Evolution, Old New | Page 9

of one
against the other, or by the pressure of unequal surfaces. It also gives to
the tendons a very considerable mechanical advantage by altering the
line of their direction, and by advancing it farther out of the centre of
motion; and this upon the principles of the resolution of force, upon
which all machinery is founded. These are its uses. But what is most
observable in it is that it appears to be supplemental, as it were, to the
frame; added, as it should almost seem, afterwards; not quite necessary,
but very convenient. It is separate from the other bones; that is, it is not
connected with any other bones by the common mode of union. It is
soft, or hardly formed in infancy; and is produced by an ossification, of
the inception or progress of which no account can be given from the
structure or exercise of the part."[16]
It is positively painful to me to pass over Paley's description of the
joints, but I must content myself with a single passage from this
admirable chapter.
"The joints, or rather the ends of the bones which form them, display
also in their configuration another use. The nerves, blood-vessels, and
tendons which are necessary to the life, or for the motion of the limbs,
must, it is evident in their way from the trunk of the body to the place
of their destination, travel over the moveable joints; and it is no less
evident that in this part of their course they will have from sudden
motions, and from abrupt changes of curvature, to encounter the danger
of compression, attrition, or laceration. To guard fibres so tender

against consequences so injurious, their path is in those parts protected
with peculiar care; and that by a provision in the figure of the bones
themselves. The nerves which supply the fore arm, especially the
inferior cubital nerves, are at the elbow conducted by a kind of covered
way, between the condyle, or rather under the inner extuberances, of
the bone which composes the upper part of the arm. At the knee the
extremity of the thigh-bone is divided by a sinus or cliff into two heads
or protuberances; and these heads on the back part stand out beyond the
cylinder of the bone. Through the hollow which lies between the hind
parts of these two heads, that is to say, under the ham, between the ham
strings, and within the concave recess of the bone formed by the
extuberances on either side; in a word, along a defile between rocks
pass the great vessels and nerves which go to the leg. Who led these
vessels by a road so defended and secured? In the joint at the shoulder,
in the edge of the cup which receives the head of the bone, is a notch
which is covered at the top with a ligament. Through this hole thus
guarded the blood-vessels steal to their destination in the arm instead of
mounting over the edge of the concavity."[17]
. . . . . .
"What contrivance can be more mechanical than the following, viz.: a
slit in one tendon to let another tendon pass through it? This structure is
found in the tendons which move the toes and fingers. The long tendon,
as it is called in the foot, which bends the first joint of the toe, passes
through the short tendon which bends the second joint; which course
allows to the sinews more liberty and a more commodious action than
it would otherwise have been capable of exerting. There is nothing, I
believe, in a silk or cotton mill, in the belts or straps or ropes by which
the motion is communicated from one part of the machine to another
that is more artificial, or more evidently so, than this perforation.
"The next circumstance which I shall mention under this head of
muscular arrangement, is so decidedly a mark of intention, that it
always appeared to me to supersede in some measure the necessity of
seeking for any other observation upon the subject; and that
circumstance is the tendons which pass from the leg to the foot being

bound down by a ligament at the ankle, the foot is placed at a
considerable angle with the leg. It is manifest, therefore, that flexible
strings passing along the interior of the angle, if left to themselves,
would, when stretched, start from it. The obvious" (and it must not be
forgotten that the preventive was obvious) "preventive is to tie them
down. And this is done in fact. Across the instep, or rather just above it,
the anatomist finds a strong ligament, under which the tendons pass to
the foot. The effect of the ligament as a bandage can be made evident to
the senses, for