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The horse anus is a marvel of evolutionary biology.
Like most mammals, rhythmic contractions of the bowels move the feces up to the end of the gut to be expelled.
What sets the horse anus apart is the bafflingly complex autonomic contractive motion of 4 muscular rings which function like sphinctoral guillotine. As the feces is squeezed out, the rings expand and the outermost ring is retracted into an interior skinfold resembling the human foreskin, leaving an interstital tissue fold to open air.
The two inner rings then contract, and push apart, severing the horse feces cleanly, allowing to drop free as the outer ring folds back through the interstital flap, creating a intrinsic wiping motion with the fourth ring as it contracts back into the horses rectum.
This advanced biological mechanism is so effective that many industrial companies have copied Nature's design for self-cleaning viscus liquid dispensers housed within complex machinery.
