connecting the fixed point it with L by the rigid body it T. It is the omission of the fulcrum lc, in calculating such oblique forces, which has hitherto obscured the explanation of the intercostal muscles.
This may be illustrated in another way (fig. 690.). Let AB and c D represent bars as before upon A C ; t' the tension ; let c n and A c be fixed ; withdraw the pin at A, and the bar A B is drawn forwards into the position a', and the tension t becomes perpendicular to the two bars. On the other hand, reverse the experiment, as in fig. 691.; supposing c D and the perpendicular body c A fixed, withdraw the pin at A, and the bar A is is drawn backwards to a'. This presupposes the bars kept apart, Now we shall suppose the bars at an angle of 90° to the body representing the spine.
A perpendicular tension (L o, fig. 694.) ad mits, of course, of no variation ; oblique ten sions admit of two variations : The perpendicular tension (L o, fig. 694.) produces but one effect, that of approxi mating the two bars A C and B D, because the force of L o is acting upon A L and n o, levers of the same length, their movements being the same they would meet in the middle dis tance at s. But if the bars are kept parallel by a rigid link like s, fig. 687. the perpendicular ten sion would produce no apparent effect upon the two bars. They might be rotated in any direc tion, and the tension would remain of the same length ; for example, in fig. 695. let t 2 be the. perpendicular tension between the bars A a C n, move the bars to s or s', and the ten sion is the same length. k k k, &c., may represent different places in the rotation, at each of which the tension t or k is the same length, although the bars at s, t 2, and s' are at different perpendicular distances from each other. A rigid connective, as wood or wire, may be substituted for the tension, an this will equally allow of the bars being rod tated, and consequently changing their per pendicular distances to each other. Hence it will be seen, that each of the lines k k k, are of the same length, although the two se micircular lines describing the revolution of the bars are constantly changing in their re lative distance to each othe We then seer.
the possibility of having a rigid body connect ing two bars, which shall nevertheless recede and approximate each other. From this we may gather, that though the sternum is rigid,. and the cartilages, perhaps, ossified, the ribs may nevertheless maintain the capability of altering the breadth of their intercostal spaces. Perpendicular tension, therefore, like L o,' (parallel to A B,) cannot rotate the bars, cause they never change their length.
All tensions are oblique, which have one of their attachments nearer to the spine than the other, therefore, in fig. 694., L K and L T are oblique tensions. An oblique tension, hence, is acting on bars at dissimilar distances from their fulcra ; thus in fig. 696., tension t' is oblique to the line a A, and the points on the lines a B, A D, to which the tension is attached, is represented by the lines a m and A M. And the law of action of such tension is, that it tends to move both bars or ribs towards that fulcrum which is near est to one of its attachments. Therefore ten sion L it (fig. 694) would rotate the bars. towards B, and tension L m towards A. The force of a given oblique tension between such, bars is modified by two circumstances,—by the degree of obliquity, and by the obliquity of the bars in reference to the body which re presents the spine.
Of the degree of obliquity of a tension. —Let fig. 697. A B, C D, represent bars as before, the different connecting lines tensions of different degrees of obliquity, but of the same power of tension. L it is perpendicular, and has rotating power. L possesses a certain_ amount of power, L 10 more power, and L the maximum power, or the power of rotating greater, than at any other intermediate posi tion of the ribs, at all of which the muscular power actually exerted is greater. All these remarks apply equally if the spine be curved ; for change of obliquity of the ribs, or change of curvature of the spine to the ribs, is the same thing.