are not irritable, therefore IRRITABILITY HAS NO

THING IN COMMON WITH SENSATION *.

MUSCLES are compofed of longitudinal fibres The Mufcles. which fhorten themfelves, and are fo difpofed that this contraction always ferves fome wife purpose. They elegantly terminate in tendons, which are braced by fheathes, and though fo numerous, each occupies its proper place and just direction.

Colour was believed to be effential to the conftitution of a MUSCLE. But in fowls, in amphibious animals, in fishes, in worms and infects, through all the gradations of animals, of different species or different fizes, the colours of the muscular fibre change. In fishes, and in infects, it is generally white; even in the human body, it is not effentially red; the fibres of the iris, and the muscular coats of the arteries, the muscles of the ftomach, of the intestines, and of the urinary bladder, are colourlefs. We cannot therefore de

*The heart is divided into two AURICLES and two VENTRICLES. The AURICLES communicate with their correfponding VENTRICLES, and have VALVES to guard this paffage. The VALVES on the right fide are called tricufpides, on the left mitrales. THESE prevent the recurrence of the blood into the AURICLES. The AURICLFS and VENTRICLES may be faid to be hollow muscles, or rather may be compared to two hollow cavities on each fide formed of one muscle, and the left cavities or muscle are more abundantly furnished with fibres, becaufe a greater force is required to propel the blood through the body than the Jungs.

fine a muscle by that property which it often wants; but we may, with the utmost propriety characterize it by its CONTRACTILE POWER, the only true evidence of its nature; for the contraction of the iris proves it to be a muscle by truer marks than its colour; and by the fame rule the muscles of a fish, or of the meanest infect, are as perfect as those of man.

Such is the connection betwixt mufcles and their contractile force, or PRINCIPLE of IRRITABILITY, that the moment it dies, all its aftonishing power is gone: and the muscle which could lift a hundred pounds while alive, can bear the weight but of a few pounds when dead. Whereas, on the contrary, all thofe parts that are IRRITABLE, as the ligaments, tendons, &c. are capable of bearing the fame weight when dead as when alive.

This IRRITABLE, or CONTRACTILE POWER, in muscles, is that property by which muscles recede from certain ftimuli, without any feeling, without creating any consciousness of action, and fo little dependant is it upon nerves, that it is found equally perfect in animals* and plants which have no nerves, and remains in parts fevered from the body to which

The polypus, according to John HUNTER, is devoid of nerves.

they

they belong. This IRRITABILITY is fo far independent of nerves, and fo little connected with feeling, which is the province of the nerves, that upon stimulating any muscle by touching it with a cauftic, or irritating it with a sharp point, or driving the electric spark through it, the muscle inftantly contracts: although the nerve of that muscle be tied; although the nerve be cut so as to separate the muscle entirely from all connection with the fyftem; although the muscle itself be feparated from the body; although the creature upon which it be performed, may have loft all sense of feeling, and have been long to all appearance dead. Thus a MUSCLE cut from the limb, trembles and palpitates long after; the HEART, separated from the body, contracts when irritated; the BOWELS, when torn from the body, continue their peristaltic motion, fo as to roll upon the table, ceafing to answer to ftimuli only when they themfelves become actually dead. Even in vegetables, as in the fenfitive plant, this contractile power lives. It is by this IRRITABLE PRINCIPLE, that a cut MUSCLE contracts and leaves a gap; that a cut ARTERY fhrinks and retires into the flesh. Even when the body is dead to all appearance, and the nervous power

gone,

gone, this contractile power remains; fo that if a body be placed in certain attitudes, before it be cold, its MUSCLES will contract, and it will be stiffened in that posture till the organization yields, and begins to be diffolved. Hence comes the diftinction betwixt the IRRITABILITY of the mufcles and the SENSIBILITY of nerves; for the IRRITABILITY of muscles furvives the animal,-as when it is active after death;-furvives the life of the part, or the feeling of the whole system, as in palfy, where the vital motions continue entire and perfect ;-and where the muscles, though not obedient to the will, are subject to irregular and violent action;-and it furvives the connection with the rest of the fyftem, as where animals very tenacious of life are cut into pieces : whereas SENSIBILITY, the property of the nerves, gives the various modifications of fenfe, as vifion, hearing, and the reft; gives alfo the general fenfe of pleasure or pain; and thus the eye feels, and the skin feels; but their appointed stimuli produce no motions in these parts; they are fenfible but not irritable. The HEART, the INTESTINES, and all the MUSCLES of voluntary motion, answer to stimuli with a quick and forcible contraction; and yet they hardly feel the sti

muli by which these contractions are produced, or at least they do not convey that feeling* to the brain. There is no consciousness of present ftimulus in those parts which are called into action by the impulfe of the nerves, and at the command of the will: fo that MUSCULAR PARTS have all the irritability of the fyftem, with but little feeling, and that little owing to the nerves which enter their fubftance; while NERVES have all the fenfibility of the fyftem, but no

motion.

The nervous influence is a mere ftimulus to the voluntary muscles, as blood is to the heart and arteries; food to the ftomach; or bile to the intestines. It lofes its influence over the fyftem fooner, than the irritable principle in the fibre fails: for the irritable state of the muscle continues long after the voluntary motion, or power of excitement from the nerves, is gone: for when we die flowly, the irritable principle of the muscles is exhaufted in the struggles of death. If, while in perfect health, we are killed by a fudden blow, the irritable power of the muscles furvives the nervous system many hours or days, and the flesh

The cause of this curious phænomenon we shall particularly explain, when we come to treat of GANGLIONS. Vide Vol. IV. Introduction.