There are two systems of anatomic orientation terms in common use, and a few of these terms have synonyms, which can cause confusion. Most neural structures were first named in quadruped vertebrates (animals with spines that walk on four legs), so that standard anatomic orientation terms were used for their names.
If a rat is standing on its four legs, which is its standard anatomic position, the entire central nervous system is basically horizontal. The central nervous system is mainly the brain and spinal cord. The brain has three main parts: the cerebrum, the brainstem, and the cerebellum. The cerebrum is attached to one end of the brainstem, the spinal cord is attached to the other end of the brainstem, and the cerebellum is attached to the back of the brainstem. The front of the horizontal cerebrum points toward the nose. Behind the cerebrum is the horizontal brainstem, and on top of this is the cerebellum. Behind the brainstem is the horizontal spinal cord, the end of which points toward the tail. In the rat in its standard anatomic position, rostral means toward the nose, caudal means toward the tail, dorsal means up, and ventral means down. For example, the cerebrum is rostral to the brainstem, the spinal cord is caudal to the brainstem, the cerebellum is dorsal to the brainstem, and the brainstem is ventral to the cerebellum.
The synonyms cranial (referring to the skull) and cephalad (referring to the head) are also sometimes used for rostral; this can cause confusion when discussing structures inside the skull and head.
With an imaginary vertical plane separating the right and left sides of the rat, any horizontal line in that plane is called a midline. Medial means toward the midline and lateral means away from the midline. For example, the nose is medial to the whiskers, and the whiskers are lateral to the nose. Proximal means toward the center of the body and distal means away from the center of the body, regardless of the position of the limb or other body part being discussed. For example, the hip is proximal to the toes, and the toes are distal to the hip.
Ipsilateral means on the same side and contralateral means on the other side. For example, the left forelimb is ipsilateral to the left hindlimb, and the left forelimb is contralateral to the right forelimb. Superficial means toward the surface and deep means away from the surface. For example, the skull is superficial to the brain, and the brain is deep to the skull.
The terms decussate (verb) or decussation (noun) are used when a bundle of axons in the central nervous system crosses the midline, either from right to left or vice versa.
Standard anatomic orientation terms are used for the embryonic human, which is during the first few weeks of development in the uterus, when there is still a tail.
After the embryonic stage, however, the standard anatomic position for humans is standing on both feet, with the arms down, and the face and palms pointing forward. Standard anatomic orientation terms are often still used, with results for structures of the head and legs that often cause confusion.
Most of the head is oriented horizontally, similar to the rat, so that for the cerebrum, as in the rat, rostral means forward, caudal means backward, dorsal means up, and ventral means down. However, the brainstem, cerebellum, spinal cord, and the rest of the body is mostly oriented vertically, so that in these structures rostral means up, caudal means down, dorsal means backward, and ventral means forward.
The second system of anatomic orientation terms used in humans could be called human anatomic position terms. This system is used more by clinicians, probably because it is simpler, as the terms always mean the same direction regardless of what body part is being discussed. In this system, the terms rostral, caudal, dorsal, and ventral are not used, and instead anterior means forward, posterior means backward, superior means up, and inferior means down.
All the other standard anatomic orientation terms are used in the same way as in the rat.
The terms for planes relative to human anatomic position are: axial for a horizontal plane that separates superior from inferior, coronal for a vertical plane that separates anterior from posterior, and sagittal for a vertical plane that separates the lateral sides (midsagittal for the midline sagittal plane).
Planes at any other angle are called oblique. These planes, or sections, are used to discuss anatomy, pathology, and imaging in two dimensional views of structures.
I will use the words arm and leg as they are used in informal English and by most clinicians, at least most of the time, instead of the formal anatomic terms upper and lower extremity. This distinction is important, because confusion often occurs when anatomists and some clinicians use the word “arm” to refer to only the part between the shoulder and elbow, and the word “leg” to refer to only the part between the knee and ankle.