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Fig. 1.40 Although they differ in form when they reach mature stages, the fundamental structure of the balloon pushed in to form a
double bag by the tissue of the organ is found around nearly every organ system, in this case with the double-layered pleura around the
lung.
Fig. 1.41 We can imagine, whether it is embryologically correct or not, that the bones and muscles share a similar double-bag pattern.
will fold in to surround the lungs with the double bag
of the visceral and parietal pleura (Fig. 1.40). The upper
and lower parts will be separated by the invasion of the
two domes of the diaphragm. The lower outside part of
each tube will fold in to form the double bag of the
peritoneum and mesentery.
The double- and triple-bagging around the brain and
spinal cord is more complex, developing from the neural
crest, the area where the mesoderm 'pinches' off the
ectoderm (with the skin on the outside and the central
nervous system on the inside), so that the meninges (the
dura and pia mater) form from a combination of these
two germ layers. 8 6
rounds the bones and the outer bag surrounds the
muscles.
To create a simple model for this idea, imagine that
we have an ordinary plastic carrier bag lying on the
counter with its open end toward us (Fig 1.42). Now lay
some wooden thread spools on top of the bag in a row
down the middle. Insert your hands into the bag on
either side of the spools, and bring your hands together
above the spools. Now we have:
1. spools
2. an inner layer of plastic fabric
3. hands
4. another outer layer of plastic fabric.
Double-bagging the muscles
Substitute 'bones' for 'spools', 'muscles' for 'hands',
and 'fascial' for 'plastic' and we are home free.
The human locomotor system is, like nearly every
other fascial structure in the body, constructed in double-
bag fashion - although this is speculative (Fig. 1.43). The
content of the inner bag includes very hard tissues -
bone and cartilage - alternating with almost totally
fluid tissue - synovial fluid; the spools and spaces
between them in our simple model. The inner fibrous
bag that encases these materials is called periosteum
when it is the cling-wrap sleeve around the bones,
We have given short shrift to this fascinating area of
morphogenesis, but we must return to the subject at
hand - the myofascial meridians in the musculoskeletal
system.
With such an 'inordinate fondness' for double-
bagging, might we not look for something similar in the
musculoskeletal system? Yes, in fact: the fibrous bag
around the bones and muscles can be viewed as having
much the same pattern as we see in the way the fascial
bag surrounds the organs (Fig. 1.41). The inner bag sur-
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