THORAX. 



1031 



set of ribs, the necks are the thickest and 

 strongest. In the llth and 12th ribs, the neck, 

 according to our definition, does not exist. 



3rd Of the tubercle. As the tubercle ar- 

 ticulates with transverse processes, those ribs 

 which have no such articulation have no tu- 

 bercle ; this is the case in the llth and 12th 

 ribs. The tubercles are most prominent in the 

 superior ribs, gradually degenerating, or be- 

 coming less apparent, down to the I Oth, 

 where it is almost rudimentary. 



4th Of the angle. In strict anatomical lan- 

 guage, the 1st rib has no angle, but the tu- 

 bercle is very prominent, and gives the bone a 

 very angular appearance, likewise in the 2nd, 

 3rd, llth, and 12th ribs the angle is almost 

 imperceptible, whereas it is well marked in 

 the 4th, 5th, 6th, 7th, 8th and Oth ribs. 



(6) Groove (specific differences}. The 

 groove is not perceptible in the 1st, llth, 

 and 12th ribs, but distinct in all the inter- 

 vening ones. 



The 1st rib has two depressions separated 

 by a tuberosity. The anterior corresponds to 

 the subclavian vein, and the posterior to the 

 artery of the same name. 



Costal cartilages. The flexibility and elas- 

 ticity of the ribs is partly owing to their struc- 

 ture, but more especially to the cartilages 

 which prolong them in front, (figs. 661, 662.) 

 There are twelve costal cartilages distinguished 

 numerically, as 1st, 2nd, 3rd, &c. ; they are 

 separated from each other by intervals which 

 are very considerable at the upper part of 

 the thorax, but gradually diminish as we 

 proceed downwards. It is not very un- 

 common to meet with thirteen cartilages on 

 one side, and at other times there are only 

 eleven. Sometimes two cartilages are joined, 

 and so aticulate with the sides of the ster- 

 num ; when there are thirteen cartilages, 

 the supernumerary one generally exists be- 

 tween the 3rd and 4th ribs ; it is thin, and as 

 it were rudimentary ; it does not form the 

 continuation of any rib, and terminates insen- 

 sibly in the muscles. The cartilages from the 

 1st to the 7th articulate immediately with the 

 sternum ; and hence the name of sternal 

 given to ribs with which they are connected. 

 Of the other five cartilages, the last two have 

 no connection with that preceding them ; and 

 from this circumstance, the name floating has 

 been given to the last two ribs. 



General characters oj the costal cartilages. 

 All the costal cartilages are flattened like 

 the ribs, and precisely ^resemble in breadth 

 and thickness the bones to which they are 

 attached. The external end is received into 

 a cavity hollowed out in the anterior extre- 

 mity of the rib; their internal or sternal ex- 

 tremity, which is much narrower than the 

 external, is angular and articulates with the 

 corresponding angular surfaces of the ster- 

 num, (fig- 661.) Their anterior or cutaneous 

 surfaces are slightly convex, and covered by 

 the muscles of the anterior region of the 

 trunk, to many of which they give attach- 

 ment. Their posterior or mediastinal surfaces 

 are slightly concave. Their superior and 



inferior edges bound the intercostal spaces, and 

 give attachment to the intercostal muscles. 

 " They are," says Cruveilhier, " altogether 

 distinct from articular cartilages, and have a 

 peculiar tendency to ossify, this process 

 taking place partly on the surface, and partly 

 from within outwards." 



Differential characters of the costal carti- 

 lages. The costal cartilages, like the ribs, 

 increase in length, from the 1st to the 7th, 

 and sometimes the 8th, which in this case 

 articulates with the sternum ; from this they 

 diminish in length to the 12th rib When 

 we recollect the conical shape of the thorax 

 this difference is to be accounted for, and 

 moreover, the osseous parts of the upper ribs 

 terminate anteriorly in a line directed ob- 

 liquely from above downwards and outwards. 

 The sternum is only about half the length of 

 the lateral pectoral space, so that only the 

 first four or five cartilages could join this 

 bone, did not the others turn upwards to 

 reach its sides (fig. 661.), by joining the lower 

 edge of the immediately superior cartilage. 

 The first three cartilages alone, therefore, fol- 

 low the same direction as the long rib to 

 which they articulate. 



The first cartilage differs from all the others 

 by its shortness, its thickness, and breadth, 

 and its tendency to ossify ; it is often, but 

 not always, continuous with the sternum. 



The 2nd and 3rd costal cartilages cannot 

 be distinguished from each other, but they 

 differ from the rest in being joined to the 

 sternum at right angles, in not being bent, and 

 in being as broad at their sternal as at their 

 costal extremities. 



The 4th cartilage becomes bent upwards, 

 after having followed the direction of the rib 

 for a little way. (fig. 661. e.) The length and 

 the curvature of the cartilages of the 5th, 

 6th, and 7th ribs progressively increase, and 

 across their intercartilaginous spaces they fre- 

 quently touch each other. Their inner ends 

 become successively narrowed so as to cor- 

 respond with the diminishing cavities on the 

 edges of the sternum. The borders of the 

 5th, 6th, 7th, and 8th costal cartilages ar- 

 ticulate together, and present for this pur- 

 pose articular facets supported by emi- 

 nences. The cartilages of the 8th, 9th, and 

 10th ribs gradually diminish in length. Ex- 

 ternally they have the same breadth as the 

 rib, and decrease as they pass inwards, so as 

 to terminate by a pointed extremity, which is 

 applied to the lower edge of the cartilage 

 above. The cartilages of the llth and 12th 

 ribs are extremely short, particularly that of 

 the 12th, which is only a mere tip to the 

 bone, and seldom exceeds four or five lines 

 in length ; their free extremity loses itself, so 

 to speak, in the substance of the abdominal 

 parietes ; so that they are altogether uncon- 

 nected with the other cartilages. 



Liability of the costal cartilages to ossify. 

 The 1st cartilage usually becomes more or 

 less ossified in adult age, and is often anchy- 

 losed to the sternum. After the middle period 

 of life, osseous matter is likewise deposited 

 3 u 4 



