Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

40
ANATOMY.
Special versi abdominis, which operate a little in drawing the chest
Anatomy^ downwards and compressing it before and on the sides, but
The aCt muc^ morf powerfully as retaining and supporting walls
muscles. abdominal viscera, counteracting by the inward and
upward action the downward impulse of the diaphragm.
In this manner the abdominal viscera, placed between two
opposing but equally balanced powers, are retained in the
cavity, and prevented from being protruded upwards or
downwards, while they are subjected to the alternate mo¬
tions of inspiration and expiration.
The muscles of the trunk are employed in retaining that
part of the skeleton in the erect attitude, in balancing it
properly on the pelvic extremities, in occasionally inflect¬
ing and extending it, in bending it to one side, or in pro¬
ducing rotation. Those of the spine and back are parti¬
cularly the agents of the erect attitude, and of extension ;
and those of the anterior region are employed in inflect¬
ing the person.
Muscles of The muscles of the superior extremities taken together
the thora- are the agents of numerous varied motions. Though the
uuties^" Pr*ncTal object of the thoracic extremities is prehension,
or embracing any object or objects firmly either by one
or both hands, this may be modified in various ways, so as
to give rise to prepulsion, traction, and constriction;
while diduction and circumduction are the result of the
combinations of the simple movements,—abduction, ad¬
duction, flexion, and extension.
Prepul- Prepulsion may be either instantaneous or continued.
Sion. The first takes place in the act of inflicting a blow or re¬
pelling an object. All the flexors are previously put in
action to shorten the member, which is then at once for¬
cibly extended, and communicates a violent shock to the
part of the object to which its extremity is applied. Of
this motion, in which the extension takes place in the
scapulo-humeral and humero-cubital articulations, the
deltoid and brachialis externus, or third head of the
triceps, are the chief agents. The wrist and fingers
are almost passive. But an analogous motion is exe¬
cuted by the latter in giving a fillip. In continuous
prepulsion, for instance, or the act of impelling an
object, the mechanism is of two kinds. In the first case,
the member being previously extended and supported on
the object, the individual inclines the trunk, and avails
himself of its weight; while the member, remaining pas¬
sive, becomes a lever moved by gravity. In the second
case, the continued action of the extensors retaining the
member forcibly extended, impels the object without in¬
terruption. When, for example, a man pushes a wheel-
carriage before him, the superior extremities are extend¬
ed and communicate motion to the carriage, while the
trunk approaching it immediately, the extremities are
again inflected, and so forth successively. In most in¬
stances this twofold mechanism is combined. When in
prepulsion the body impelled is fixed, the impulse is
thrown back on the person of the prepelling agent. Ex¬
amples of this effect of prepulsion are observed in the act
of rising from a seat by the occasional use of the thoracic
extremities, and in pushing a vessel from the shore by
means of an oar or pole.
Traction. Traction, which consists in a general action of the
flexors of the thoracic extremities, is directly the reverse
of prepulsion. Its effect is to diminish the space between
the agent and the object drawn, which takes place by
shortening the member; while in prepulsion this space is
enlarged by elongating or extending the member. In the
case of a very great effort, for instance that of detach¬
ing a piece of wood strongly fixed in a wall, the action of
the flexors is aided by the weight of the trunk, which is
instinctively inclined in the opposite direction ; and if the
body drawn yields at once, a fall is often the result, be- Special
cause in this inclination the centre of gravity is subverted. Anatomy.
In another form of traction, in which the body grasped
does not yield, and when the action takes place upwards,
the effect is to elevate the person of the agent by the
flexors of the superior, and even occasionally of the inferior
extremities. Familiar examples of this are afforded in the
act of climbing walls, trees, and occasionally rocks, in as¬
cending the rigging of a ship, and still more forcibly in the
manner in which the active seaman ascends a single rope.
Constriction consists in the forcible and continued in-Constric-
flection either of a single hand or of the whole of bothtion.
members. In the first case the agents are the superficial
and deep flexors, and the flexors of the thumb and little
finger; and in the second, with the action of these muscles,
that of the biceps, brachialis internus, and coraco-brachialis
is combined. This motion can neither be so sudden as
that of prepulsion, nor, like it, can it be aided by the weight
of the person.
Diduction, which consists in the forcible separation of Diduction.
the upper extremities from each other, as in swimming,
is effected partly by the latissimus dorsi and teres major,
partly by the posterior fasciculi of the deltoid moving the
whole extremity in the scapulo-humeral articulation.
Circumduction, which also is exclusively confined toCircum.
the scapulo-humeral articulation, is effected principally duction.
by the deltoid, large pectoral, latissimus dorsi, teres major,
&c.; while rotation is performed by the supraspinatus,
infraspinatus, teres minor, subscapular is, &c.
Besides these general classes of motion executed by Gesture,
the thoracic extremities, to them also belongs the power
of assuming many of those attitudes and all the varieties
of gesture which man employs for the purpose either of
expressing his feelings or giving significance and ani¬
mation to the language which he adopts for that purpose.
How much these gestures, when well chosen and properly
introduced, aid both the expression of the countenance
and the language of the lips, is w^ell known to the public
speaker and the dramatic performer.
Besides the ordinary flexors of the wrist (radialis ra-Motions of
ternus et internus, ulnaris internus), there are two common band and
flexors of the fingers, one superficial {sublimis), the other ^nger&
deep-seated (profundus); the thumb has a long and short
flexor, and the little finger has a short flexor. The flexor
muscles, therefore, may be distinguished into four orders ;
those common to the wrist and hand, those common to
the hand and fingers, those proper to the fingers, and
those proper only to some of the fingers. Though the
radialis internus and ulnaris internus chiefly bend the wrist
and hand, yet both the superficial and deep flexors, by
passing beneath the annular ligament, co-operate in the
same motion, and necessarily bend the hand previous to
their final action on the digital phalanges. In this they
are considerably aided by the action of the palmaris lon-
gus and the palmaris brevis, which render the palmar apo¬
neurosis tense, and enable it to afford the necessary resist¬
ance to the subjacent flexor tendons.
The superficial flexor, the tendons of which are insert¬
ed into the anterior and citerior part of the second row of
phalanges, has the effect of bending that part of the fingers;
and further, by being bound down by a ligamentous sheath
to the first phalanx, inflects them at the same time into
the palm. The slits in each tendon allow those of the deep
flexor to pass forward on the median line of each phalanx,
to be inserted in the unguinal phalanges, and thereby to
operate most directly and perfectly in inflecting them on
the palm ; and, by being confined also in the same sheath
by strong ligamentous bands, aid in inflecting the second
and first range of phalanges. (Plate XXVIII. fig. 2 and 3.)