2519. FEET. Latin verse (like English) is measured by groups of
syllables called _Feet_. Each of these groups has a definite length of
so many _morae_ (2515).
It is theoretically more accurate to make the foot purely a
time-division, as some authorities do; but the definition given above is
sanctioned by established usage.
2520. ARSIS and THESIS. Every complete foot consists of two parts, an
accented and an unaccented. The part on which the rhythmical accent or
_ictus_ falls is called the _Thesis_ (Gr. θέσις, _a setting down_). The
unaccented part of the foot is termed the _Arsis_ (Gr. ἄρσις,
_a raising_).
The name _Thesis_ originally referred to the setting down of the foot in
beating time or marching, or to the movement of the leader’s hand in
making the downward beat; and _Arsis_ in like manner meant the raising
of the foot or hand. But the Roman grammarians misunderstood the Greek
terms, supposing them to refer to the lowering and raising of the voice,
and so interchanged them. Hence many modern writers prefer to use
_Arsis_ to denote the accented, and _Thesis_ the unaccented, part of the
foot.