From multiple knowledgeable gurus, lecturers and content experts I have heard the same story, as humans developed a bipedal stance our legs rotated giving rise to the spiralling dermatomal distribution found in the lower limb.
I am sure that many of you have heard this common story and then looked at your beloved pet at home and thought, “seems right” and moved on with your life with a better understanding of the dermatomal map and how to locate issues in patients. That story is a lie and comes from the anatomical position which was adopted from the famous DaVinci Anatomical Man which is used for describing anatomical positions.
“The anatomical position, Leonardo DaVinci 1490”
The anatomical position was based on DaVinci’s drawing (Le proporzioni del corpo umano secondo Vitruvio) and was made around 1490. This then went on to become the standard position for anatomical descriptions and was well established by the time Charles Darwin came up with the idea of natural selection in 1859.
Because its hard to change the status quo and so much had been discovered and described based on DaVincis anatomical man, despite the discovery of natural selection the human anatomical position has remained the same.
“the dermatomal pattern spirals down the legs”
The story goes that during evolution mans (not sure about women at this stage) legs rotated from the position of a quadruped to that of our current position. This rotation is what causes the dermatomal pattern to spiral down the legs, whereas in the upper limb they are nicely arranged is a neater pattern.
“Horses, cats and dogs, monkeys, mice and elephants”
To break down this myth lets first look at quadruped legs, I am going to show pictures of horses but cats and dogs, monkeys, mice and elephants have the same layout. The trick is realising where the hip is located and where the ankle starts. Horses for example have particularly robust long and fused metatarsal bones (called a cannon bone in horses), this places the ankle joint or hock almost midway down the horses leg where it can be mistaken for a backwards knee. As you can see the knee joint is much higher with only a very short femur above it. Looking at the picture you can identify the toes, metatarsals, calcaneus, tib and fib, the patella and femur (though not all those names are correct in a horse).
“change the anatomical position”
A better lens to look at dermatomes and their wrapping pattern down the human leg might be with a change in the anatomical position. If we change the human posture from upright with arms supinated and instead in a quadruped position with the arms pronated, we get a picture were the spiral of our lower limb dermatomes matches the upper limbs.
It turns out that when looking at dermatomes we have been misled by horses and dog ankles. With this realisation we can re-evaluate the anatomical position and see that both the upper and lower limb dermatomes spiral distally in the same pattern when in a quadruped position. This understanding hopefully will make it easier to know where the different dermatomes are and as a gives rise to the question; surely its beneficial to have the spiral arrangement if its in both upper and lower limbs?
Next time you hear someone say that our legs rotated with evolution point them here and help educate people about an easier way of learning the dermatome arrangement
