 Lymphatic vessels. In general, there are no lymphatic vessels in the core of lamina propria between the intestinal glands and none that extend toward the luminal surface of the large intestine. Only recently, using new very selective markers for lymphatic epithelium, researchers have found occasional small lymphatic vessels at the bases of the intestinal glands. These lymphatic vessels drain into the lymphatic network within the muscularis mucosae. The next step in lymph drainage occurs in the lymphatic plexuses in the submucosa and muscularis externa before lymph leaves the wall of the large intestine and drains into the regional lymph nodes. To understand the clinical significance of the lymphatic pattern in the large intestine, see Folder 17.6.
As noted, in the cecum and colon (the ascending, transverse, descending, and sigmoid colons), the outer layer of the muscularis externa is, in part, condensed into prominent longitudinal bands of muscle, called teniae coli, which may be seen at the gross level (see Fig. 17.27). Between these bands, the longitudinal layer forms an extremely thin sheet. In the rectum, anal canal, and vermiform appendix, the outer longitudinal layer of smooth muscle is a uniformly thick layer, as in the small intestine.
Bundles of muscle from the teniae coli penetrate the inner, circular layer of muscle at irregular intervals along the length and circumference of the colon. These apparent discontinuities in the muscularis externa allow segments of the colon to contract independently, leading to the formation of haustra colli, sacculations of the colon wall.
The muscularis externa of the large intestine produces two major types of contraction: segmentation and peristalsis. Segmentation is local and does not result in the propulsion of contents. Peristalsis results in the distal mass movement of the colonic contents. Mass peristaltic movements occur infrequently; in healthy persons, they usually occur once a day to empty the distal colon.
The submucosa of the large intestine corresponds to the general description already given. Where the large intestine is directly in contact with other structures (as on much of its posterior surface), its outer layer is an adventitia; elsewhere, the outer layer is a typical serosa.
The cecum forms a blind pouch just distal to the ileocecal valve; the appendix is a thin, fingerlike extension of this pouch. The histology of the cecum closely resembles that of the rest of the colon; the appendix differs from it in having a uniform layer of longitudinal muscle in the muscularis externa (Fig. 17.30 and Plate 63, page 624). The most conspicuous feature of the appendix is the large number of lymphatic nodules that extend into the submucosa. In many adults, the normal structure of the appendix is lost, and the appendage is filled with fibrous scar tissue. Blockage of the opening between the appendix and the cecum, usually due to scarring, buildup of thick mucus, or stool that enters the lumen of the appendix from the cecum, may cause appendicitis
FIGURE 17.30 • Photomicrograph of a cross section through the vermiform appendix. The vermiform appendix displays the same four layers as those of the large intestine except that its diameter is smaller. Typically, lymphatic nodules are seen within the entire mucosa and usually extend into the submucosa. Note the distinct germinal centers within the lymphatic nodules. The muscularis externa is composed of a relatively thick circular layer and a much thinner outer longitudinal layer. The appendix is covered by a serosa that is continuous with the mesentery of the appendix (lower right). 10.
 FOLDER 17.6 Clinical Correlation: The Pattern of Lymph Vessel Distribution and Diseases of the Large Intestine