The classroom skeleton is a political statement – join my ‘Equal Rights for Cartilage! movement.
I am fond of making this observation – made it this weekend in San Francisco. Of course it’s a bit of a ploy, but here’s why I say so:
1) The main point is that ‘the skeleton’ doesn’t exist as a framework in the real body. Absent the soft-tissue, the skeleton would clatter to the floor in irretrievable pieces – even the cranial bones would scatter and the pelvis fall apart without the ‘grout’ of the sutural fascia. The ‘shingles’ of the facet joints lock over each other, but without the ligaments and the discs, this would not be enough to hold them together.
Add the ligaments
and the bones would mostly hold in a general relationship, but of course the whole thing would sag and fall. The muscles would drip off without the restraining (tensegrity) of the superficial and attaching fasciae. The body is a tensegrity (or at least ‘tension dependent’) dance between the myofascialature and the skeleton. The classroom skeleton makes it seem a steady frame on which the muscles can be hung, on-by-one, till it can move. Not so. Bones float in the soft-tissue, pushing out against it; soft-tissue pulls in, holding it together and holding it up in a tough but delicate tension-compression balance.
2) The hardware on a skeleton gives many false impressions – the bones of the feet wired together with no movement at the sub-talar joint, giving the casual observer a false idea about the ankle in eversion and inversion. The fibula would fall without wire ties, etc. And then there’s that metal rod up its bum from sacrum to skull. Modern skeletons seem to have given up wiring the ribs to the neck (which at least imitated the scalenes) for an entirely unrealistic rod between the front of the throacic spine and the back of the sternum – talk about a spear through the heart!
The scapula is usually nailed to the rib cage, giving the impression that the arm appendage starts at the gleno-humeral joint, instead of the sterno-clavicular joint.
Most cheap skeletons have U-joint hardware at the shoulder and hip, which works well enough for most demonstration purposes, but spoils our sense of the biomechanics of the rotator cuff and the psoas complex. More expensive models have gone to flexible spines and bungee cord joint connectors, which, though they give up the ghost sooner than the metal hardware, are much more accurate in ball and socket mechanics.
3) But the worst of all is the choice of which cartilage to include and which to ignore. Most include the pubic symphysis, the intervertebral discs, and the costal cartilages between the ends of the ribs and the sternum. Some plastic skeletons include the knee menisci.
Most skeletons wire the hyoid bone to the front of the neck vertebrae, which is accurate positioning, but inaccurate in function. None to my knowledge include the laryngeal crichoid, or tracheal cartilages under the hyoid, which extend down into the bronchae and bronchioles before terminating in the (epithelial) alveoli. Neither are the ear nor nose cartilages or the linings of the eustachian tubes included.
Of course most bones start as cartilage and are progressively ossified, so the decision of which cartilages to include is a matter of convenience and production costs, which I appreciate. But nevertheless, it is a political statement of inclusion and exclusion. An inclusive skeleton would have these cartilages of the throat and chest, and would suggest a ‘visceral’ skeleton (an idea I lifted from Jon Zahourek) to add to the axial and the two parts of appendicular skeleton. This visceral skeleton would include the ‘breastplate’ of the sternum and costal cartilages, the pulmonary cartilages mentioned – the entire upside-down tree of the respiratory and vocal superstructure, plus the hyoid, mandible, maxillary, palatine, zygomatic, nasal, and lacrimal bones – mostly derived from the proto-gill arches of the embryo. The head scratcher in this would be the vomer: firmly in the nose, it seems part of this visceral skeleton, but it could also be seen as the ‘spinous process of the sphenoid ‘vertebra’.