THORAX. 



1053 



Fig. 697. 



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 : 



TABLE H. Power gained by a given Tension, 

 as an intercostal Muscle, in relation to its 

 Obliquity. 



Resistance. 

 



1-50 

 2-25 

 4-25 

 5-50 



The power descreases as the tension ap- 

 proaches the perpendicular L K', 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 

 90 to the spine act more powerfully than 

 when the lever is at an 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. i") 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 



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. 



TABLE I. Change of Power, from the Obli- 

 quity of Bars or Ribs to the Spine. 



Angles of the Bars with the 



Body representing the Spine. Tension. Resistance. 



30 2'5 4 



60 2-5 12 



90 2-5 22 



120 2-5 33 



Of oblique tensions in contrary directions. 

 We 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 K 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 D and v' B, v' B becomes short- 

 ened to n b', and v D, on the contrary, becomes 

 lengthened to n b' ; and on the other side 

 of 90, v D is shortened to * 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 in' it shortens, while t lengthens to- 

 wards n, and shortens towards n'. We re- 

 present the tensions t t, &c., and t' 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 



