseases of the intestines resemble in many-respects those of the stomach, but there are important differences. In structure the intestine differs from the stomach in several respects. We no longer have the specific glands peculiar to the stomach, but, on the other hand, the intestine presents numerous closed lymphatic follicles in its mucous membrane, and these are only present to a very slight extent in the stomach. The lymphatic follicles are solitary or collected into groups, in the latter case forming the well-known Peyer's patches.
After leaving the stomach the food passes rapidly through the upper part of the small intestine, occupying on an average two and a half to three hours in doing so, and it is at the same time rendered alkaline and partially protected from further decomposition by the pancreatic fluid and the bile. The movement of the intestinal contents is effected by the peristaltic contraction of the bowel, and the rapid passage of the contents through the small intestine indicates that here the peristalsis is peculiarly active, whereas, in the large intestine, it is slow. When the faeces reach the large intestine they are still fluid, and the chief function of the colon seems to be to complete the absorption of the fluid, and allow the faeces to become thicker. But if the peristaltic action of the large intestine be increased, then there will be no time for the faeces to become thick, and fluid evacuations will be the result. This will be still more the case should the movement of the small intestine be increased, and the contents carried through it even more quickly than usual.
It will be seen that Diarrhoea results from increased peristaltic movement, and that the evacuations will be more fluid the higher up the increased movement begins. Certain medicinal agents produce fluid motions, and these seem to act generally by increasing the peristalsis, although some appear to produce their effects by causing a transudation of fluid into the canal (Hay). Irritating articles of food produce a like increase of the peristalsis and consequent diarrhoea, and so may ulcers and inflammations.
In the stools in diarrhoae we may expect to find chemical constituents which normally are present in the higher parts of the intestine, but are absorbed before reaching the rectum. If the diarrhoea arise from increased peristalsis of the colon, then we shall find material which is normal in the caecum, such as undecomposed bile, leucin, chloride of sodium, peptones, and sugar, some of which are present in appreciable quantities in normal faeces. But if the diarrhoea has involved the small intestine, then we shall find these constituents much more abundantly, and also remains of undigested food.
We have already seen in the case of the stomach that many of its diseases are connected with the fact that the food stagnates and decomposes in that viscus. It will be seen from what has gone before that the intestinal contents stagnate chiefly in the Large intestine, and next to that in the lower part of the small intestine. It is probably due to this that we find the jejunum peculiarly free from all forms of disease; in this respect contrasting with the lower part of the small intestine, the ileum, but still more with the large intestine. Hence it is that the diseases of the large intestine resemble those of the stomach much more than dp those of the small intestine. This is especially true in regard to simple inflammations, which very often are concentrated on those parts where the intestinal contents most readily stagnate, namely, the caecum and the rectum. It is true also of cancer, which is very rare in the small intestine but common in the large, especially in the caecum and rectum.
It is to be remembered, further, that the intestine is a comparatively narrow tube, and is subject to obstruction in various ways.
These are not so important as those of the stomach. After death the blood is apt to gravitate towards the more dependent parts of the wall of the intestine, and the colouring matter being dissolved out and staining the mucous membrane, it may give rise to a deceptive appearance of inflammation. Similarly the intestine may be stained with the biliary colouring matter in the neighbourhood of the gall-bladder.
Cohnheim, Allg. Path., 1882, ii., 132; Nothnagel, Phys. u. Path, des Darmes, 1884; Hay, Jour, of Anat. and Phys., xvii., 1883; Markwai.d, Virch. Arch., lxiv., 505, 1875.