THORAX. 



1051 



Fig. 688. 



K to L in order to produce this motion : half 

 of it might be wood, bone, or iron, provided 

 the other half retained its elastic power. The 

 effect would be the same, and the bar A B at N 

 would be elevated by the tension between T L, 



Fig. 689. 



connecting the fixed point K with L by the 

 rigid body K T. It is the omission of the 

 fulcrum K, 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 D and A 

 c be fixed ; withdraw the pin at A, and the bar 

 A B is drawn fonvards into the position B', and 

 the tension t becomes perpendicular to the 

 two bars. On the other hand, reverse the 

 experiment, as in Jig. 691.; supposing c D and 

 the perpendicular body c A fixed, withdraw the 

 pin at A, and the bar A B is drawn backwards 

 to B'. This presupposes the bars kept apart, 



Fig. 690. 



B' 



otherwise the free bar would approximate the 

 fixed bare D. Therefore, one fulcrum is pushed 

 upon by one bar, and pulled upon by the other. 

 If the bars were kept fixed, and the body re- 

 presenting the spine was left free, the tension 

 would draw this last mentioned body into the 



Fis. 691. 



t 



B; 



position of c c and c f c' fig. 678. Therefore, the 

 element of the two fulcra is the chief agent for 

 directing their upward or downward move- 

 ment, under an oblique tension. If we arrange 

 two bars with one fulcrum {fig. 692.), and allow 



Fig. 692. 



a 







the tension to act as before, then the effect 

 is only to draw the two bars together, as o b 

 and o' d(fig. 693.). If we have an arrange- 

 ment to substitute two fulcra at a a' fig. 692. 

 and withdraw the centre fulcrum, then the 

 two bars rise as before. 



Fig. 693. 



Now we shall suppose the bars at an 

 angle of 90 to the body representing the spine. 



A perpendicular tension (L o,fig. 69-t.) ad- 

 mits, of course, of no variation ; oblique ten- 

 sions admit of two variations : 



