The reason why : $b a careful collection of many hundreds of reasons for things which, though generally believed, are imperfectly understood. A book of condensed scientific knowledge for the million
John Stuart Mill · en
[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_.
CHAPTER XLVII.
951. _What are nerves?_
The nerves are branches of the _brain_ and the _spinal cord_; they are
distributed in great numbers to all the active and sensitive parts of
the body.
952. _What is the spinal cord?_