Human Anatomy (with the Fundamentals of Dynamic and Sports Morphology) - Ivanitsky, M. F. 2008

The Doctrine of Internal Organs
General Part

It is customary to refer to the Organs located predominantly within Body Cavities (the thoracic, abdominal, and pelvic cavities) as Internal Organs.

The viscera (internal organs) encompass the Components of the digestive, respiratory, and urogenital systems. The Digestive System supplies the body with nutrients and eliminates waste products. The Respiratory system provides the body with oxygen and removes carbon dioxide. The Urinary Organs excrete harmful Metabolic waste products from the body. The reproductive organs perform the function of propagation and the continuation of the species.

All germ layers participate in The Development of internal organs (see p. 44).

Internal organs are divided into hollow (tubular) and parenchymal organs, or glands. The former serve to transport and process contents, while the latter secrete substances (juices). All hollow, or tubular, organs share a common structural plan. Their wall consists of three main membranes: the mucous membrane, the muscular coat, and the serous membrane (Fig. 70).

The mucous membrane (mucosa) performs protective, secretory, absorptive, and other Functions. The lamina propria of the mucosa is formed by loose Fibrous Connective Tissue containing Blood Vessels and nerves. Facing the lumen of the organ, the mucous membrane is covered by an epithelium, which may be stratified squamous in some organs, columnar in others, and cuboidal in still others. It forms glands that release their products—secretions—into the lumen of the given organ. Structurally, glands are classified as alveolar, tubular, or mixed, as well as simple, branched, or compound. The simplest glands are unicellular (for example, goblet Cells of the intestine that secrete mucus). In large, complexly structured glands, the secretory epithelium is located only at the blind end of the gland, whereas the release of secretion into the organ's lumen occurs through excretory ducts whose walls contain a layer of Cytology/cytology/32.html">Smooth Muscle tissue. Glands of the digestive, respiratory, and urogenital systems are classified as exocrine, releasing their secretions into the lumen of an organ. Glands that release incretions (Hormones) into the blood or tissue fluid constitute an independent apparatus of endocrine organs (see p. 408).

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Fig. 70. General structural plan of the digestive tract:

I - mucous membrane (mucosa); II - submucosa; III - muscular coat (muscularis); IV - serous membrane or adventitia; 1 - epithelium; 2 - muscular layer of the mucosa (muscularis mucosae); 3 - lamina propria of the mucosa; 4 - goblet cells; 5 - glands in the lamina propria of the mucosa; 6 - glands located in the submucosa; 7 - vascular plexuses; 8 - submucosal nerve plexus (Meissner's plexus); 9 - myenteric nerve plexus (Auerbach's plexus); 10 - gastric pits; 11 - intestinal crypts; 12 - intestinal villi (after V. G. Eliseev et al.)

Aggregations of lymphoid tissue performing a protective function are frequently found within the mucous membrane. These aggregations may be diffuse (solitary lymphatic follicles) or concentrated (aggregated follicles).

The muscular coat in most organs is constructed of smooth muscle tissue and consists of two layers: an outer longitudinal layer and an inner circular layer. In certain organs (such as The Stomach), it consists of three layers, with an additional layer formed by oblique fibers. The simultaneous contractions of the fibers of the longitudinal and circular layers produce the so-called peristaltic movement (Fig. 71). This is most clearly observed in the stomach, intestines, and the excretory ducts of various glands. Its essence lies in the fact that when the circular layer contracts in one region, it simultaneously relaxes in an adjacent region. This contraction moves along the digestive tube, gland ducts, and the like, forming a peristaltic wave. Peristaltic waves facilitate the movement of the organ's contents.

The serous membrane, which forms the outer layer, is connective tissue in nature. It covers many organs, particularly those of the abdominal and thoracic cavities. In the Abdominal cavity, it is termed the Peritoneum, and in the thoracic cavity, the Pleura and Pericardium. Its free surface is lined with mesothelium, which gives it a smooth, shiny appearance. Each serous membrane has two layers (leaves): one of them—the parietal layer—lines the walls of the cavity (such as the abdominal or thoracic cavity) in which the respective organs are located, while the other—the visceral layer—covers the organs themselves. Between these two layers, and between individual organs covered by the serous membrane, There is a slit-like space termed the serous cavity (peritoneal cavity, pleural cavity). At the sites where one layer transitions into the other, the serous membrane forms folds known as mesenteries. The smoothness of the mesothelial covering and the moistening of its surface by serous fluid, produced by mesothelial cells, reduce friction during organ sliding.

In certain instances (in relatively immobile organs), the outer layer is represented by the adventitia—loose fibrous connective tissue that covers the organ and transitions directly into the connective tissue septa between bundles of its muscle fibers.

Fig. 71. Diagram of peristaltic movement (wave of peristaltic contraction):

A — Small Intestine; B — Large Intestine; C — direction of contraction of the circular muscle layer of the intestine; in regions a—b — narrowing, b—c — widening of the intestinal lumen (after Braus)