Page 34 - My FlipBook

18 OF THE MUSCLES OF THE LOWER-JAW.
half from the angle towards the Chin. In consequence of this
extent of insertion, the fibres of this portion divaricate very
considerably. They are mostly fleshy at their insertion, a few
only being tendinous, particularly those that are inserted back-
wards. The posterior and inner portion of the Masseter is
narrower at its insertion than at its origin ; its posterior fibres
running forwards, as well as downwards, while its anterior
run almost directly downwards. It occupies in its insertion
the remaining part of the scabrous surface, above the angle
of the Lower-Jaw, which lies between the anterior portion
and the two upper processes, viz., the Condyle and Coronoide.
As the anterior fibres of this portion rise on the inside of the
posterior fibres of the other portion ; and as its posterior fibres
run forwards as well as downwards, and its anterior run almost
directly downwards, while the fibres of the other portion radiate
both forwards and backwards ; these two portions in some
measure decussate, or cross one another. The anterior fibres,
which run farthest and lowest down, are tendinous at their
insertion, while the posterior and shortest are fleshy.
The use of the whole Muscle is to raise the Lower-Jaw ; and
when it is brought forwards, the posterior and inner portion will
assist in bringing it a little back ; so that this Muscle becomes
a rotator, if the Jaw happens to be turned to the opposite side.
We may observe, that this Muscle is intermixed with a
number of tendinous portions, both at its origin and its insertion
;
which give rise to a greater number of fleshy fibres, and thereby
add to the strength of the Muscle, (y)

(y) [The masseter muscle exists in all the inferior Mammalia. It may
be said generally to vary in size and strength with the amount of re-
sistance it is destined to overcome in the act of mastication. In those
animals, also, in which the other elevators of the jaw, especially the
temporal, are of small size, the masseter takes on increased development
and power (e. g., Rodentia, Ruminantia). The size, strength, and shape of
the zygomatic liar, together with the condition of the angle ami postero-
external surface of the inferior maxilla, may he taken as indices of the
development and powers of this muscle. In the typical Carnivora, the
itic arch i. ; of yery greal strength and depth ; below it is formed