beart is of a mufcular texture, forming what is called a hollow muscle; and the urinary bladder, ftomach, inteftines, &c. are enabled to act upon their contents, merely because they are provided with mufcular fibres. Thefe are called INVOLUNTARY muicles, because their motions are not dependent on the will. The mufcles of refpiration, being in fome measure influenced by the will, are ad to have a MIXED motion.

(181.) MUSCULAR FIBRES are bundles of redh coloured threads, which perform all the motoas vifible in the human body. When many of thee fibres are collected together, and appear more evidently red, they are called a MUSCLE In every mufcle we meet with long foft threads of fbres, fomewhat elaftic, or extenfible, and almost conftantly difpofed parallel with each other; and thefe, being furrounded with a good deal of cellular fubftance, are by that fastened together into Ette bundles, called lacertuli; which are again tied together into larger bundles, by a more loofe celular net-work, that contains fome fat; and between these we conftantly perceive membranous partitions and stripes of the cellular fubftance removing them farther from each other, till at laft a number of them combined together, either parallel or inclined, are furrounded with a more thin and denfe cellular membrane continuous with that of their partitions; and this being again furrounded by a thicker plate of the cellular fubfance, parts the whole bundle from the adjacent feh, and gives it the denomination of a jingle or entire muscle.

(182.) In every one of these threads there appears a letter feries of filaments, which, by oblique extremities, are cemented to others of the fame kind, forming together a large fibre. The generality of the mufcles, but more especially thofe which are inferted into the bones, have other bres fixed to them; but these are condensed into a more flender, hard, and fhining fubftance, of a fher colour, which has the name of TENDON. When the tendinous fibres expand into a broad flat furtace, they form what is called an oponeurofis.

(183.) Some anatomifts have defcribed the mufcular fibres to be ftrings of bladders, and have endeavoured to account for mufcular motion by an expantion made from an influx of blood and animal fpirits into thefe bladders; but as the mufCes do not increase their bulk fenfibly in contracting, this hypothefis cannot be correct. Other anatomifts thought that in this way the mufcles might be contracted by a fwelling, fcarce fenfible, the bladders were but very finall; for, fay they, fuppofing a bladder of any determined bignefs Can raile a weight a foot, a hundred bladders, whofe diameters are each a hundredth part of the former, will raife the weight to the fame height. But the force of inflation, and the fwelling of all together, will be ten thousand times lefs, and it will alfo raife ten thousand times lefs weight, which they have not obferved. Therefore not one fach ftring of bladders, but ten thoufand, must be applied to do the fame thing that the one bladder will do; and they will have the fame fwelling; therwife, it would be eafy to fhew' how to make a perpetuum mobile of almost any force. VOL. II. PART 1.

(184.) Mufcles are of two forts, rectilinear and penniform. The RECTILINEAR MUSCLES have their fibres almoft parallel, in the fame or near the fame direction with the axis of the muscle.The latter have their fibres joined, in an oblique direction, to a tendon paffing in or near the axis, or elfe on their outfide. Rectilineal mufcles, if their origins and infertions lie in little compafs, are never of any confiderable thicknefs, unles they are very long, because the outer fibres would comprefs the inner ones, and render them almost ufelefs. Every rectilineal mufcle, therefore, whose inner fibres are comprelled by the outer, has its inner fibres longer than its external fibres, in or der that they may be capable of an equal quantity of contraction.

(185.) The PENNIFORM MUSCLES, thongh in a manner free from the inconvenience of one fi bre compreffing another, and though by the ob liquity of their fibres nothing is abated of their moment, yet the fibres of the penniform muscles becoming more and more oblique as they con tract their strength decreases, and their velocity increafes, which makes them lefs uniform in their actions than the rectilineal muscles: For in all cafes, juft fo much more weight as rectilineal bres will raife them oblique ones, the oblique will move their weight with fo much greater velocity than the rectilineal: which is making their moments equal: So that in the structure of an ani mal, like all mechanic engines, whatever is gained in ftrength is loft in velocity, and whatever is gained in velocity is loft in ftrength. A penniform mufcle, therefore, is never exerted where a rectilineal muscle can be ufed; and the cafes in which a rectilineal muscle cannot be used, are where the fhape of a mufcle is fuch as that the inward fibres would be too much comprefed, or where the rectilineal fibres could not have a lever to act with, fuitable to their quantity of contraction, which is the cafe with all the long mufcles of the fingers and toes.

(186.) Every muscle must be inferted in, or pafs over the centre of the motion of the joint it moves, at a diftance fuitable to its quantity of contraction, and the quantity of motion in the joint moved; for if it were to be inferted too near, the motion of the joint would be performed before the muscle became completely contracted; if too far off the mufcle would ceafe to contract before the whole motion of the joint was performned. And though the quickness and quantity of motion in a mufcle will be, cæteris paribus, as the length of its fibres; (for if a fibre 4 inches long will contract one inch in a given time, a fbre 8 inches long will contract two inches in the fame time; and the ftrength of a muscle or power to raife a weight, cæteris paribus, will be as th number of its fibres; for if one fibre will raife grain weight, 20 fibres will raife 25 grains:) Y.t two mufcles of equal magnitude, the one long, and the other fhort, will both move the farie weight with the fame velocity when applied to a bene; because the levers they act with must b as their lengths, and therefore the penniform and thort thick mufcles are never applied to a bone for the fake of ftrength, nor long fibred mufeles for quickncfs; for whatever is gained by the form

D

of the mufcle, whether ftrength or quickness, must be loft by their infertion into the bone. Either the mufcles must not perform all that they can, or the bones must have lefs motion than they are capable of.

(187.) In the limbs feveral muscles pafs over two joints, both of which are liable to move at once, with a force proportionable to the levers they act with, upon each joint; but either joint being fixed with an antagonist mufcle, the whole force of fuch mufcles will be exerted upon the other joint; which in that cafe may be moved with a velocity equal to what is in both joints, when thefe mufcles act upon both at once. This mechanifm is of the utmoft importance. (188.) The proper use and action of any mufcle is that which it has without the necessary affiftance of any other muscle, and what that is, in a muscle moving a joint, we may always know in any fituation, and with what force it acts, cæteris paribus, by dropping a line from the centre of motion of the joint it moves, perpendicular into the axis of the mufcle. But in a joint which admits only of flexion and extenfion, this line muft alfo be perpendicular to the axis of motion in that joint, and the action of the mufcles will be in the direction of that particular line, and the force with which it acts in any fituation will be, cæteris paribus, as the length of that perpendicular line.

(189) Every mufcle, fo far as it is diftinct, and is moved against any part, is covered with a fmooth cellular membrane to make the friction cafy; but where the muscles are externally tendinous, their furfaces are generally,fmooth enough to make fuch a covering unneceffary. Befides this membrane there is another, known by the name of fafcia tendinofa, which deferves to be particularly confidered. The ftrong one on the outfide of the thigh, which belongs to the fafcialis and gluteus mufcles, is of great ufe, in raifing the gluteus farther from the centre of motion of the joint it moves to increafe its force. In like manner the fafcia detached from the tendon of the biceps cubiti alters its direction for the fame purpofe; but thofe on the outside of the tibia and cubit, &c. are only flat tendons from which the fibres of the mufcles arife as from the bones. There exift alfo in many places tendons between the mufcles, from which each mufcle arifes in like manner; for the bones themfelves are not fufficient to give origin to half the fibres of the mufcles that belong to them.

Name.

talis.

(190) We cannot close this fection without taking notice of the IRRITABILITY of the mut cles. If a mufcle is pricked or irritated, it im mediately contracts. This is called its irritable principle, and this irritability is to be confidcred as the characteristic of mufcular fibres, and may ferve to prove their exiftence in parts that are too minute to be examined by the eye. This power, which difpofes the mufcies to contract when ftimulated, independent of the will, is fuppofed to be inherent in them; and is therefore named by Halier vis infita. This property is not to be confounded with elafticity which the membranes and other parts of the body poffeffes in a greater or lefs degree in common with the mufcles; nor with fenfibility, for the heart, though the most irritable, seems to be the leaft fenfible of any of the mufcular parts of the body. After a mufcu lar fibre has contracted, it foon returns to a flate of relaxation, till it is excited afresh, and then it contracts and relaxes again. We may likewife produce fuch a contraction, by irritating the nerve leading to a muicle, although the nerve itfelf is not affected. This principle is found to be greater in fmall than in large, and in young than in old, animals. In the voluntary mufcles thefe effects of contraction and relaxation of the flefly fibres are produced in obedience to the will, by what may be called the vis a rvosa, a property that is not to be confounded with the vis infita. The existence of a vis infita, however, different from a vis nervea, has been called in queftion by feveral, particularly by Doctor Monro, who has flated very formidable objections against it, in his Obfervations on the Nervous System, of which we fhall afterwards have occafion to take particular notice, under NEUROLOGY.

SECT. II. Of the PARTICULAR MUSCLES. (191.) To prevent tautologyy and circumlocu tion, we have thrown the enumeration and defcription of the particular mufcles into the form of a table; in which the name, origin, infertion, and principal ufe of each mufcle, will be seen at one glance, defcribed in a few words. It is however to be understood, that this does not include all the mufcles of the body; fince thofe belonging to the eyes, internal ear, inteftinum rectum, &c. are mentioned in other parts of the work. We must alfo remark, that though almost all the mufcles are in pairs, mention is here made only of the mufcles of one fide.

Origin.

Infertion.

Ufe.

occipitis.

eye-brows.

the head backwards, and to raife the eyebrows and fkin of the fore-head. Into the inner part of To draw the eye

the occipito-fronta- brows towards each

lis.

other, and to wrin

kle the forehead.

4. Helicis major. From the upper, anterior, and acute Into the cartilage of part of the helix.

the helix, a little above the tragus.

5. Helicis minor. From the lower and fore part of the Into the helix, near helix.

the fiffure in its cartilage. THE NOSE. Into the nafal procefs of the os maxillare, and anterior extre mity of the os nafi, MOUTH AND LIPS, Into the upper lip

and ala of the nofe.

Into the orbicularis oris at the angle of the mouth.

From the os malaæ, near the zygo- Into the angle of the

matic future.

mouth.

Ufe. To shut the eye.

To open the eye.

To raise the ear. To raife this eminence, and to pull it forwards.

To ftretch the concha, and pull the ear backwards.

To deprefs the concha, and pull the point of the tragus a little outwards. To dilate the mouth of the concha. To ftretch the concha and fcapha, and likewife to pull the parts it is connected with towards each other.

To deprefs the upper part of the helix.

To contract the fiffure.

To ftraighten the noftrils, and likewife to Corrugate the skin of the nofe.

To draw the upper lip and fkin of the nofe upwards and outwards. To raise the corner of the mouth.

To raise the angle of the mouth, and make the cheek prominent, as in laughing.

the mouth obliquely outwards.

Into the root of the ala nafi and upper lip.

7.