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the principle of life may in some instances be suddenly removed, or have its power abolished, whilst in general it is lost by degrees.
in; 1. The contraction of irritability takes place in some animals in a very slow and gradual manner, and their muscles in general are incapable of sudden contraction. -- Yet though the action of their muscles is very slow, it is very powerful and very permanent. The American sloth, supports its weight for a very long time in one attitude by fixing its claws into the branches of trees; an act which would speedily weary muscles of an ordinary character. The muscles of the legs of birds that roost, em to have a similar power of permanent contraction.
Mr. Carlisle has lately demonstrated a peculiar distribution of the arteries in the
limbs of these tardigrade animals, as they are called, and Doctor Macartney has shewn that a similar arrangement of véssels exists in the legs of fowls. Such a distribution of the arteries may be subservient without being essential to these modes of action.
In the human body we
see instances of irritability exerting itself after the manner it does in general in tardigrade animals. If the iris had possessed the ordinary powers of muscles, and none else, it could not have remained, as it is known to do, permanently contracted in a strong light, and permanently dilated in a weak
Indeed, an anatomist who is fond • of tracing structure as connected with
function, might readily persuade himself, that there is in the iris a distribution of arteries, similar to that which Mr. Carlisle has demonstrated in the limbs of
sloths. We find, however, that sphincter muscles in general have the power of continuing their contraction, though no peculiar distribution of vessels is discoverable in them. In the gall bladder, the function of which requires this slow but permanently acting irritability, in order to express its contents in small and equal quantities into the bowels, as the digested aliment passes into them, we discover no peculiar arrangement of arteries. Though we cannot excite any sudden contraction of that bag, yet we know that it can gradually reduce itself into a very small compass. The skin has every where this slow but permanently acting, and gradually relaxing irritability, the effects of which are most evident in lax and pendulous portions of it. Accordingly we sometimes observe the scrotum and prepuce condensed into à surprizingly small and very compact mass.
Thus have we even in the human body evidences of irritability acting in various modes, whilst we can equally perceive that in tardigrade animals some of their muscles act with celerity. In the Lori, of whose habits. Vosmaer has given so interesting an account, and which manifested no signs of alacrity, save in eating the food that it liked, no stimulation nor injury could induce it to mend its pace, but it shewed its resentment of the attempt to make it perform impossibilities, by suddenly snapping at the stick or instrument with which it was goaded; and thus again demonstrated that the muscles of its jaw were endowed with an irritability of the more common character.
Having thus briefly described the principal phænomena of muscular action, for I forbear to notice others of less impor
- tance, I proceed to review the conjectures that have been formed as
to the cause of these curious, sudden, and powerful contractions. Not to speak of exploded hypotheses, I trouble you only with those which are modern.
First, then, the contraction has been supposed to be the effect of some chemical change occurring in the part. This opinion is I think invalidated by the reiterated contractions which may be produced in the limbs of some animals when removed from the body, even during twenty-four hours, if excited by voltaic electricity, and consequently when no supply of materials can be supposed to exist within the limb, to produce such reiterated chemical changes. The opinion is still further refuted by observing, that these vivacious contractions will equally take place, upon the same excitement, in the exhausted receiver' of an air