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a distinct class, and subservient alone to what he calls the organic life. The contractile power of muscles is also capable of remaining in vehement action for a great length of time, as we see in some cases of cramps, and still more in some cases of tonic tetanus,

1 Yet though the irritable power is' noe incapable of continued exertion, it seems evidently to be in general susceptible of fatigue, and inclines to be

to be at rest. If we stimulate the muscles of a limb of a frog severed from the body, by voltaic eleetricity, the muscular actions are at first vivid and forcible, but they grow fainter and feebler on repeated excitement. Yet if we wait a little 'till they seem to regain their power, they become vivid and forcible as at first from the same de gree of excitement." Such actions may be

the principle of life may in some instances be suddenly removed, or have its power abolished, whilst in general it is lost by degrees.

6. 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

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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

one. 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

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creased nor diminished during its contraction, what is lost in length being gained in bulk. The, voluntary .contraction of muscles cannot be long continued ; they become weary and painful, the contraction remits and recurs, causing a tremulous motion. Yet this phænomenon does not seem to be the effeet of absohute inability, in the irritable : property, to continue in action, for some muscles continue to act without experiencing fatigue. For instance, those of the jaws and back; for whenever they relax, the jaw drops, and the head and body fall forwards, las we see in persons who are going to sleep in a sitting posture. Certain sphincter muscles likewise remain in action without experiencing fatigue. Some sphincters also, I may add, are disposed to yield considerably without impatience; so that their irritability resembles that of those muscles which Bichật has considered as

a distinct class, and subservient alone to what he calls the organic life. i The contractile power of muscles is also capable of remaining in vehement action for a great length of time, as we see in some cases of cramps, and still more in some cases of tonic tetanus,

Yet though the irritable power is noe incapable of continued exertion, it seems evidently to be in general susceptible of fatigue, and inelines to be at rest. If we stimulate the museles of a limb of a frog severed from the body, by voltaic electricity, the muscular actions are at first vivid and forcible, but they grow fainter and feebler on repeated excitement, Yet if we wait a little till they seem to regain their power, they become vivid and forcible as at first from the same der gree of excitement." Such actions may be

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