Body Cavities and Membranes
During embryonic development, the body is first divided into two internal cavities: the posterior (dorsal) body cavity and the anterior (ventral) body cavity. Each of these major cavities is then subdivided into smaller cavities. The cavities, as well as the organs in the cavities (called the viscera), are lined by membranes.
Posterior (Dorsal) Body Cavity
The posterior body cavity is subdivided into two parts: (1) The cranial cavity, enclosed by the bony cranium, contains the brain. (2) The vertebral canal, enclosed by vertebrae, contains the spinal cord (Fig. 1.5a) The posterior body cavity is lined by three membranous layers called the meninges. The most inner of the meninges is tightly bound to the surface of the brain and the spinal cord. The space between this layer and the next layer is filled with cerebrospinal fluid. Spinal meningitis, a serious condition, is an inflammation of the meninges usually caused by an infection.
Anterior (Ventral) Body Cavity
The large anterior body cavity is subdivided into the superior thoracic cavity and the inferior abdominopelvic cavity (Fig. 1.5a). A muscular partition called the diaphragm separates the two cavities. Membranes that line these cavities are called serous membranes because they secrete a fluid that has just about the same composition as serum, a component of blood. Serous fluid between the smooth serous membranes reduces friction as the viscera rub against each other or against the body wall.
Figure 1.5 The two major body cavities and their subdivisions. a. Left lateral view b. Frontal view.
To understand the relationship between serous membranes and an organ, imagine a ball that is pushed in on one side by your fist. Your fist would be covered by one membrane (called a visceral membrane), and there would be a small space between this inner membrane and the outer membrane (called a parietal membrane).
The thoracic cavity is enclosed by the rib cage, and has three portions: the left, right, and medial portions. The medial portion, called the mediastinum, contains the heart, thymus gland, trachea, esophagus, and other structures (Fig. 1.5b). The right and left portions of the thoracic cavity contain the lungs. The lungs are surrounded by a serous membrane called the pleura. The parietal pleura lies next to the thoraic wall, and the visceral pleura adheres to a lung. In between the two pleura, the pleural cavity is filled with pleural fluid. Similarly, in the mediastinum, the heart is covered by the two-layered membrane called the pericardium. The visceral pericardium which adheres to the heart is separated from the parietal pericardium by a small space called the pericardial cavity (Fig. 1.5b). This small space contains pericardial fluid.
The abdominopelvic cavity has two portions: the superior abdominal cavity and the inferior pelvic cavity. The stomach, liver, spleen, gallbladder, and most of the small and large intestines are in the abdominal cavity. The pelvic cavity contains the rectum, the urinary bladder, the internal reproductive organs, and the rest of the large intestine. Males have an external extension of the abdominal wall, called the scrotum, where the testes are found. Many of the organs of the abdominopelvic cavity are covered by the visceral peritoneum, while the wall of the abdominal cavity is lined with the parietal peritoneum. Peritoneal fluid fills the cavity between the visceral and parietal peritoneum. Peritonitis, another serious condition, is an inflammation of the peritoneum, again usually caused by an infection.
Figure 1.6 Clinical subdivisions of the abdomen into quadrants. These subdivisions help physicians identify the location of various symptoms.
Clinically speaking, the abdominopelvic cavity is divided into four quadrants by running a transverse plane across the midsagittal plane at the point of the navel (Fig. 1.6a). Physicians commonly use these quadrants to identify the locations of patients’ symptoms. The four quadrants are: (1) right upper quadrant, (2) left upper quadrant, (3) right lower quadrant, and (4) left lower quadrant. Figure 1.6b shows the organs that lie within these four quadrants.