Two distinct varieties of gland are found in the small intestine. Those known as Brunner's glands are localized to the submucosa of the duodenum; they are insignificant in number when compared with the sound variety, called Lieberkiihn's glands, which are distributed over the entire intestinal tract and are closely set in the mucous membrane.

Portion of the Wall of the Small Intestine laid open to show the valvulae conniventes.

Fig. 76. Portion of the Wall of the Small Intestine laid open to show the valvulae conniventes. {Brinton).

Drawing of transverse section of the duodenum showing Brunner's Glands (b) opening into Lieberkiihn's follicles (l), (v) villi, (m) muscular coats.

Fig. 77. Drawing of transverse section of the duodenum showing Brunner's Glands (b) opening into Lieberkiihn's follicles (l), (v) villi, (m) muscular coats.

Brunner's glands form, in some animals, a dense layer in the submucous tissue of the beginning of the duodenum; they are small, branched saccular glands resembling mucous glands in structure. Owing to their small size the secretion cannot be obtained in sufficient quantity to make satisfactory experiments in respect to its properties. It is said to dissolve albumin and to have a diastatic fermentative action, so that probably the secretion is analogous to that of the pancreas, as Brunner originally supposed. The quantity of fluid secreted by these glands is so small that its existence is not taken into account in speaking of the intestinal juice, by which is meant the fluid poured out by the innumerable short tubes or follicles of the intestine.

These Lieberkiihrfs follicles belong to a very simple form of gland, each one being a single straight cavity in the mucous membrane hardly deep enough to deserve the name of a tube. In the small intestine they are set as closely as the villi permit. In the large intestine, where the villi are absent, they are more closely set and are also deeper (Fig. 78). They are bounded by a thin basement membrane which is embraced by a close capillary network of blood vessels, and are lined by a single layer of cylindrical or spherical epithelial cells.

Section of the Mucous Membrane of small intestine, showing Lieberkiihn's follicles.

Fig. 78. Section of the Mucous Membrane of small intestine, showing Lieberkiihn's follicles (a) with their irregular epithelium, and the villi {6) passing out of view; (c) Muscularis mucosae; {d) Submucous tissue. (Cadiat).

The epithelial covering of the processes known as villi, which are studded all over the mucous membrane of the small intestine, produce some mucus.

Method Of Obtaining Intestinal Secretion

Considerable difficulty has been found in obtaining the proper intestinal juice free from admixture with the secretions of the liver and pancreas which are carried along and mixed with it. A short portion of the small intestine has been successfully isolated from the rest without injuring the mesentery or its blood vessels. One of the extremities of the isolated portion was closed, and the other was retained by sutures at an opening in the abdominal wall. The cut ends of the remainder of the intestine were at the same time united, so that the continuity of the alimentary tract was preserved. Thus, a limited piece of gut formed a cul-de-sac from which the fluid could be collected through a fistulous opening.

Villus with the capillaries injected, showing their close relation to epithelium, some ot the cells of which are distended with mucus.

Fig. 79. Villus with the capillaries injected, showing their close relation to epithelium, some ot the cells of which are distended with mucus. {Cadiat).

Characters Of The Secretion

The liquid obtained from such a fistula is thin, opalescent and yellowish, with a strong alkaline reaction and a specific gravity of ion. It contains some proteid and other organic material, a ferment and inorganic salts in which sodium carbonate preponderates.

Mode Of Secretion

The secretion flows slowly from such a fistula, but the amount increases during digestion, showing that the secretion of the intestine is under the control of some nerve centre which can call the entire tract into action when one part is stimulated. The local stimulation of the mucous membrane makes it red, and causes it to pour out a more abundant secretion. Beyond this little is known of the nervous mechanism or the local cell changes which accompany the formation of the secretion.