Anatomy Q & A

[Colin McHenry <cmchenry@westserv.net.au>]

Dear VrtPaleos and DMLers,

I have to give a lecture next week to some first year medical students 
about the evolution of the verbrate body plan, basic mammalian anatomy, 
and the more recent evolutionary history of humans.  Obviously, this is 
a great opportunity for me to open up texts that have been gathering 
dust for a decade or more (I'm resisting the temptation to work in 
plesiosaurs somewhere),  and I'm having fun putting the material 
together.  However, from the (suggested) list of the topics the course 
coordinator would like me to cover, there are still a few items that the 
textbooks have refused to shed light on.  At this point, I've got plenty 
of material, so it's no disaster if I don't inlcude them, but a couple 
of things on the list have me curious.   Any insights would be much 
appreciated.

Okay, in no particular order;

1. The M. palmaris longus is not an important muscle in humans, and is 
apparently absent in 10 - 20% of people.  As far as I can work out, it 
is considered to be the displaced flexor of the proximal phalanges, now 
inserting on the palmar aponeurosis.  So, why it is being lost in 
humans?  Presumbably it is drifting out, rather than being actively 
selected against, but what role does it play in other mammals that it no 
longer does in us?  My best guess so far is that, as a flexor of the 
digits, it is important in bracing the phalanges against the wieght of 
the body in digitigrade animals, but I have not been able to discover if 
the pattern of its development in other animals is consistent with that 
idea.  If it is as unimportant in apes as in humans then its is possible 
that, with the evolution of the brachiating hand, it decreased in 
importance compared with more digitigrade ancestral primates.  Anyway, 
that's the limit of my speculations.

2. Similarily, the M. plantaris of the human leg is reduced / 
occasionally absent.  In humans, this inserts onto the achilles tendon 
(along with the more developed Gastrocnemus and Soleus).  However, it 
appears that the the plantaris originally inserted upon the plantar 
fascia, and is the hind limb equivalent of the palmaris longus.  IN 
which case, does the logic presented above apply for the plantaris also?

3. Fingernails in primates....brace the dorsal surface of the fingertips 
when climbing (i.e. hanging on by the fingertips...)?

4. Dorsal ribs in tetrapods.....are homologous with what structure in 
fish?  Sarcopterygians don't have ossified ribs (or, if they do, they 
are very short).  Actinoptergians can have dorsal and ventral ribs in 
the trunk - is either of these homologous to tetrapod ribs?  If so, do 
they just look equivalent because they are an ossification of a 
cartilage that is growing in the fascia between myotomes (i.e. a product 
of a constrained process that has a common heritage), rather than being 
structures that have a direct common heritage (and how do you tell the 
difference anyway?).

5. Why the sexual dimorphism of the 'carrying angle' of the elbow in 
humans (bigger in women than men)?  Presumably this is to do with an 
larger trochlear condyle in females?  And it is in any way linked to the 
larger angle (in the coronal plane) of the knee joint (which is a 
mechanical result of women having a wide pelvis) - perhaps some sort of 
pleiotrophic link between the two?

6. Why does digit I of the ancestral pentadactyl limb have two 
phalanges, whilst all the others have three or more?

7.... and lastly for now, a question that is either surgically incisive, 
or just plain stupid (!).  Why is it that elbows bend forwards, and 
knees bend backwards?   Usually no attempt is made to explain it - the 
only offering I found was that 'pushing is more effecient that pulling' 
(explaning the knee, at least), but I don't see mechanically why this 
should be true.  The only exception to this rule that I can think of is, 
of course, turtles, where the elbow bends back in at least some species 
(do they all do this?).  Which just proves that turtles really are from 
Mars......

Perhaps others will find answers to these questions of interest also. In 
any case, many thanks in advance for any insights/explanations, and 
sorry for the cross posting.

Cheers
Colin 'how can I draw the dermatone patterns if you kids have used up 
the purple texter' McHenry

--
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Colin McHenry
School of Environmental and Life Sciences (Geology)
University of Newcastle
Callaghan NSW 2308
Tel: +61 2 4921 5404
Fax: + 61 2 4921 6925

******************
Colin McHenry & Sarah Johnston
14 Summer Place
Merewether Heights  NSW 2291
+61 2 4963 2340
mob: 0423 081683

cmchenry@westserv.net.au
Colin.Mchenry@newcastle.edu.au