Billeder på siden
PDF
ePub

This opinion is countenanced by the experiments of Baron de HALLER; fome of which are recorded in one of the early volumes of the Philofophical Tranfactions, for it appears that a paralysis of the posterior extremities of animals was induced by tying their aorta.

The learned Dr. MONRO, who was not much inclined to give up the pre-eminence of the brain and nervous system to the vascular, after many experiments was obliged to conclude, "that concomitant arteries, fomehow or other, tune the nerves, fo as to fit them to convey impreffions."

In order to decide, fays the ingenious Dr. Fowler, whether a greater detriment to that condition of a limb, upon which contraction depends, is induced by interrupting its circulation, or by intercepting its communication with the brain, I refolved upon trying the powers of the nervous electricity as difcovered by GALVANI.

Having tied the crural artery on one fide, and divided the fciatic nerve on the other, on three full grown frogs, I cut off their heads with a pair of fciffars, to preserve the circulation of the blood as entire as poffible, and at the fame time to prevent the continuance of pain, which might exhaust the power of the body, and defeat the experiment.

When

When these frogs were laid upon a surface of zinc, and excited by means of a rod of filver, the contractions were found extremely feeble in the legs whofe artery had been tied, and ceafed altogether in about twenty-two hours after their death. But in the legs, whofe nerve had been divided, they appeared very vigorous, and continued excitable upwards of two days after they had ceased

to be fo in the other.

Having thus found, that a diminution of the circulation of a part, was accompanied with a proportionable diminution of the refpective powers of nerves and mufcles in that part, I next proceeded to examine if an increafed circulation would be attended with a proportionable increase of these powers; and that this is actually the cafe, the facts I am about to relate will prove.

I have before fhewn, that if a living and entire frog be set upon a plate of zinc, contractions can very feldom be produced in any part of its body by paffing a rod of filver over it, fo that the filver, the frog, and the zinc, may be all in contact with each other. But I have found, in upwards of twenty experiments, that when inflammation had been excited in one of the hind legs of a frog, by irritating it with a brush, contractions uniformly took place in that leg when the metals were applied

Z z

applied to it, although none had been produced in it before it was inflamed, nor could ftill be produced in the other leg which remained in its natural state.

Having previously, continues Dr. FOWLER, excited inflammation, by means of a brush, in the foot and leg of a healthy and large frog, I cut off its head. The contractions excited by the different metals in the inflamed leg were in vigorous and instantaneous jerks; those in the found leg more languid, and difficultly excited. Spontaneous motions continued at this time nearly the fame in both. Even till the end of the second day, after the frog's head had been taken off, the contractions excited in the inflamed leg continued uniformly, and beyond all comparison more vigorous than what I could by any means excite in the found leg. It then became very stiff.

Dr. FOWLER, from these experiments, has judicioufly concluded, that the fanguiferous fyftem contributes more immediately than the nervous to the support of that condition of muscles and of nerves, on which depend all the phænomena of contraction in the animated fibre.

This fubject, it must be confeffed, is after all exceedingly abftrufe, and the ideas of the best physiologists are not so clear and diftinct, fo complete and comprehensive,

as

as we could with: but, as Dr. BEDDOES has judiciously

obferved,

WE SHOULD SET A PROPER VALUE ON OUR PRESENT KNOWLEDGE, ALTHOUGH IT BE IMPERFECT, AND RESTRAIN THOSE RUDE HANDS, THAT ARE EVER READY TO PLUCK UP THE TENDER PLANTS OF SCIENCE, BECAUSE THEY DO NOT BEAR RIPE FRUIT AT A SEASON, WHEN THEY CAN ONLY BE PUTTING FORTH THEIR BLOSSOMS.

[ocr errors]

Of the importance of the IRRITABLE PRINCIPLE, derived probably from oxygenated blood, we shall foon learn, if we confider the vast expence of it continually going on in the fyftem. First, the IRRITABLE PRINCIPLE is expended in giving perpetual and strong motion to the heart, which overcomes the elafticity and vis inertia of the whole arterial fyftem :-next the expense of the IRRITABLE PRINCIPLE in moving, with great force and velocity, the innumerable trunks and ramifications of the arterial fyftem: the expenfe of the IRRITABLE PRINCIPLE in circulating the whole mass of blood through the long and intricate contortions of the very fine veffels, which compose the glands and capillaries;-then the expense of the IRRITABLE PRINCIPLE in the exertions of the absorbent extremities of all the

[blocks in formation]

laeals, and of all the lymphatics, which open their mouths on the external surface of the skin, and on the internal furfaces of every cell or interftice of the body;-then the expense of the IRRITABLE PRINCIPLE in the venous abforption, by which the blood is received from the capillary veffels, or glands, where the arterial power ceases, and is drank up, returned to the heart;-next the expenfe of the IRRITABLE PRINCIPLE used in the muscles of refpiration in their office of perpetually occafioning the expansion of the bronchia, or air-veffels, of the lungs ;-and laftly, in the unceafing periftaltic motions of the ftomach and whole fyftem of inteftines, and in all the fecretions of bile, gaftric juice, mucus, perfpirable matter, and the other excretions from the fyftem.

If we confider the ceaseless expense of irritable principle thus perpetually employed, it will appear to be much greater in one day, than all the voluntary exertions of our muscles and organs of fenfe in a week. Hence when the vital organs are too forcibly put into action by any ftimulus, without a proportionally increafed production of the irritable principle in the fyftem, it is evident that a great deficiency of action soon enfues, or A STATE OF EXHAUSTION*. But of this we are to consider more at large in the following fections.

*Dr. DARWIN.

SECT.

« ForrigeFortsæt »