In equine anatomy, deep describes the position of a structure located below the surface of the body or further from the skin than another structure being used as a reference point. A muscle, tendon, or bone described as deep lies beneath a more superficial (surface-closer) layer. The term is always relational: a structure is deep relative to something else, not deep in absolute terms.
Anatomical position terms like deep and superficial allow veterinarians and farriers to communicate precisely about where a problem or structure is located without ambiguity. For example, the deep digital flexor tendon (DDFT) runs deep to the superficial digital flexor tendon in the lower limb, passing behind the cannon bone and through the pastern region to attach to the coffin bone. In a lameness evaluation, distinguishing which tendon is affected — deep or superficial — determines both diagnosis and treatment.
The same directional vocabulary applies when describing wounds and abscesses. A wound that penetrates to the deep layer has breached the superficial soft tissue and reached structures closer to bone or joint, which carries greater risk of serious infection or hoof damage. See also the points of the horse for anatomical orientation, and hoof abscess for a clinical context where deep versus superficial location determines urgency.
Further Reading: The directional terminology used in equine lameness evaluation is grounded in horse anatomy (Wikipedia); for clinical application to tendon and foot injuries, the Merck Veterinary Manual covers the components of the equine musculoskeletal system in detail.