A presentation in sectional view is sometimes the only way to illustrate the relationships among the parts of a three-dimensional object. An understanding of sectional views has become increasingly important since the development of electronic imaging techniques that enable us to see inside the living body without resorting to surgery.

Planes and Sections

Any slice through a three-dimensional object can be described with reference to three sectional planes, indicated in Figure 1-9 and Table 1-3. The transverse plane lies at right angles to the long axis of the body, dividing it into superior and inferior sections. A cut in this plane is called a transverse section, or cross section. The frontal plane, or coronal plane, and the sagittal plane are parallel to the long axis of the body. The frontal plane extends from side to side, dividing the body into anterior and posterior sections. The sagittal plane extends from front to back, dividing the body into left and right sections. A cut that passes along the midline and divides the body into left and right halves is a midsagittal section, or median section; a cut parallel to the midsagittal line is a parasagittal section.

Body Cavities

Many vital organs are suspended in internal chambers called body cavities. These cavities have two essential functions: (1) They protect delicate organs, such as the brain and spinal cord, from accidental shocks and cushion them from the thumps and bumps that occur when we walk, jump, or run; and (2) they permit significant changes in the size and shape of internal organs. For example, because they are inside body cavities, the lungs, heart, stomach, intestines, urinary bladder, and many other organs can expand and contract without distorting surrounding tissues or disrupting the activities of nearby organs.

The dorsal body cavity contains the brain and spinal cord. The much larger ventral body cavity contains organs of the respiratory, cardiovascular, digestive, urinary, and reproductive systems. The relationships among the dorsal and ventral body cavities and their subdivisions are indicated in Figure 1-10 and shown in Figure 1-11.

Dorsal Body Cavity The dorsal body cavity (Figure 1-11a,c) is a fluid-filled space whose limits are established by the cranium, the bones of the skull that surround the brain, and the vertebrae of the spinal column. The dorsal body cavity is subdivided into the cranial cavity, which contains the brain, and the spinal cavity, which contains the spinal cord.

Ventral Body Cavity As embryological development proceeds, internal organs grow and their relative positions change. These changes lead to the subdivision of the ventral body cavity, or coelom. The diaphragm, a flat muscular sheet, divides the ventral body cavity into a superior thoracic cavity, bounded by the chest wall, and an inferior abdominopelvic cavity, enclosed by the abdominal wall and by the bones and muscles of the pelvis.

Many of the organs in these cavities change size and shape as they perform their functions. For example, the lungs inflate and deflate as you breathe, and your stomach swells at each meal and shrinks between meals. These organs are surrounded by moist internal spaces that permit expansion and limited movement but prevent friction. The thoracic cavity is subdivided into three separate spaces, whereas the abdominopelvic cavity contains a single, extensive chamber. The internal organs that are partially or completely enclosed by these cavities are called viscera. A delicate layer called a serous membrane lines the walls of these internal cavities and covers the surfaces of the enclosed viscera. Serous membranes secrete a watery fluid that coats the opposing surfaces and reduces friction.

The Thoracic Cavity The walls of the thoracic cavity surround the lungs and heart; associated organs of the respiratory, cardiovascular, and lymphatic systems; the inferior portions of the esophagus; and the thymus. The thoracic cavity contains three internal chambers: a single pericardial cavity and a pair of pleural cavities (Figure 1-11a). These cavities are lined by shiny, slippery serous membranes.

The heart is surrounded by the pericardial cavity. The relationship between the heart and the cavity resembles that of a fist pushing into a balloon (Figure 1-11b). The wrist corresponds to the base (attached portion) of the heart, and the balloon corresponds to the serous membrane that lines the pericardial cavity. The serous membrane is called the pericardium. The layer covering the heart is the visceral pericardium, and the opposing surface is the parietal pericardium. The space between the visceral pericardium and the parietal pericardium is the pericardial cavity. During each beat, the heart changes in size and shape. The pericardial cavity permits these changes, and the slippery pericardial lining prevents friction between the heart and adjacent structures in the thoracic cavity.

The pericardium lies within the mediastinum, the portion of the thoracic cavity between the left and right pleural cavities (Figure 1-11d). The connective tissue of the mediastinum surrounds the pericardial cavity and heart, the large arteries and veins attached to the heart, and the thymus, trachea, and esophagus.

One pleural cavity lies on each side of the mediastinum. Each pleural cavity encloses a lung. The relationship between a lung and a pleural cavity is comparable to that between the heart and the pericardial cavity. The serous membrane lining a pleural cavity is called a pleura. The visceral pleura covers the outer surfaces of a lung, and the parietal pleura covers the opposing mediastinal surface and the inner body wall.

The Abdominopelvic Cavity The abdominopelvic cavity extends from the diaphragm to the pelvis. It is subdivided into a superior abdominal cavity and an inferior pelvic cavity (Figure 1-11a,c). The abdominopelvic cavity contains the peritoneal cavity, a chamber lined by a serous membrane known as the peritoneum. The parietal peritoneum lines the inner surface of the body wall. A narrow space containing a small amount of fluid separates the parietal peritoneum from the visceral peritoneum, which covers the enclosed organs.

The abdominal cavity extends from the inferior surface of the diaphragm to an imaginary plane extending from the inferior surface of the lowest vertebra to the anterior, superior margin of the pelvis. This cavity contains the liver, stomach, spleen, small intestine, and most of the large intestine. (The positions of most of these organs are shown in Figure 1-7c.) These organs are partially or completely enclosed by the peritoneal cavity, much as the heart or lungs are enclosed by the pericardial or pleural cavities. A few organs, such as the kidneys and pancreas, lie between the peritoneal lining and the muscular wall of the abdominal cavity. Those organs are said to be retroperitoneal (retro, behind).

The pelvic cavity is the portion of the ventral body cavity inferior to the abdominal cavity. The bones of the pelvis form the walls of the pelvic cavity, and a layer of muscle forms its floor. The pelvic cavity contains the last portion of the large intestine, the urinary bladder, and various reproductive organs. For example, the pelvic cavity of females contains the ovaries, uterine tubes, and uterus; in males, it contains the prostate gland and seminal vesicles. The pelvic cavity contains the inferior portion of the peritoneal cavity. The superior portion of the urinary bladder in both sexes, as well as the uterine tubes, the ovaries, and the superior portion of the uterus in females, are covered by peritoneum.

Which type of section would separate the two eyes?

If a surgeon makes an incision just inferior to the diaphragm, which body cavity will be opened?

This chapter provided an overview of the locations and functions of the major components of each organ system. It also introduced the anatomical vocabulary needed for you to follow more-detailed anatomical descriptions in later chapters. Many of the figures in later chapters contain images produced by the procedures outlined in Figure 1-12 through Figure 1-14 (see "FOCUS: Sectional Anatomy and Clinical Technology,"), which summarize the most common methods of visualizing anatomical structures in living individuals.

Chapters 2–4 will take you on a tour of the principal levels of organization, from individual atoms to individual humans. As we proceed through the text, we will emphasize major structural and functional patterns. To sharpen your analytical skills, we have included critical-thinking questions at the end of each chapter and in the Applications Manual. Chapters 5–29 will focus on the individual organ systems and their interrelationships.

FIGURE 1-9 Planes of Section. The three primary planes of section, defined and described in Table 1-3
FIGURE 1-10 Relationships of the Various Body Cavities
FIGURE 1-11 Body Cavities. (a) The dorsal body cavity is bounded by the bones of the skull and vertebral column. The muscular diaphragm divides the ventral body cavity into a superior thoracic (chest) cavity and an inferior abdominopelvic cavity. The pericardial cavity is inside the chest cavity. (b) The heart is suspended within the pericardial cavity like a fist pushed into a balloon. The attachment site, corresponding to the wrist of the hand, lies at the connection between the heart and major blood vessels. (c) Anterior and (d) sectional views of the ventral body cavity, showing the central location of the pericardial cavity within the chest cavity. The sectional plane shows how the mediastinum divides the thoracic cavity into two pleural cavities.
FIGURE 1-12 X-Rays (a) X-rays of the skull, taken from the left side. X-rays are a form of high-energy radiation that can penetrate living tissues. In the most familiar procedure, a beam of X-rays travels through the body and strikes a photographic plate. Not all of the projected X-rays arrive at the film; some are absorbed or deflected as they pass through the body. The resistance to X-ray penetration is called radiodensity. In the body, radiodensity increases in the following sequence: air, fat, liver, blood, muscle, bone. The result is an image with radiodense tissues, such as bone, appearing in white, and less-dense tissues in shades of gray to black. (b) A barium-contrast X-ray of the upper digestive tract. Such an X-ray is produced by introducing a relatively radiodense material into the body to provide sharp outlines and contrast and to check the distribution of fluids or the movements of internal organs. In this instance, the patient swallowed a solution of barium. Barium is very dense, and the contours of the stomach and intestinal linings are clearly outlined against the white of the barium solution.
FIGURE 1-13 Common Scanning Techniques (a) The relative position and orientation of the scans shown in parts (b)(d). (b) A color-enhanced CT scan of the abdomen. Computerized tomography (CT), formerly called computerized axial tomography (CAT), uses computers to reconstruct sectional views. A single X-ray source rotates around the body, and the X-ray beam strikes a sensor monitored by the computer. The source completes one revolution around the body every few seconds; it then moves a short distance and repeats the process. The result is usually displayed as a sectional view in black and white, but it can be colorized for visual effect. CT scans show three-dimensional relationships and soft-tissue structure more clearly than do standard X-rays. (c) A color-enhanced MRI scan of the abdomen. Magnetic resonance imaging (MRI) surrounds part or all of the body with a magnetic field about 3000 times as strong as that of Earth. This field affects protons within atomic nuclei throughout the body. The protons line up along the magnetic lines of force like compass needles in Earth's magnetic field. When struck by a radio wave of a certain frequency, a proton will absorb energy. When the wave pulse ends, that energy is released and the source of the radiation is detected. Each element differs in terms of the radio frequency required to affect its protons. (d) An ultrasound scan of the abdomen. In ultrasound procedures, a small transmitter contacting the skin broadcasts a brief, narrow burst of high-frequency sound and then picks up the echoes. The sound waves are reflected by internal structures. An echogram, or ultrasound picture, can be assembled from the pattern of echoes. These images lack the clarity of other procedures, but no adverse effects have been reported, and fetal development can be monitored without a significant risk of birth defects. Special methods of transmission and processing permit analysis of the beating heart, without the complications that can accompany dye injections. Note the differences in detail among this image, the CT scan, and the MRI image.
FIGURE 1-14 Special Scanning Methods. (a) A spiral-CT scan of the chest. Such an image is created by special processing of CT data to permit rapid three-dimensional visualization of internal organs. Spiral-CT scans are becoming increasingly important in clinical settings. (b) Digital subtraction angiography (DSA) is used to monitor blood flow through specific organs, such as the brain, heart, lungs, or kidneys. X-rays are taken before and after radiopaque dye is administered, and a computer "subtracts" details common to both images. The result is a high-contrast image showing the distribution of the dye.
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