is possessed by many animals. A slight degree of
information will also instruct him, that there are certain motions of
his animal frame, over which he has no immediate control. The motions
which he is able to direct and regulate, have been termed voluntary; and
those over which he possesses no influence or command, have been
denominated involuntary motions. The most perfect instances of the
latter are the pulsations of the heart, and the movements of the
intestines, usually called peristaltic. The curiosity which is natural
to man as an intelligent being, would of course prompt him to enquire
into the cause of these phenomena, although the result of his
investigations might be inadequate to the toil of his research: for, he
would be as much puzzled to account for the influence by which certain
muscles are moved at will, as he would at others which possess a
determinate motion, and are not subject to this direction. While man
continues in a healthy state, he is enabled to move at pleasure those
muscles or instruments of motion which are subject to his will; and the
involuntary muscles continue duly to perform their appropriate office;
but in certain morbid states it sometimes occurs, that the exertion of
the will to move a leg or arm is ineffectually directed, and however
much we desire, wish, or will such motion, these limbs are
disobedient.[12] This condition of the members has been termed
paralytic: the will to move remains perfect; but the organs to be acted
on are insensible to that influence which, in a sound state, excited
them to motion. As in the healthy state the will has the power to
produce motion, so it is also competent to prevent it; therefore to
move or to abstain from motion, are equally the dictates of the will.
But it not unfrequently happens, when we intend to thread a needle, to
write our name, or to perform some surgical operation, that the will
exerts all its influence to keep the hand steady for the due performance
of these necessary acts; yet, notwithstanding these implicit commands,
the hand continues to move in all directions, but those which could
accomplish the object. So, that these muscles, ordinarily voluntary,
become, in a certain degree, converted into involuntary muscles. A
higher degree of this state prevails in the affection called St. Vitus'
Dance, and likewise in some convulsive symptoms attendant on locked
jaw, where the body is drawn with incredible violence. It may be
noticed, tha