1052 



THORAX. 



a, obliquity in contrary directions. 



b y degrees of obliquity in each direction. 



Fig. 694. 



AO 



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 B 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 ^.695. let t 2 be the 

 perpendicular tension between the bars A B 

 c D, move the bars to s or s', and the ten- 



Fig. 695. 



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, be-' 

 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 Jig. 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 Jig. 696., 



Fig. 696. 



Diagram shewing that perpendicular fibres never 

 /alter their length. 



tension if is oblique to the line a A, and 

 the points on the lines as, AD, to which 

 the tension t f 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 K (Jig. 694.) would rotate the bars 

 towards B, and tension L T 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 K is perpendicular, and has no 

 rotating power. L K 1 possesses a certain 

 amount of power, L K 2 more power, and L A 

 the maximum power, or the power of rotating 



