HUMAN MEDICAL BIOLOGY, ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedonyuk 2010

ANATOMY, PHYSIOLOGY, PATHOLOGY

CHAPTER 7. THE NUTRITIONAL PROCESS AND ITS PATHOLOGY

ANATOMY AND PHYSIOLOGY OF THE DIGESTIVE ORGANS

LARGE INTESTINE

1. PARTS OF THE LARGE INTESTINE

The Large Intestine (intestinum crassum) is the direct continuation of the Small Intestine, measuring 1.5–2 m in length. It begins with the cecum, located in the right iliac fossa. The vermiform Appendix extends from the cecum. The large intestine continues as the colon, which comprises ascending, transverse, and descending parts. The descending part (descending colon) passes into the sigmoid colon and terminates in the rectum, which opens outward through the anus.

Characteristic Features of the large intestine include not only its larger diameter compared to the small intestine, but also the presence of three teniae (taeniae coli) formed by the longitudinal fibers of the muscular coat (Fig. 7.24).

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Fig. 7.24. Fragment of the transverse colon

Because the length of the intestine exceeds the length of the taeniae, the intestinal wall bulges out to form haustra. Additionally, throughout the entire large intestine, there are fat-containing Appendices epiploicae (fat-filled tags of the serous coat). These features are absent in the rectum.

The mucosa of the large intestine features semilunar folds and lacks villi. Lymphoid tissue forms only isolated lymphoid follicles.

The muscular coat consists of two layers: an inner circular layer and an outer longitudinal layer that forms the three taeniae.

The cecum (caecum) is the section of the large intestine located below the opening of the small intestine in the right iliac fossa. Its length is about 6 cm. The vermiform appendix (appendix) extends from the posteromedial surface of the cecum (Fig. 7.25). It can occupy various positions: 1) descending, in which its tip reaches the pelvic inlet; 2) lateral, where the appendix is deflected laterally; 3) ascending, where the appendix lies posterior to the cecum, parallel to it, and extends upward. The lumen of the appendix may be partially or completely obliterated. Its opening is covered by a mucosal fold known as the valve of the vermiform appendix. The mucosa of the appendix contains an Abundance of lymphoid tissue, which is why it is often referred to as the "abdominal tonsil." It is covered by Peritoneum on all sides and has a mesentery.

Fig. 7.25. The cecum

The junction between the small and large intestine is called the ileocecal junction (ileocecal angle). At this site, the mucosa forms a valve (ileocecal valve, or valve of Bauhin), which prevents the reflux of large intestine contents back into the small intestine.

The ascending colon (colon ascendens) is a continuation of the cecum. It ascends to the Inferior surface of the Liver, where it forms a bend (right colic flexure) and transitions into the transverse colon (Fig. 7.26). The ascending colon is covered by peritoneum on three sides and lacks a mesentery.

Fig. 7.26. The large intestine

The transverse colon (colon transversum) possesses a mesentery, lies intraperitoneally, and extends from the right colic flexure to the inferior end of the Spleen, where it forms the left colic flexure and transitions into the descending colon.

The descending colon (colon descendens) courses downward and, at the level of the iliac crest, transitions into the sigmoid colon. It is covered by peritoneum on three sides and lacks a mesentery.

The sigmoid colon (colon sigmoideum) extends to the level of the 3rd sacral vertebra, where it becomes the rectum. It is covered by peritoneum on all sides (lies intraperitoneally) and has a mesentery.

The rectum (rectum) is the terminal segment of the large intestine. It is situated in the lesser pelvis and forms two flexures: the sacral flexure and the perineal flexure. Its terminal part pierces the pelvic floor and ends at the anus.

In the upper part of the rectum, There is a dilation known as the ampulla (Fig. 7.27). At this Location, the mucosa forms three transverse folds corresponding to the flexures of the gut. The lower part of the rectum is narrowed and termed the anal canal. It contains longitudinal folds known as anal columns, between which lie the anal sinuses. The area between the sinuses and the anus is called the hemorrhoidal zone, which contains dense venous plexuses.

Fig. 7.27. Rectum

The muscular coat of the rectum consists of longitudinal and circular layers. The latter thickens in the area of the anal canal to form the involuntary internal anal sphincter. Externally, around the anus, striated circular Muscle fibers form the external anal sphincter, the contraction of which is under voluntary control. In its upper part, the rectum is covered by the peritoneum on all sides, in the middle part — on three sides, and in the lower part — it lies completely outside the peritoneal cavity.