THORAX. 



1027 



the 7th rib and place it on a table it will be 

 observed that the extremities cannot simul- 

 taneously rest upon one plane, because it is 

 twisted upon itself. This is due to what is 

 called the curve of torsion. The rib is twisted 

 at each end ; hence a posterior and an ante- 

 rior torsion. The posterior torsion is most 

 conspicuous, and is therefore more commonly 

 noticed. 



() Posterior torsion. This torsion is 

 marked at the angle of the rib (h,fg. 663.), 

 particularly upon the convex surface, by an 

 oblique line or a series of faint lines directed 

 from above downwards and forwards. Like 

 other features of the ribs, it passes through 

 gradations, being scarcely perceptible in 

 the 2nd rib, more so in the 3rd, and in- 

 creasing to the maximum in the 8th, in the 

 9th and 10th it quickly decreases, until it is 

 lost in the llth and 12th. The greater the 

 torsion the more distinct is the angle ; where 

 the angle is rudimentary the torsion is imper- 

 ceptible, as in the llth and 12th ribs. 



The degrees of this torsion as it passes 

 through the ribs are most distinctly seen by 

 having the ribs separate, and placing them on 

 a flat surface with their superior edge upper- 

 most, arranging them so that they do not quite 

 touch each other, when the heads of the dif- 

 ferent ribs will stand up at different heights 

 from the table, forming a somewhat regular 

 wave. This is solely produced by this posterior 

 torsion. 



(1)) Anterior torsion. "Near the anterior 

 extremity, on the convex surface, in well- 

 developed ribs, we observe an oblique line 

 analogous to that at the angle of the rib, but 

 much less distinctly marked. This line may 

 be considered as forming the anterior angle 

 and corresponding torsion of the ribs, which, 

 like the posterior, is intended for more favour- 

 able muscular insertions. Although the 

 anterior angle is comparatively feebly marked, 

 the anterior torsion of the ribs is as well 

 defined as the posterior torsion. It will 

 be seen in jig. 6G7, that while the posterior 

 extremity of the rib curls upwards, the an- 

 terior extremity curls downwards. In like 

 manner as we notice the posterior torsion 

 by placing the separate ribs on a table, 

 so may the anterior torsion be as strikingly 

 seen, taking care to place the ribs upon their 

 superior edge so that the sharp inferior edge 

 is turned upwards. The anterior ends will 

 be seen to stand up in different degrees from 

 the table according to their torsion, commenc- 

 ing with the 3rd or 4th and terminating with 

 the 10th rib. 



Articulations of the ribs. The ribs are 

 articulated behind with the dorsal vertebras, 

 and in front with the sternum through the 

 medium of the costal cartilages. This has 

 alreadj' been noticed under Extremities of 

 the ribs. 



Position of the ribs. The ribs are arranged 

 more or less obliquely, about midway 

 between the perpendicular and horizontal 

 (Jigs. 662. and 083.) ; and they somewhat 



diverge from each other as they approach the 

 sternum. (Jigs. 662. 680. and 681.) Not one 

 of them is horizontal, though commonly re- 

 presented as if they were. Their position is 

 given iiy%. 662. This is important to remem- 

 ber, because we shall see that costal breathing 

 altogether depends upon their obliquity. 



Structure. The compact and spongy sub- 

 stances are so distributed throughout the 

 whole length of the ribs that they possess 

 a certain degree of flexibility, with great 

 power of resistance. In young subjects the 

 compact substance is in excess ; in the aged, 

 and in certain diseases, the opposite is the 

 case ; hence the extreme fragility of these 

 bones, which are then broken by the least 

 effort. 



Development. The ribs are amongst the 

 earliest developed bones, ossification com- 

 mencing in them even somewhat before it 

 has made its appearance in the vertebrae. 

 The deposition of osseous matter extends 

 rapidly throughout them. Each rib (with 

 exceptions to be noticed) is formed from 

 one principal piece ; and two epiphyses. Of 

 these two epiphyses, one forms the head of 

 the rib, and the other the tubercle : their 

 ossification commences between the sixteenth 

 and twentieth years of age ; and they are united 

 to the rest of the bone a few years after, 

 about the twenty-fifth year. 



II. Special characters of different ribs. The 

 differential characters of the ribs, when mi- 

 nutely examined, are well marked ; for, strictly 

 speaking, no two ribs on the same side are of 

 the same shape and dimensions. Although 

 the difference is very small between two con- 

 tiguous ribs, as, for instance, between the cen- 

 tral ribs, i. e. between the 6th, 7th, and 8th, 

 yet it is very conspicuous between those of 

 the top compared with those of the bottom of 

 the thorax. Whatever be the peculiarity under 

 examination, we find it most developed in 

 the 6th or 7th rib ; and below this it 

 becomes less and less marked, until, in the 

 12th rib, it appears rudimentary. In fact, the 

 12th rib may be considered little more than a 

 prolonged transverse process ; but not so 

 with the 1st rib, which possesses all the marks 

 and uses necessary to the character of a rib. 



The ribs differ in their length, and in their 

 chord and versed-sine measurements, and 

 consequently in the area of thorax which 

 they encompass. The thoracic dimensions 

 vary considerably in different men and in the 

 two sexes ; yet the relative measurements and 

 weight of the ribs will be found useful to 

 our comprehending more perfectly the res- 

 piratory movements. 



These relative measurements are from a 

 well-formed male thorax. The area-measure- 

 ment is calculated from an internal cast of 

 the thorax, cut up slice by slice through 

 each intercostal space. These slices were 

 traced upon paper and measured, giving the 

 absolute area of thoracic cavity encompassed 

 by each pair of ribs, their cartilages, and the 

 sternum. (See^/zg. 668.) 



3 u 2 



