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pairs。 Fore and hind may bend either both backwards； as the figures
marked A； or in the opposite way both forwards； as in B； or in
converse ways and not in the same direction； as in C where the fore
bend forwards and the hind bend backwards； or as in D； the opposite
way to C； where the convexities are turned towards one another and the
concavities outwards。 Now no biped or quadruped bends his limbs like
the figures A or B； but the quadrupeds like C； and like D only the
elephant among quadrupeds and man if you consider his arms as well
as his legs。 For he bends his arms concavely and his legs convexly。
In man； too； the flexions of the limbs are always alternately
opposite； for example the elbow bends back； but the wrist of the
hand forwards； and again the shoulder forwards。 In like fashion；
too； in the case of the legs； the hip backwards； the knee forwards；
the ankle in the opposite way backwards。 And plainly the lower limbs
are opposed in this respect to the upper； because the first joints are
opposites； the shoulder bending forwards； the hip backwards； wherefore
also the ankle bends backwards； and the wrist of the hand forwards。
14
This is the way then the limbs bend； and for the reasons given。
But the hind limbs move criss…cross with the fore limbs； after the off
fore they move the near hind； then the near fore； and then the off
hind。 The reason is that （a） if they moved the forelegs together and
first； the animal would be wrenched； and the progression would be a
stumbling forwards with the hind parts as it were dragged after。
Again； that would not be walking but jumping； and it is hard to make a
continuous change of place； jumping all the time。 Here is evidence
of what I say； even as it is； all horses that move in this way soon
begin to refuse； for example the horses in a religious procession。 For
these reasons the fore limbs and the hind limbs move in this
separate way。 Again； （b） if they moved both the right legs first the
weight would be outside the supporting limbs and they would fall。 If
then it is necessary to move in one or other of these ways or
criss…cross fashion； and neither of these two is satisfactory； they
must move criss…cross； for moving in the way we have said they
cannot possibly experience either of these untoward results。 And
this is why horses and such…like animals stand still with their legs
put forward criss…cross； not with the right or the left put forward
together at once。 In the same fashion animals with more than four legs
make their movements； if you take two consecutive pairs of legs the
hind move criss…cross with the forelegs； you can see this if you watch
them moving slowly。 Even crabs move in this way； and they are
polypods。 They； too； always move criss…cross in whichever direction
they are making progress。 For in direction this animal has a
movement all its own； it is the only animal that moves not forwards；
but obliquely。 Yet since forwards is a distinction relative to the
line of vision； Nature has made its eyes able to conform to its limbs；
for its eyes can move themselves obliquely； and therefore after a
fashion crabs are no exception but in this sense move forwards。
15
Birds bend their legs in the same way as quadrupeds。 For their
natural construction is broadly speaking nearly the same。 That is；
in birds the wings are a substitute for the forelegs； and so they
are bent in the same way as the forelegs of a quadruped； since when
they move to progress the natural beginning of change is from the
wings （as in quadrupeds from the forelegs）。 Flight in fact is their
appropriate movement。 And so if the wings be cut off a bird can
neither stand still nor go forwards。
Again； the bird though a biped is not erect； and has the forward
parts of the body lighter than the hind； and so it is necessary （or at
least preferable for the standing posture） to have the thigh so placed
below the body as it actually is； I mean growing towards the back。
If then it must have this situation the flexion of the leg must be
backwards； as in the hind legs of quadrupeds。 The reasons are the same
as those given in the case of viviparous quadrupeds。
If now we survey generally birds and winged insects； and animals
which swim in a watery medium； all I mean that make their progress
in water by dint of organs of movement； it is not difficult to see
that it is better to have the attachment of the parts in question
oblique to the frame； exactly as in fact we see it to be both in birds
and insects。 And this same arrangement obtains also among fishes。
Among birds the wings are attached obliquely； so are the fins in water
animals； and the feather…like wings of insects。 In this way they
divide the air or water most quickly and with most force and so effect
their movement。 For the hinder parts in this way would follow forwards
as they are carried along in the yielding medium； fish in the water；
birds in the air。
Of oviparous quadrupeds all those that live in holes； like
crocodiles； lizards； spotted lizards； freshwater tortoises； and
turtles； have their legs attached obliquely as their whole body
sprawls over the ground； and bend them obliquely。 The reason is that
this is useful for ease in creeping into holes； and for sitting upon
their eggs and guarding them。 And as they are splayed outwards they
must of necessity tuck in their thighs and put them under them in
order to achieve the lifting of the whole body。 In view of this they
cannot bend them otherwise than outwards。
16
We have already stated the fact that non…sanguineous animals with
limbs are polypods and none of them quadrupeds。 And the reason why
their legs； except the extreme pairs； were necessarily attached
obliquely and had their flexions upwards； and the legs themselves were
somewhat turned under （bandy…shape） and backwards is plain。 In all
such creatures the intermediate legs both lead and follow。 If then
they lay under them； they must have had their flexion both forwards
and backwards； on account of leading； forwards； and on account of
following； backwards。 Now since they have to do both； for this
reason their limbs are turned under and bent obliquely； except the two
extreme pairs。 （These two are more natural in their movement； the
front leading and the back following。） Another reason for this kind of
flexion is the number of their legs； arranged in this way they would
interfere less with one another in progression and not knock together。
But the reason that they are bandy is that all of them or most of them
live in holes； for creatures living so cannot possibly be high above
the ground。
But crabs are in nature the oddest of all polypods