On a horse, front and back become cranial and caudal, because anatomy depends on posture
Is your back posterior or dorsal? For a human it is both, but on a dog those words point in different directions. Anatomists anchor every direction to a fixed reference pose, and the pose itself changes between people, four-legged animals and fish, which is why vets and doctors speak slightly different dialects.
Directions only make sense against a default body. For humans that is standing with arms by the sides and palms turned forward. A quadruped stands on all four feet with its head facing ahead, and a fish rests belly down with its fins in neutral. Whatever position a patient or animal is actually in, structures are described as if it were in that pose, so a raised arm does not scramble the vocabulary.
Because a walking person's spinal axis is upright while a dog's runs horizontally, the same words can diverge. In humans, superior means toward the head and inferior toward the feet. Veterinarians drop those terms except for the eye, eyelids, lips and inner ear, using dorsal and ventral, back and belly, instead. Likewise anterior and posterior are kept for parts of an animal's head, with cranial and caudal, toward skull and tail, used for the rest. Rostral, from the Latin for beak, means toward the nose; in a horse the eyes are caudal to the nose.
Other pairs measure distance from a midline or a trunk. Medial lies nearer the body's central plane, lateral further out, so the arms are lateral to the torso. Ipsilateral means same side, contralateral opposite, as when the left half of the brain controls the right limbs. Proximal and distal are relative: the elbow is distal to a cut on the upper arm yet proximal to one on the forearm. Varus and valgus describe a joint bent toward or away from the midline, the Latin for bow-legged and knock-kneed.
Three planes slice the body for imaging and embryology. Sagittal planes split left from right, the coronal or frontal plane separates front from back, and the transverse plane cuts across both. The matching body axes are set early, before and during gastrulation in the embryo. Animals without bilateral symmetry break the scheme: they lack a front and back altogether, though proximal and distal still apply.
Source: Anatomical terms of location
