cantle(n = noun.artifact) - the back of a saddle seat;
is a kind of back, backrest
is a part of saddle

1.  A corner or edge of anything; a piece; a fragment; a part. Milton. [1913 Webster]
"Cuts me from the best of all my land
A huge half moon, a monstrous cantle out." [1913 Webster]

2.  The upwardly projecting rear part of saddle, opposite to the pommel. [1913 Webster]

   To cut in pieces; to cut out from. [1913 Webster]

N  part, portion, dose, item, particular, aught, any, division, ward, subdivision, section, chapter, clause, count, paragraph, verse, article, passage, sector, segment, fraction, fragment, cantle, frustum, detachment, parcel, piece, lump, bit cut, cutting, chip, chunk, collop, slice, scale, lamina, small part, morsel, particle, installment, dividend, share, debris, odds and ends, oddments, detritus, excerpta, member, limb, lobe, lobule, arm, wing, scion, branch, bough, joint, link, offshoot, ramification, twig, bush, spray, sprig, runner, leaf, leaflet, stump, component part, sarmentum, compartment, department, county, fractional, fragmentary, sectional, aliquot, divided, in compartments, multifid, disconnected, partial, partly, in part, partially, piecemeal, part by part, by by installments, by snatches, by inches, by driblets, bit by bit, inch by inch, foot by foot, drop by drop, in detail, in lots.