THORAX. 



1029 



As the chord length is to the antero-posterior 

 movement, so is this measurement to the 

 lateral movement of the thorax. This is the 

 most uniform measurement in the ribs. If the 

 1st rib be T75, the 2nd rib is to that as 3; 

 and from this rib to the 9th rib, the versed 

 sine never exceeds 3'5 ; hence the curve c, 

 Jig. 665. is more horizontal than any of the 

 other curves. So likewise it will be found 

 that the lateral enlargement of the thorax in 

 deep breathing is more uniform than the 

 antero-posterior enlargement. (See dotted 

 line, jti*s. 71 1, 712. compared with dotted line, 

 Jigs. 713, 714.) The space gained by the ribs 

 rotating must he strictly in relation to the 

 length between the deepest part of the arch 

 and the chord line. (D c,Jig. 664.) This mea- 

 surement is greatest relatively in the 2nd rib, 

 and absolutely smallest in the 12th rib, the 

 curve here being very small. 



The versed sine of the ribs corresponds 

 with the great curve: if a rib were not curved 

 at all, there would be no versed sine. The 

 versed sine does not increase after the 4th 

 rib; and the curvature of the first four ribs 

 forms smaller circles than the rest. The 5th 

 and 6th ribs, although rapidly increasing in 

 absolute length (see Table A.), yet present 

 nearly the same versed sine; and while the 

 chord line increases up to the 9th rib, from 

 the 6th to the 9th the versed sine decreases, 

 showing that the circle or arch of the rib 

 becomes larger as we descend, until the 

 12th rib, which describes the greatest curve 

 and the shortest versed sine. 



In fact there is little difference in the versed 

 sine length from the 3rd to the 10th rib, as 

 described by the double line c,fig. 665. This 

 difference of arching of the ribs constitutes the 

 conical form of the thorax, the smaller circles 

 being at the apex, and the larger at the base 

 of the cavity. 



(2) Weight. The ribs increase not only in 

 their various measurements but also in their 

 weight up to the 7th or 8th where they attain 

 their maximum development. The faint con- 

 tinuous curve, e, Jig. 665. is the line of the 

 relative weight. 



(3) Torsion of the ribs (special characters'). 

 We have already mentioned that the 

 ribs have two torsions, an anterior and a pos- 

 rerior. No rib is entirely free from this twist. 

 It is incorrect to believe that the 1st rib is 

 without any torsion, and therefore that the 

 whole rib in its length can touch the same 

 plane. In fact the 1st rib may in one sense 

 be looked upon as the most twisted of all 

 the set, inasmuch as the flat sides which are 

 internal and external in other ribs are in this 

 rib inferior and superior. 



Like other features, the degree of torsion 

 in the different ribs is progressive towards a 

 maximum, and then towards a minimum. The 

 2nd, llth, and 12th ribs are most devoid of 

 torsion. In the last two ribs the torsion ap- 

 pears less than it really is, because of their 

 shortness and the large circle which they de- 

 scribe. 



The two torsions in each rib are always in 



contrary directions, except in the 2nd rib, 

 where they are both downwards, but the tor- 

 sion is here very slight. In the 1st rib, as in 

 the ten inferior ribs, they are in contrary di- 

 rections to each other, so that its two ends 

 cannot touch the same plane at the same time ; 

 but its anterior and posterior torsions are 

 respectively contrary to those of the ten in- 

 ferior ribs ; its posterior torsion being, like 

 that of the second rib, downwards, and its an- 

 terior upwards. 



The relative torsion of the ribs maybe ex- 

 pressed by giving the respective elevations of 

 each extremity from the plane upon which 

 they rest. Thus the posterior torsion is seen 

 when the rib rests upon its inferior edge, and 

 the anterior torsion when it rests upon its su- 

 perior edge. In this way the following table 

 is calculated. 



TABLE B. Torsions of the Ribs. 



If the posterior twist of the ribs upwards be 

 expressed conjointly as 114, the anterior tor- 

 sion downwards is 153 ; therefore the anterior 

 torsion is greater than the posterior. The 

 torsions in the 2nd, 3rd, llth, and 12th 

 are the least ; and they are the greatest in the 

 7th, 8th, and 9th ribs. Moreover as the tor- 

 sion increases at one extremity of the rib, for 

 the most part it increases at the other like- 

 wise. 



The ribs of some persons are much less 

 twisted than those of others. This is parti- 

 cularly the case in young subjects before the 

 age of puberty. In infants the torsions are 

 imperceptible ; therefore they increase as we 

 advance to maturity. These torsions afford 

 more favourable traction for the muscles. 

 Where the respiratory movement is most ap- 

 parent, as from the 4th to the 9th rib, the 

 torsion is highest. 



t The posterior torsion gives the ribs, when 

 placed in their natural position, aver}' oblique 

 direction with reference to the spine. This is 

 very important to remember; for the more 

 oblique, the more favourable are they for mo- 

 bility or for increasing the thoracic cavity. 

 For the same reason the anterior torsion, being 

 in a contrary direction to the posterior, increases 

 still more the obliquity of the rib with refer- 



* Downwards. f Upwards. 



3 u 3 



