The small intestine is composed of four principal coats: the serous, muscular, submucous, and mucous (Fig. 2). The serous coat is formed by the visceral layer of the peritoneum. The muscular coat consists of an internal circular and an external longitudinal layer (Fig. 3). The former is usually considerably thicker than the latter. They both consist of bundles of unstriped muscular tissue supported by connective fibres. The submucous coat consists of connective tissue in which numerous blood-vessels and lymphatics ramify. The mucous membrane is the most important coat with regard to the function of digestion. It consists of a very thin muscular layer (muscularis mucosae) containing circular and longitudinal fibres, the tunica propria of the mucosa, a tissue made up principally of reticular connective tissue with numerous leucocytes, glands, and the epithelial covering. The mucous membrane of the small intestine is of a grayish-red color and has a velvety appearance. It possesses certain large folds of valvular flaps (valvulae conniventes Kerkringi) (Fig. 4). These are-permanent crescent ic folds of mucous membrane set transversely to the long axis of the intestine. Each one extends from one-half to two-thirds of the distance of the lumen.

The largest are more than two inches long and about one-third of an inch wide. They begin somewhat below the pylorus, are very large just below the entrance of the bile duct, remain conspicuous until the middle of the jejunum is reached, then become smaller and gradually disappear at the lower part of the ileum. They serve to increase the surface of the mucous membrane.

Longitudinal Cross section through the Wall of the Small Intestine

Fig. 2. - Longitudinal Cross-section through the Wall of the Small Intestine (Ileum). Solitary lymph nodules (nodulus lymphatious solitarius). Intestinal glands (Lieber-kuehni) (Toldt). a. The mucous layer; h. the muscularis mucosae; c, the submucous layer; d', the muscular layer; e. thesubserosa; f. the serous layer; g, intestinal villi: h, intestinal glands (Lieberkuehn); i, blood-vessels; k, a solitary lymph nodule; I, its centre.

Longitudinal Cross section through the Wall of the Duodenum

Fig;. 3. - Longitudinal Cross-section through the Wall of the Duodenum. Brunner's glands (glandulae duodenales) (Toldt). a. The mucous layer; b, the muscularis mucosae; c. the submucous layer; d, the circular muscular layer; e, the longitudinal muscular layer; f, intestinal villi; g, intestinal glands (Lieberkuehn); h, Brunner's duodenum glands: i, serous layer.

The microscopical anatomy of the mucous membrane reveals the following: The entire inner surface of the small intestine is composed of villi, certain papilliform processes, and glands; an epithelial layer containing columnar epithelial cells with a striated border, and some goblet cells cover the entire surface. The villi are formed principally by elevations of the tunica propria of the mucous membrane (Fig. 5). They are about 0.5 to 0.7 mm. in height and about 0.1 to 0.2 mm. wide and number almost ten millions. Each villus possesses a centrally located space for chyle which is covered with endothelial cells and connected with the lymphatics of the intestinal mucosa. Each villus contains a perfect arrangement of blood-vessels and muscular fibres which originate in the muscularis mucosa. When filling up with blood each villus expands, while under the contraction of its muscle it shrinks. Thus it is enabled to perform the function of suction and pumping. The villi form the main organ for the absorption in the small intestine.

Jejunum Partly Opened

Fig. 4. - Jejunum Partly Opened (Toldt). a. Serosa; b, mucosa: c, circular folds of Ker-kring.

Mucous Membrane of the Ileum with a Solitary Lymph Nodule (Toldt)

Fig. 5. - Mucous Membrane of the Ileum with a Solitary Lymph Nodule (Toldt). a. Intestinal glands (Lieberkuehn): b, intestinal villi: c, a solitary lymph nodule.

Ileum Partly Opened (Toldt)

Fig. 6. - Ileum Partly Opened (Toldt). a. Solitary lymph nodules; b, serosa; c, mucosa.

Around the villi lie their glands. First, there are tubular glands (of Lieberkuhn), and, secondly, acinous glands of Brunner. The former are similar in structure to the tubular glands in the stomach. They cover almost the entire surface of the whole small and large intestine. Each glandular tubule is about 0.3 to 0.4 mm. long and opens without forming any ramifications. They number over forty millions and form the principal organ of intestinal secretion. Brunner's glands are confined to the duodenum. They are most abundant at the commencement of this portion of the intestine, diminishing gradually as the duodenum advances. They are situated beneath the mucous membrane and embedded in the submucous tissue. Each gland is a branched and convoluted tube lined with columnar epithelium. In structure they are very similar to the pyloric glands of the stomach, but are more branched and convoluted, and their ducts are longer. The duct of each gland passes through the muscularis mucosae and opens on the surface of the mucous membrane. Solitary follicles or glands are found scattered throughout the mucous membrane of the small intestine. They are most numerous in the lower part of the ileum. Each one has a diameter of from 3 to 6 mm.

The structure of the solitary follicle is similar to that of the lymph nodes and consists of a dense reti-form tissue packed with lymph corpuscles and permeated by fine capillaries. There are no ducts. The interspaces of the retiform tissue are continuous with larger lymph spaces at the base of the gland, by which they communicate with the lacteal system. The base of the nodules is in the submucous tissue. It penetrates the muscularis mucosae and enters the mucous membrane forming a slight projection of its epithelial layer. The solitary follicles are the breeding place of the lymph cells. They are met with in two conditions, namely, either scattered singly, in which case they are termed glandulae solitariae (Fig. 6), or aggregated in groups varying from one to three inches in length and about one-half inch in width. The surface of the solitary follicles is free from villi. Chiefly of an oval form, their long axis is parallel with that of the intestine. In this state they are called glandulae agminatae or Peyer's patches or plaques (Fig. 7). They are almost always placed opposite the attachment of the mesentery.

Peyer's patches number about twenty to twenty-eight. In some cases they are already found in the jejunum, but they are most prevalent in the ileum.

Peyer's Patch (Noduli Lymphatici Aggregati) in the Ileum (Toldt)

Fig. 7.-Peyer's Patch (Noduli Lymphatici Aggregati) in the Ileum (Toldt). a, Peyer's patch; b, solitary lymph nodules.