the bars increases with the increasing obliquity of these tensions. By experiment we found that equal tensions at the following angles, pro duced the following difference of power in rotating the bars : — The power descreases as the tension ap proaches the perpendicular L te, and increases as it approaches L A ; this is the maximum point : if the tension be attached to the body E E, either above A or below it, the system is changed into that of a single lever. From this we gather, that the power of an intercostal muscle, as an elevator and depressor of the ribs, increases with its obliquity ; and that this movement entirely depends upon its obliquity. This is the only instance in the body where the power of a muscle increases with its obliquity.
Of the obliquity of the ribs or bars with re ference to the spine.— A given tension, say at the angle of 45°, will, when the lever is at to the spine act more powerfully than when the lever is at au angle similar to that of the ribs. We found by experiment, that the bars in the following positions required an increased power to sustain them. The tension being uniform, and the resistance to be overcome acting from the same point.
This gain of power is dependent upon the obliquity of the bars and change in the direc tion of the tension, for in each of these posi tions the tension was maintained the same. But if the tension be not kept uniform, still the resistance is increased as the bars rise ; thus, if the bars are at the angle of 50° (fig. 699. ni") somewhat similar to the position of the ribs, and these under a certain tension allowed to resist a power of 4, when they are moved upwards to 90°, through which revolution the tension has kept diminishing (because it has kept shortening), it will resist a power of 5. In this we see a beautiful compensation to the muscular contraction, viz. that while an in tercostal muscle is losing power as it con tracts, this loss is made up by the change in position of the ribs. We feel conscious that we can exert a retaining power at the ter mination of a deep inspiration as great, if not Of oblique tensions in contrary directions. —W have shown that an oblique tension between parallel levers moves them in a cer tain course. Now it is evident that tension in a contrary direction (all other things remain ing the same) must likewise move such levers in a contrary direction. This is so clear that
although Haller asserted absolutely that cross ing muscles have the same action, yet he was not comfortable under such an opinion, "for,' says he, " why do they cross ?" We have shown that a tension in the di rection of L T fig. 694., will raise the bars, and one in the direction of L lc will depress them; they are, therefore, antagonistic forces, and when the tensions are similar, they pro duce an equilibrium of contrary force. If the bars A B, C D, (fig. 676.) be rotated, the lines will have directions contrary to each other, and will lengthen and shorten inversely to each other. Thus of the crossing tensions (fig. 676.) v n and v' B, B becomes short ened to n b', and v n, on the contrary, becomes lengthened to n b' ; and on the other side of 90°, v n is shortened to s d, and V' B lengthened to s' b. Therefore muscles circum stanced like the intercostals and crossing each other, or observing contrary directions to each other, cannot be associates in action, for when one contracts, the other must relax, as the ribs move. We represent this more clearly in fig. 698. where A B and C D represent bars as before, rotating upon E, E, and t t' two ten sions in contrary directions. As the bars are raised towards m, t' lengthens, and if depressed towards m' it shortens, while t lengthens to wards n, and shortens towards n'. We re present the tensions t t, &c., and t', &c., by the white lines, in the different positions they would assume if the two bars were rotated to those places in the half cir cle. It will likewise be observed, that while either tension gradually lengthens :or shortens, the two bars pass through their maximum perpendicular distance from each other at B D, on either side of which they attain their minimum distance. Therefore, if we examine tension t at n', the bars would there be closer to each other than they are at B D ; nevertheless, this tension in ascending must contract while the bars are increasing their perpendicular distances as they move to B D ; beyond which the tension still shortens, while the bars now decrease their perpendicular distance. Therefore it is per fectly compatible for an intercostal fibre to separate the two ribs, between which it is at tached, by its contracting ; and, if above a certain point (90° to the spine), to approxi mate the same ribs by its further contraction.