937. _What are ligaments?_
Ligaments consist of bands and cords of a _tough_, _fibrous_, _and
smooth substance_, by which the bones are bound together and held in
their places, allowing them freedom to move, and supplying smooth
surfaces over which they glide.
938. _Why are the joints bound with ligaments?_
Because the bones would otherwise be constantly liable to _slip from
their places_.
[Verse: "That which is born of the flesh is flesh; and that which is
born of the Spirit is spirit."--JOHN III.]
939. _What are tendons?_
Tendons are _long cords_, of a substance similar in its nature to
_cartilage_, by which _the muscles are attached to the bones_.
[Illustration: Fig. 56.--SHOWING A BALL AND SOCKET JOINT, AND THE
MANNER IN WHICH LIGAMENTS ARE EMPLOYED TO HOLD BONES IN THEIR
POSITIONS.]
A. The _ball_, or _head_ of the thigh bone.
B. The _socket_, showing the ligament in the socket, which holds
the head of the bone in its place, but allows it free motion.
C. _Ligaments_ tied from bone to bone, giving firmness to the
parts.
940. _Why are tendons used to attach the muscles to the bones?_
Because, by this arrangement, the large muscles by which the
extremities are moved, _may be placed at some distance_ from the bones
upon which they act, and thus the extremities, instead of being large
and clumsy, are _small_ and _neat_.
941. _How many muscles are there in the human body?_
There are about _four hundred and forty-six muscles_ that have been
dissected and described, and the actions of which are perfectly
understood. But there is probably a much larger number of muscles, and
of compound actions of muscles, than the skill of man has been able to
recognise.
[Verse: "All flesh is not the same flesh: but there is one kind of
flesh of men, another flesh of beasts, another of fishes, and another
of birds."--CORINTHIANS XVI.]
[Illustration: Fig. 57.--ILLUSTRATION OF THE RELATION OF MUSCLES,
TENDONS, AND BONES.]
942. _What is the constitution of a muscle?_
Every muscle is made up of a number of _parallel fleshy fibres_, or
threads, which are bound together by a smooth and soft tissue, forming
a sheath or case to the muscle, and enabling it to _glide freely_ over
the surfaces upon which it moves.
A. Lower extremity of the muscle which draws the fore-arm towards
the upper-arm, bends the elbow, raises the hand to the head, and
is powerfully exerted in pulling, lifting, &c.
C. A muscle which gives off four long _tendons_, which pass under
the _ligaments_ of the wrist, one to each finger, and by which the
fingers are bent upon the palm of the hand, as in grasping, &c.
F. _Tendon_ of a muscle which draws the little finger and the
thumb towards each other.
The _ligaments_ may be seen enfolding the finger-joints, and also
crossing the wrist, underneath the _tendons_.
The muscles are compressed into _tendinous cords_ at their ends, by
which they are _united to the bones_.
They are arranged in _pairs_, having reciprocal actions--each muscle
having _a companion muscle_ by which the part which it moves is
restored to its original position, when the influence of the first
muscle is withdrawn, and the stimulus given to bring back the part.
943. _Why can we raise our fingers?_
Because muscles which lie _on the fore-arm_, and have their tendons
fastened at the ends of the fingers, _contract_, and by becoming
shorter, _draw the fingers upward_, and towards the arm.
[Verse: "Thou hast clothed me with skin and flesh, and hast fenced
me with bones and sinews."--JOB XI.]
944. _Why can we throw back the fingers after they have been raised?_
Because the muscles at the back of the arm, _whose tendons are attached
to the back of the fingers_, contract and restore them to their former
position.
945. _What degree of strength do the muscles possess?_
The degree of strength of a muscle depends upon the _healthy condition_
of the muscle, the _amount of stimulus_ which it receives at the time
of exertion, and the manner in which _its powers are applied_.
The great muscle of the calf of the leg has been found, when removed
from a dead body, to be capable of sustaining a weight equal to _seven
times the weight of the entire body_.
_But the contractile power of the living muscles_ is very great: the
thigh bone has frequently been broken by muscular contractions in fits
of epilepsy. And in cases where there has been a dislocation of the
thigh, the head of the thigh-bone being thrown out of its socket, (Fig.
56) it has been found necessary to employ strong ropes, attached to a
wheel turned by several hands, in order to _overcome the contraction of
the excited muscles, and to enable the operator to restore the bone to
its place_.
946. _What is the stimulus which sets the muscles in action?_
The muscles are excited to action by _the nerves_, which they receive
from the _spinal cord_.
947. _Why does it require the influence of the will to set the arms in
motion?_
Because the muscles which form their mechanism are _voluntary_
muscles--that is, they are subject to the _will of man_, and
influenced by impulses directed to them through the nervous system _by
the mind_, which is the governing power.
[Verse: "And he took him by the right hand, and lifted him up; and
immediately his feet and ankle bones received strength."--ACTS III.]
948. _Why does the heart beat without any effort of the will?_
Because the muscles of the heart are _involuntary muscles_--that is,
they are _independent of the will_, and receive _a continuous nervous
stimulus_ which is not _under the controul of the mind._
[Illustration: Fig. 58.--MUSCLES AND VESSELS OF THE LEG AND FOOT.]
A. A large _ligament_, which covers the knee pan, or moveable bone
of the knee, by which the ends of the bones of the thigh and leg
are kept from slipping over each other.
B. A muscle which passes underneath the cartilages of the ankle,
and gives off four _tendons_, which are distributed to the toes,
and by which they are extended in elongating the foot, walking, &c.
C. Part of the muscle which forms the fleshy bulb of the calf of
the leg, and which terminates in the large _tendon_ attached to
the heel, called the _tendon of Achilles._
D. One of the ligaments which bind the tendons and the bones of
the ankle.
E. _Arteries_ proceeding from the large vessel descending the leg,
by which the toes are supplied.
949. _Why are the muscles of the arms, &c., made subject to the
controul of the will?_
Because, as they supply the mechanism through which we adapt ourselves
to our varying wants and circumstances, it was necessary that they
should be placed under the controul of the mental power, and be moved
_only in accordance with man's necessities._
[Verse: "If thou sayest, Behold, we knew it not; doth not he that
pondereth the heart consider it? and he that keepeth thy soul, doth
not he know it? and shall not he render to every man according to his
works?"--PROVERBS XXIV.]
950. _Why are the motions of the heart, &c., made independent of the
will?_
Because, as the necessity for the heart's motion is _fixed and
unalterable_, the constant motion of the heart could be best secured by
giving it a _fixed nervous influence_, by which it might be unfailingly
prompted to fulfil its functions.
If the movements of man's heart were _subject to his will_, he would
be constantly required to regard the operations of that organ; and so
large an amount of mental care and physical exertion would have to
be employed in that direction, that man's sole work would be to keep
himself alive. Hence we see the goodness of the Creator in _giving_
life to man, and in _keeping the vital impulses under his divine care_.