2. The division of the tendo Achillis is the result, either, 1st, of
the action of a cutting instrument; or, 2dly, of muscular action: hence
two very different modes of its production, the one by a wound, the
other by a rupture. The first is not a very rare accident, because
the projection of the tendon exposes it oftentimes to the stroke of
external bodies: the second, though but little noticed by the ancients,
has been frequently observed by the moderns, since their attention was
called to it by Petit.
3. The manner in which the division is produced by a wound, has nothing
particular in it; that by a rupture, takes place in the following
manner. A man leaps over a ditch, but his spring or exertion is too
weak; he reaches the opposite bank only with the ends of his feet:
the line of gravity not falling on the ground, the weight of the body
throws the feet into a state of violent flexion, the muscles contract
with great force, to prevent a fall backwards, and, at that instant,
the tendon is ruptured, in consequence of being drawn downwards by the
violent flexion of the foot, and upwards by the effort of the muscles:
hence it appears that Petit was deceived with regard to the mechanical
cause of the rupture, which he considered as taking place at the moment
of the patient’s alighting on his feet, when, as he said, the tendons
were surprised, so to speak, into a state of too great tension. It is
easy to apply the principles of this particular case to others that
may happen, and where the position may not be the same; such as, when
we leap on a table, &c. Sometimes slighter efforts have produced the
effect; and, as Louis observes, dancers have sometimes ruptured the
tendo Achillis by making a powerful exertion on the point of the foot,
as well as by other motions.
4. Divisions produced in the first mode, may be situated in any part
of the tendon. Those produced in the second, occur more particularly
about its middle: to that part the effort or strain is most forcibly
determined, and there the resistance is the weakest. The rupture of the
tendon may, according to Petit, be either complete or incomplete; but,
if we consider the simultaneous contraction of the gastrocnemii and
soleus muscles, and the intimate manner in which their two tendons are
united at a considerable distance above the heel, it will be difficult
to conceive how these tendons can be ruptured separately. With regard
to divisions produced by cutting instruments, the case is different:
there, the weapon may pass half way through the tendon either from
behind or laterally; and perhaps divisions of this kind are much more
frequently incomplete than otherwise, in consequence of the great
resistance of the tendinous fibres.
§ II.