Re: What are the chestnuts on horses for?

Trevor,

The question is less of what the Chestnut is for and more of why it is 
there.  The Chestnut is located on the inside (medial) of the front legs 
just above the carpus (knee) joint, and in the rear legs just below the 
hock.  It is not a blemish but a remnant of evolution.  The Chestnut, along 
with a similar horny feature called the Ergot (a small outgrowth on the 
rear of the fetlock joint where feathering, if present, is also found), are 
thought to be remnants of pads such are found on dogs and bears.

A horse now only has a single hoof per leg but this wasn't always the case. 
 The ancestor of the modern horse was a four-toed, fox-sized animal called 
Eohippus.  As the horse evolved, it got bigger, longer, and began to lose 
toes, until eventually they had only one per leg.  The modern horse 
essentially runs on its middle finger with the hoof wall being the 
fingernail.  Now this is a simplified version, obviously, but there still 
are remnants that support this.  For example, the splint bones are parallel 
to the cannon bone but don't run the entire length of the cannon and 
therefore have no actual structural purpose.  (They may help some in shock 
absorption and some other minor functions).  They correspond to the 2nd and 
4th metacarpal and metatarsal bones in the human hand and foot.  (They are 
the long bones in the palm and foot).  Whereas the 3rd 
metacarpal/metatarsal is the cannon bone on which the horse runs.  This 
helps shows a common a link in ancestry for most mammals.  The Chestnut and 
Ergot are also remnant features such as this and are similar in composition 
to the hoof wall.  They help support the evidence of other toes once 
being present.  They are unique in shape and are sometimes used as 
identifiers in thoroughbreds.  Hope this answers your question!  Any other 
questions feel free to e-mail me at bradk@jymis.com.
BK

References:  "Horse Anatomy.  A Coloring Atlas" Robert Kainer and Thomas 
McCracken.  2nd Edition.  Alpine Publications. 1998

"Evolution and Biomechanics of the Horse" No Foot-No Horse II.  Jeff 
Thomason.  The Equine Research Centre, University of Guelph, Ontario.  
Sept. 1994 pg. 8-14.