Unit 7: Body Structure

Certain directional anatomical terms appear throughout this and any other anatomy textbook (Figure 4). These terms are essential for describing the relative locations of different body structures. For instance, an anatomist might describe one band of tissue as “inferior to” another or a physician might describe a tumor as “superficial to” a deeper body structure. Commit these terms to memory to avoid confusion when you are studying or describing the locations of particular body parts.

• Anterior (or ventral) describes the front or direction toward the front of the body. The toes are anterior to the foot.
• Posterior (or dorsal) describes the back or direction toward the back of the body. The popliteus is posterior to the patella.
• Superior (or cranial) describes a position above or higher than another part of the body proper. The orbits are superior to the oris.
• Inferior (or caudal) describes a position below or lower than another part of the body proper; near or toward the tail (in humans, the coccyx, or lowest part of the spinal column). The pelvis is inferior to the abdomen.
• Lateral describes the side or direction toward the side of the body. The thumb (pollex) is lateral to the digits.
• Medial describes the middle or direction toward the middle of the body. The hallux is the medial toe.

• Intermediate describes a position between a more medial and a more lateral structure. The middle finger is intermediate between the ring and index fingers.
• Proximal describes a position in a limb that is nearer to the point of attachment or the trunk of the body. The brachium is proximal to the antebrachium.
• Distal describes a position in a limb that is farther from the point of attachment or the trunk of the body. The crus is distal to the femur.
• Central describes a position towards the middle (centre) of a structure or organ system. The central nervous system is contained within the skull and vertebral column.
• Peripheral describes a position towards the outer edge (periphery) of a structure or organ system. The peripheral nervous system is found outside the skull and vertebral column.
• Superficial describes a position closer to the surface of the body. The skin is superficial to the bones.
• Deep describes a position farther from the surface of the body. The brain is deep to the skull.

Body Planes

Sectioning, or cutting, is frequently used in the study of Anatomy. The body can be sectioned in various ways to produce a plane, this is a two-dimensional surface of a three-dimensional structure that has been cut. A body structure is often cut into thin sections before macroscopic viewing to allow visualization of the structure’s interior and assist with identification of local disease or infiltration as these pathologies may not be obvious when observing the surface anatomy alone. Modern medical imaging devices enable clinicians to obtain “virtual sections” of living bodies. We call these scans. Body sections and scans can be correctly interpreted, however, only if the viewer understands the plane along which the section was made. A plane is an imaginary two-dimensional surface that passes through the body. There are three planes commonly referred to in anatomy and medicine (Figure 5).

• A sagittal plane is a plane that divides the body or an organ vertically into right and left sides. If this vertical plane runs directly down the middle of the body, it is called the midsagittal or median plane. If it divides the body into unequal right and left sides, it is called a parasagittal plane or (less commonly) a longitudinal section.
• A frontal plane is a plane that divides the body or an organ into an anterior (front) portion and a posterior (rear) portion. A frontal plane is often referred to as a coronal plane (“corona” is Latin for “crown”).
• A transverse plane is a plane that divides the body or organ horizontally into upper and lower portions. Transverse planes produce images referred to as cross sections.

Body Cavities and Serous Membranes

The body maintains its internal organization by means of membranes, sheaths, and other structures that separate compartments. The dorsal (posterior) cavity and the ventral (anterior) cavity are the largest body compartments (Figure 6). These cavities contain and protect delicate internal organs, and the ventral cavity allows for significant changes in the size and shape of the organs as they perform their functions. The lungs, heart, stomach, and intestines, for example, can expand and contract without distorting other tissues or disrupting the activity of nearby organs.

Subdivisions of the Posterior (Dorsal) and Anterior (Ventral) Cavities: The posterior (dorsal) and anterior (ventral) cavities are each subdivided into smaller cavities. In the posterior (dorsal) cavity, the cranial cavity houses the brain, and the spinal cavity (or vertebral cavity) encloses the spinal cord. Just as the brain and spinal cord make up a continuous, uninterrupted structure, the cranial and spinal cavities that house them are also continuous. The brain and spinal cord are protected by the bones of the skull and vertebral column and by cerebrospinal fluid, a colorless fluid produced by the brain, which cushions the brain and spinal cord within the posterior (dorsal) cavity.

The anterior (ventral) cavity has two main subdivisions: the thoracic cavity and the abdominopelvic cavity (Figure 6). The thoracic cavity is the more superior subdivision of the anterior cavity, and it is enclosed by the rib cage. The thoracic cavity contains the lungs and the heart, which is located in the mediastinum. The diaphragm forms the floor of the thoracic cavity and separates it from the more inferior abdominopelvic cavity. The abdominopelvic cavity is the largest cavity in the body. Although no membrane physically divides the abdominopelvic cavity, it can be useful to distinguish between the abdominal cavity, the division that houses the digestive organs, and the pelvic cavity, the division that houses the organs of reproduction.

Abdominopelvic Regions and Quadrants: To promote clear communication, for instance about the location of a patient’s abdominal pain or a suspicious mass, health care providers typically divide up the abdominopelvic cavity into either nine regions or four quadrants (Figure 7).

The more detailed regional approach subdivides the cavity with one horizontal line immediately inferior to the ribs and one immediately superior to the pelvis, and two vertical lines drawn as if dropped from the midpoint of each clavicle (collarbone). There are nine resulting regions. The simpler quadrants approach, which is also commonly used in medicine, subdivides the cavity with one horizontal and one vertical line that intersect at the patient’s umbilicus (navel).

These regions can be used to identify the location of abdominal organs more precisely.  For example:

• The right hypochondriac region contains the gall bladder and part of the liver, and the right kidney
• The epigastric region contains part of the liver and part of the stomach
• The left hypochondriac region contains part of the spleen and part of the stomach, and the left kidney
• The right lumbar region contains most of the ascending colon
• The umbilical region contains the transverse colon and part of the small intestine
• The left lumbar region contains most of the descending colon
• The right iliac region contains the appendix and caecum
• The hypogastric region contains the lower small intestine, the distal sigmoid colon and anus, and the urinary bladder, as well as the uterus and ovaries in females and the prostate in males
• The left iliac region contains the proximal sigmoid colon

Membranes of the Anterior (Ventral) Body Cavity: A serous membrane (also referred to as a serosa) is one of the thin membranes that cover the walls and organs in the thoracic and abdominopelvic cavities. The parietal layers of the membranes line the walls of the body cavity (pariet- refers to a cavity wall). The visceral layer of the membrane covers the organs (the viscera). Between the parietal and visceral layers is a very thin, fluid-filled serous space, or cavity (Figure 8).

There are three serous cavities and their associated membranes. The pleura is the serous membrane that surrounds the lungs in the pleural cavity; the pericardium is the serous membrane that surrounds the heart in the pericardial cavity; and the peritoneum is the serous membrane that surrounds several organs in the abdominopelvic cavity.

The serous membranes form fluid-filled sacs, or cavities, that cushion and reduce friction on internal organs when they move, such as when the lungs inflate or the heart beats. Both the parietal and visceral serosa secrete the thin, slippery serous fluid located within the serous cavities.

The pleural cavity reduces friction between the lungs and the body wall. Likewise, the pericardial cavity reduces friction between the heart and the wall of the pericardium. The peritoneal cavity reduces friction between the abdominal and pelvic organs and the body wall. Therefore, serous membranes provide additional protection to the viscera they enclose by reducing friction that could lead to inflammation of the organs.