1054. 



THORAX. 



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. 



Fig. 698. 



In deep inspiration it will be found that the 

 ribs increase the breadth of their intercostal 

 spaces (as was mentioned years ago by Ham- 

 berger) ; and that by the contraction of an 

 intercostal fibre. The bars (fig. 698.) in ro- 

 tating, twice attain a minimum, and once a 

 maximum, distance from each other; while 

 the oblique tension in that revolution once 

 attains its maximum, and once its minimum 

 length. 



Of tensions at different parts of the bars or 

 ribs. Parallel tensions of equal power pro- 

 duce the same effect, whether near the ful- 

 crum or more distant from it ; an intercostal 

 fibre near the vertebrae, has the same power 

 as a fibre near the anterior extremity of the 

 ribs. 



Let Fig. 696. represent ribs as before, with 

 two parallel tensions at different distances 

 from the bod}', E E, then 



t'+A-M t' +am=t'+x'(A.w am)=t'+A o 

 t + A N t-\-a ?z=t+(A N a n) = t + A O. 

 Therefore the tension L T (fig. 69i.) acts with 



the same power as tensions p p', which is 

 much nearer to the fulcra A B. 



Knowing now the effect of a single oblique 

 tension, it is easy to consider an indefinite 

 number of tensions, for they follow the same 

 law of action. 



lnfig.699. the tensions x acting on A c, /j/i 



Fig. 699. 



the bars to m', and, by the same reason, ten- 

 tions observing a contrary direction and acting 

 upon A' c' lift the bars to m'". Therefore 

 tensions, although observing contrary direc- 

 tions, may be made to conspire to the same 

 action, and may therefore be associates when 

 acting upon different fulcra. We have shown 

 that tensions in contrary directions, but acting 

 upon the same fulcra, are antagonists. If we 

 join the levers ( fig. 699.) and increase their 

 number, we may represent the thorax as in 

 fig. 700., kk representing the spine, b b the 

 sternum, the bars a a the ribs, and the bars 

 a' of the costal cartilages united to the bars 

 representing ribs by a movable joint ; let r 

 represent the external intercostals, these, we 

 know, will act as elevators, while those at h, 

 representing the internal intercostals, are asso- 

 ciates in action, although they observe a con- 

 trary direction, because they act upon the 

 fulcra in b b ; in fact, they are elevators of 

 the levers representing the cartilages. 



What now is the combined action of a 

 series of two such tensions ? The whole body 

 of levers will be raised, and the part b b re- 

 presenting the sternum will have two motions : 

 it will be raised and moved forwards into the 

 position of b' b'. This is precisely the mo- 

 tion of the sternum in deep inspiration. 

 In a model of this kind, certain means 

 must be used to limit the motion, or the 

 movement is continued until the tensions 

 are at rest. Or, if the bars representing the 

 ribs a a (fig. 700.) be fixed, then the tensions 

 representing the internal intercostals h would 

 depress the short bars representing the car- 

 tilages, because b b is free, and k k is fixed. 

 And were either set of tensions continued 

 over the joints representing the union of the 

 cartilages with the ribs, such fibres would 



