4 8 



Anatomy of Skeleton 



usually there are additional interchondral articulations between the fifth, sixth and 

 seventh cartilages. The cartilages run at first in the direction of the front ends of 

 their ribs, so that, from the fourth down, they go at first downwards and then come up 

 with an increasing curve, towards the sternum : the third and second are nearly 

 horizontal, and the first is directed downwards as well as inwards. 



The second is most prominent anteriorly, and can be recognised at once in the 

 living subject : the succeeding cartilages can be counted down from this. 



Having become familiar with the names and positions of the various parts and with 

 the general shape of the ribs, it is as well to consider them as a whole in the articulated 

 skeleton before proceeding to study the details of the individual bones. 



Looking at them in position, it at once becomes evident that the ribs are placed 



obliquely in the wall of the thorax, and there is a 

 slight increase in their obliquity from above down- 

 wards, so that the intercostal spaces are somewhat 

 wider in front than behind : the upper spaces are 

 also rather wider than the lower ones. The bony 

 thorax, the greater part of the walls of which is made 

 by the ribs, is somewhat barrel-shaped, flattened 

 from before backwards, and broader below than 

 above. There is an evident convexity from above 

 downwards in the line of its side wall, and this line 

 turns in markedly at the top. Fig. 38 shows the 

 effect of this form of the thorax on the direction in 

 which corresponding surfaces of the ribs are turned : 

 it is seen that the highest part of the curve is about 

 the ninth rib, and that the line, turning in below this, 

 causes the pleural surface of the lower ribs to look 

 upwards as well as inwards, while above the ninth 

 they look down. This downward aspect of the pleural 

 surface reaches its acme in the upper two ribs, owing 

 to the increased curve in the line, and on the first rib 

 the pleural surface looks almost directly downwards. 

 But it is clear, notwithstanding, that the lower sur- 

 face of the first rib corresponds with the inner (and 

 upper) surface of the last rib. 



FIG. 38. Scheme to show the effect 

 of the shape of the thorax on the 

 direction of the surfaces of the 

 ribs. 



The antero-posterior compression of the thorax is a 

 human character, a greater comparative depth being found 



in lower animals and in the foetus.* The adult measurement between the lower end of the sternum 

 and the vertebral column is about 8 inches, and at the top of the sternum about 2 2 J inches, while 

 the broadest part of the cavity, about the level of the ninth rib, is about 1 1 inches. Owing to the 

 anterior deficiency that forms the costal arch or subcostal angle, the length of the posterior part of 

 the cavity (12 13 inches) is about twice as long as its vertebral measurement in front. 



These last measurements, with the obliquity of the ribs, account for the upper aperture, bounded 

 by the first ribs and dorsal vertebra and top of manubrium, sloping downward, while the large lower 



* Like other recently acquired characters, this shallowness of the thorax is variable. It can be expressed 



depth x 100 

 by an index brea( ^ tll (at the nipple level). The normal average is about 71, but varies upwards to 80. 



or even 90 or more. These high indices are frequently if not usually in tuberculous chests, and may represent 

 an arrest in evolution of the human thorax : the evolution goes on during ontogeny, the index being con- 

 siderably over 100 before birth, decreasing to about 80 at puberty, and then reaching the normal level in 

 the next eight to ten years. 



