Muscles, Action of the. All the muscles act, by the in-flation of their bellies; in consequence of which they are com-pressed or shortened. Thus, in muscular contraction, the more-able bone only is drawn in the direction of the fibres, while theother bone, to which the muscle is attached, remains fixed.Hence they operate in some measure as a lever; the two ends ofevery muscle being fastened to the bones by means of the ten-dons; but, if any fibre or nerve of the muscle be divided, or onlyobstructed by a ligature, the power of contraction instantly ceases.

Muscles, Insertion and E'orce of the. The all-wise Au-thor of nature has furnished animals with limbs, moveable aboutthe joints by means of muscular cords, inserted near the joint orcentre of motion ; the great wisdom of which will appear, fromsupposing the insertion to be at E, Plate CXVIII. fig. 3, near thewrist B, the muscle DE being either loose and separate from thebones DAB, or bound down to it by some ligament or fascia R ;in either of which cases the bone AB cannot be turned up quite tothe situation AH, unless the muscle DE be contracted or shorten-ed to DM, which would not only be troublesome, but even im-possible. It would be troublesome because the breadth andthickness of the arm would be vastly increased, so as to become asbig as the belly of an animal. On the other hand, the structureof a muscle being such that it cannot be contracted but a little,seldom above two or three fingers-breadth ; such an insertion asthat at E, which requires a contraction of above a foot and a half,would be altogether impossible. Therefore, in fact, we find themuscles inserted near the centre of motion, as at I, fig. 4. In or-der to calculate the force of any muscle, we are to consider thebones as levers; and then the power or force of the muscle willbe always to the resistance or weight it is capable of raising, as thegreater distance of the weight from the centre of motion is to theless distance of the power. Hence, it being found by experi-ments, that a robust young man is able to suspend a weight R,fig. 4, equal to twenty-eight pounds, when the arm is extended ina supine and horizontal situation, we have this proportion, viz. theforce of the muscle ID is to the weight R, = 28lb. as the distanceRC is to the distance IC. But it is found, that BC, the length ofthe cubit and hand, is more than twenty times greater than IC, thedistance of the muscle from the centre of motion. Therefore theforce of the muscle ID, must be more than twenty times greaterthan the weight R, or more than 28 x 20 = 56011). Again, tofind the force which the biceps and brachiteus muscles exert, whenthe humerus EA, fig. 5, is perpendicular to the horizon, we arefirst to consider what weight a man is capable of sustaining in thisposture, viz. R = 351b. and next the quantity of the distances CB,Cl, which, in this case, are as 16 to 1. Therefore the force ofthese muscles is to the weight R = 351b. as the distance CB = 16is to the distance IC = 1 ; or the force is equal to 560, as before.But what appears most wonderful, is the force of the muscles thatmove the lower jaw; which, when taken altogether, do not in aman exceed the weight of lib. and yet exert a force equal to■5341b. and in mastive dogs, wolves, bears, lions, &c. their force isVastly superior, so as to break large bones, as they practise dailyin their feeding. Thus we see that, on account of the insertion ofthe muscles being so near the centre of motion, it is necessary forthem to exert a much greater force than would otherwise be re-quired. And as Dr. Blane observes, there is another circumstancewhich tends to waste the power, and that is the obliquity withWhich they are inserted into their bones ; so that the greater partQf the force is expended in pressing one bone against another atdie articulation, and only a small part of it in making the flexuresa nd extensions. These disadvantages, however, are compensatedBy a number of eonveniencies, which could not have been obtain-ed on any other plan. We must distinguish between those actionswhich consist in pressure, and those which depend on percussion;for as the momentum of this last depends on velocity, it is evident,that there must be a great advantage from the insertion of the ten-don being near the centre of motion, as greater velocity, with lesse xpence of contraction, will thus be communicated to the extre-mity. The muscles, for instance, which are attached to the ole-cranon, in performing those actions with the hand which requireTubbing, act with a disadvantage exactly proportional to the ine-quality of the distance from their insertion to the joint of thedhow, and that from the same joint to the hand. This is an act®f pressure. But in the case of percussion, as in the action ofvol. in_ no. 151.

using a hammer, there is an evident advantage resulting from thevelocity communicated to the extremity; for, in order to haveproduced the same velocity, with the insertion at a greater dis-tance from the centre of motion, a much greater degree of con-traction would have been necessary ; and the doctor shews that fa-tigue principally depends on a contraction of the muscles. “ Ifany one (says he) will take the trouble of comparing the fatigue ofthe biceps muscle, in bearing a weight in the hand with the elbow-joint bent to a right angle, with that of bearing the same weightfor the same length of time with the joint at an acute angle, lie willbe sensible how much the degree of fatigue depends on the extentof contraction ; and, by attending to the relative situation of mus-cular fibres, it will appear that Nature , in distributing the fibres ofmuscles obliquely, has had it in view not only to increase theirnumber, but to save contraction.” In considering the actions ofthe various muscles in producing the different actions of the bodywe find scarcely one produced that can be called direct. In someinstances we find two muscles, or two sets of muscles, co-operat-ing, so that the motion effected by them shall be in the diagonal oftheir direction. This is the case of the oblique muscles of the ab-domen in some of their actions, and of the intercostal muscles inall theirs. Sometimes different portions of llie same muscle com-bine in like manner to produce a similar effect; and in all thelong muscles, however simple their origin and insertion may be,there is an internal obliquity of their fibres with regard to one an-other ; for these do not run from end to end, but there are partsof the tendon running into the belly of the muscle, so as to divideit into penniform and rhomboidal portions. This distribution ofthe fibres takes off from the length ; but as it takes place in thosecases where the origin and insertion are at a considerable distance,this can be afforded: and Ibis, as well as the waste, of power, inconsequence of oblique action, is more than compensated by theincreased strength from the fibres being multiplied ; for, in conse-quence of this structure, there is an extent of tendon afforded suf-ficient for the insertion of a greater number of fleshy fibres. Thedoctor thus shews that, by the obliquity of the muscles, a veryconsiderable quantity of contraction is saved, and consequently aproportional degree of fatigue prevented. “ Let the line AB(says he) in the annexed diagram, represent a moveable bone, andthe line CD a fixed bone parallel to it. Let FE, perpendicularto these lines, represent a muscle acting in its own direction, andthe lines GE, HE, represent two muscles acting obliquely, andproducing by a diagonal-action the same effect as the other. Ifthe bone AB be brought to the situation ab by the action of themuscle FE, the muscle will then be in the situation FK. If thebone is brought into the same situation by the action of the mus-cles GE, HE, these muscles will then be in the situation GK,HK, Plate CXVIII. fig. 6. The proposition to be demonstratedis, that the line GK bears a greater proportion to the line GE,than the line FK does to the line FE ; for EK is to FE as GL is toGE (Euc. Elem. B. vi. Prop. 2) ; and the angle ELK beiim lessthan a right angle, the angle GLK, which is adjacent to it, mustbe greater than a right angle; and the angle GKL being in thesame triangle with GLK, must be less than aright angle. Theline GK, therefore, which subtends the greater angle, is greaterthan the line GL, subtending the less, and therefore bears a greaterproportion to GE. But the line GL is to GE as FK is to FE -and therefore GK bears a greater proportion to GE than FK doesto FE ; that is, the fibres of the muscles acting obliquely, suffer aless proportional decurtation than those of the muscle actino- di-rectly. It is farther obvious, that the more oblique the action be-comes, the greater saving there will be of contraction; for in mov-ing the line ab towards CD, the line FK diminishes in a swifterratio than the line GK; and when the former has vanished, thelatter is in the situation GF.” Besides these advantages in pointof diminishing fatigue, there are others relating to the shape orthe members. Thus, if the insertions of the muscles had been ata great distance from the joints, they must, upon every occasion,have passed like bow-strings from one bone to the other, and thelimbs must have been exceedingly clumsy and unwieldy ; all themotions must also have been extremely slow : and notwithstand-ing the superior strength which people would then have enjoyed,it is very plain, that they would scarcely have been fit for any of theoffices of life which they now perform. The motions of'the fargreater part of the muscles are voluntary, or dependent on ourk 8 D will;.