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succession. Perhaps the original creator for this movement
(and thus the deepest expression of the lateral line) is found
in the tiny intertransversahi muscles that run from transverse
process to transverse process in the spine. When one side
contracts, it stretches the corresponding muscle on the other
side (Fig. 5.19). The spinal stretch reflex, an ancient spinal
cord movement mediator, causes the stretched muscle to
contract, thus stretching the first muscle on the opposite side,
which contracts in its turn, and so on. In this way, a coordi-
nated swimming movement (in other words, coordinated
waves running down the lateral musculature) can occur with
minimal involvement by the brain. A lamprey eel, a modern
equivalent to ancient fish, can be decerebrated, and when it
is placed in flowing water, it will still swim upstream in a blind,
slow, but coordinated fashion, working only through spinal
mechanisms - the stimulation from the vibratory sensors on
the lateral skin linking to the stretch reflex.
Of course, corresponding movements remain in humans.
There are many movements such as walking that work through
reciprocal stretch reflexes. The side-to-side motion itself is not
so visible in regular adult walking, but its underlying primacy
is indicated in the infant at about three to six months, when
the side-to-side movement of creeping begins. This move-
ment will later be replaced by more sophisticated crawling
movement, which combines flexion/extension and rotation
along with the lateral flexion.
Discussion 2
The Lateral Line and fish: vibration, swimming, and
the development of walking
Sensing vibration
The top of the LL embraces the ear, located in the temporal
bone on the side of the head; indeed, the ideal of Lateral Line
posture is always described as passing through the ear. The
entire ear, of course, contains structures sensitive to vibratory
frequencies from about 20 to 20000 Hz, to gravitational pull,
and to acceleration of motion. The ear is a sophisticated ren-
dering of vibratory sensors that are set along the entire lateral
line of many ancient and some modern fish, such as sharks,
who 'hear' the thrashing of their prey from these lines (Fig.
5.18). Later vertebrates seem to have concentrated most of
their vibratory sensitivity at the leading end of the organism.
Some connection seems to remain, however, in the way that
left/right differences can reflect balance problems more than
front/back differences.
Swimming
Almost all fish swim with a side-to-side motion. This obviously
involves the contraction of the two lateral muscle bands in
Walking
When we assess adult walking, excessive side-to-side motion
is seen as an aberration. We expect to see the head and even
the thorax moving along fairly straight ahead, with most of the
side-to-side accommodation handled at the waist and below.
From the point of view of myofascial meridians, the entire LL
is involved in such adjustments, and should be considered in
correcting deviations of too much or too little lateral flexion in
the underlying pattern of walking.
For our primary forward motivating force we humans use
flexion/extension, sagittal motion (as the dolphins and whales
do as well), not side-to-side motion as the fish do. Our walking
involves a little side-to-side accommodation, as we have
noted, but the contralateral motion of human walking involves
a lot of rotation, especially through the waist and lower rib
cage, which mediate between opposed oscillations of the
pelvic girdle and the shoulder girdle.
The series of 'X's or the 'basket weave' that characterizes
the LL in the trunk and neck are perfectly situated to modulate
and 'brake' these rotatory movements. Therefore, the woven
structure of the LL in the trunk can be seen as partial arcs of
spirals that are used like springs and shock absorbers to
Fig. 5.18 Some fish such as sharks have a line of vibratory
sensors running down their lateral line. Humans seem to have
concentrated most of that vibratory sensitivity in the ear at the top
of the line.
Fig. 5.19 Lateral movement, the kind involved in the swimming motions of a fish or forward motion of an eel or snake, consists of
reciprocal reflexes flowing down the musculature in waves. When one side is contracted, the other side is stretched, inducing a
contraction in it, which stretches the first side, so it contracts, and so on and on upstream.
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