The human body is an intricate three-dimensional structure. To precisely describe the location of organs, the orientation of structures, and the mechanics of movement, scientists and medical professionals require a universal language of reference. This standardization is achieved through the use of imaginary lines and surfaces, known as anatomical axes and planes. These conceptual tools function like a fixed, internal coordinate system, allowing for consistent communication regardless of a person’s actual posture or activity. Defining these axes provides a framework for all anatomical measurement and description.
Establishing the Foundational Anatomical Position
All anatomical descriptions begin from a single, universally accepted reference posture called the anatomical position. This posture is defined by the body standing upright and facing directly forward. The feet are positioned slightly apart and parallel, with the toes pointing straight ahead. The arms hang down at the sides, but the palms of the hands are rotated to face forward, with the thumbs pointing away from the body. This specific stance establishes a baseline for directional terms like anterior (front) and posterior (back). Even if a person is lying down or performing an athletic maneuver, any analysis of their anatomy or movement is still mentally referenced back to this standardized, erect position.
The Three Cardinal Anatomical Axes
The three cardinal anatomical axes are imaginary lines that pierce the body, creating a three-dimensional reference system, much like the X, Y, and Z axes in geometry. These three lines are always perpendicular to one another, providing a fixed framework for describing rotation and movement.
Longitudinal Axis
The Longitudinal Axis, also called the vertical axis, runs straight down the body from the top of the head to the space between the feet. This axis is oriented superiorly (toward the head) and inferiorly (toward the feet), and is the axis around which rotational movements occur.
Transverse Axis
The Transverse Axis, often referred to as the mediolateral or horizontal axis, runs horizontally from left to right, passing through the body side-to-side. This axis pierces the body from one side of the torso or limb to the other, making it the reference line for movements that occur forward and backward.
Anteroposterior Axis
The third axis is the Anteroposterior Axis, also known as the sagittal axis, which runs horizontally from front to back. This line enters the body from the anterior (front) surface and exits at the posterior (back) surface. The anteroposterior axis provides the pivot point for side-to-side movements.
How Axes Define Anatomical Planes
The relationship between the anatomical axes and the anatomical planes is one of strict perpendicularity. An anatomical plane is an imaginary flat surface that slices through the body. Each plane is defined by the two axes that lie within it, while being perpendicular to the remaining third axis. These planes are crucial for visualizing internal structures and classifying movement.
Sagittal Plane
The Sagittal Plane is a vertical plane that divides the body into left and right sections. This plane is formed by the longitudinal (vertical) and the anteroposterior (front-to-back) axes, and is perpendicular to the transverse (side-to-side) axis.
Coronal Plane
The Coronal Plane, or frontal plane, is a vertical plane that divides the body into anterior (front) and posterior (back) sections. This plane is formed by the longitudinal (vertical) and the transverse (side-to-side) axes, and is perpendicular to the anteroposterior axis.
Transverse Plane
The Transverse Plane, also known as the axial or horizontal plane, divides the body into superior (upper) and inferior (lower) sections. This plane is formed by the transverse (side-to-side) and anteroposterior (front-to-back) axes, and is perpendicular to the longitudinal axis.
Using Axes to Understand Body Movement
The primary application of the anatomical axes is in the analysis of human movement, where they serve as the axes of rotation for joints. A fundamental principle of movement description is that any motion occurs within a specific plane and rotates around the axis that is perpendicular to that plane. This consistent relationship provides a clear method for describing the range of motion at a joint.
For example, movements like flexion (bending) and extension (straightening) of the elbow or knee occur in the sagittal plane. These motions rotate around the transverse axis, which runs horizontally through the joint.
Similarly, side-to-side motions, such as the abduction (moving away) and adduction (moving toward) of the arm or leg, take place in the coronal plane. These coronal plane movements rotate around the anteroposterior axis, which passes from front to back through the joint.
Finally, rotational movements, like twisting the torso or turning the head, occur in the transverse plane. These actions rotate around the longitudinal axis, which is the vertical line piercing the joint from top to bottom.