56 Anatomy of Skeleton 



sternum on one or both sides, more frequently on the right. The increasing obliquity 

 of the cartilages downwards only extends to the tenth : the last two not having 

 indirect connection with the sternum are not turned up but are sharp-pointed tips 

 embedded in the muscles of the body wall. 



Interchondral and chondro-sternal articulations are secured by anterior and 

 posterior accessory bands on the capsules of the joints : in the sternal joints these 

 radiate on to the corresponding bony surfaces. The anterior bands are stronger. 

 Synovial cavities are in all the interchondral and in most of the chondro-sternal joints 

 except the first : double in the second and occasionally in the fourth and third. The 

 connection with the ribs is by continuity of covering membrane and some amount of 

 fixation between the cartilage and bone : a synovial cavity has been occasionally 

 found in the first costo-chondral joint, but not in any of the others. 



The cartilages increase in length from the first to the seventh and then decrease : 

 their thickness is in general greater as they are followed upwards ; they become 

 somewhat smaller as they approach the sternum. 



The cartilages afford attachment above and below to the Internal Intercostal 

 muscles and anterior intercostal membranes, which fill up the interchondral intervals. 



The deep aspect of the true cartilages, from the second down, is covered by 

 Triangularis sterni, which arises from them, and crossed by the internal mammary 

 vessels : that of the first cartilage (and part of the second and third) is covered by 

 pleura and crossed by the vessels. In the case of the seventh cartilage the vessels are 

 the superior epigastric, and the outer part of the cartilage, outside the area of Triangu- 

 laris, is, like the remaining cartilages, taken up (see Fig. 46) by the alternate origin of 

 digitations of Diaphragm and Transversalis : these cartilages that form the costal arch 

 are crossed on their inner side, between the digitations, by intercostal nerves and 

 vessels from musculo-phrenic running into the abdominal wall. 



The lower edges of the false cartilages, where these edges form the costal arch, 

 receive the insertion of the upper fibres of the Internal Oblique, and where the cartilages 

 are not joined, in the last two spaces, the fibres of this muscle becorne directly continuous 

 with those of the Internal Intercostals. 



The front surfaces of the true cartilages are covered by Pectoralis major, which 

 also arises from them, with the occasional exception of the first and seventh. Outside 

 and below the pectoral area, the Rectus abdominis covers and is attached to the fifth, 

 sixth and seventh cartilages, and may even extend higher : thus its upper part may 

 come under cover of the pectoral muscle, which gains some origin from its sheath. 



In this position the Rectus lies immediately on the cartilage, but when it comes 

 down over the costal arch it lies on the aponeurosis of Internal Oblique inserted into 

 the border of the arch, and this aponeurosis separates it from muscular fibres of Trans- 

 versalis arising from the deep aspects of the cartilages. 



Outside the part covered by Rectus the cartilages of the false ribs are covered by 

 External Oblique, but do not give origin to any of its fibres (see Fig. 39). 



The first costal cartilage requires some further notice. Its upper surface has an 

 articular surface for the clavicle, on the inner margin of which the inter-articular 

 nbro-cartilage is attached. Outside and rather behind the joint is the rhomboid 

 ligament : in front of this the cartilage is covered by Pectoralis major and, deep to 

 this muscle, gives part origin to Subclavius. The subclavian vein lies behind the 

 ligament and is an upper and back relation of the cartilage to a small extent, and it is 

 below the vein that the pleura and internal mammary artery come against the cartilage. 

 The Sterno- thyroid muscle arises from the inner end of the cartilage (see Fig. 47), so 

 that this part has no pleura in relation with it, and the deep layer of the omohyoid 



