i6o HUMAN ANATOMY. 



' strengthening bands require no further description. A cavity is often found in the 

 cartilage, making a typical half-joint. At what time it appears is unknown. Some- 

 times it is so developed that the joint is practically a true one, with articular carti- 

 lage ; this exceptional arrangement is more common in women than in men, being 

 especially adapted to the female type of respiration. The cartilage persisting 

 between body and ensiform is strengthened in a similar manner. A cavity rarely 

 occurs in the cartilage, which, on the contrary, often undergoes ossification. 



THE COSTO-STERNAL JOINTS. 



The first costal cartilage joins directly, without interruption, the lateral expan- 

 sion of the sternum ; the following costal cartilages articulate at the points already 

 mentioned by synovial joints. Those that come between different sternebrae — that 

 is, from the second to the fifth — often have the joint subdivided by a band into an 

 upper and a lower half. This is usual in the joint of the second cartilage ; progres- 

 sively rare as we descend. The sixth and seventh cartilages frequently have no 

 true joint.' Each of these joints is enclosed by a capsule, the front and back fibres 

 of which radiate over the sternum. 



THE INTERCHONDRAL JOINTS. 



The seventh, eighth, ninth, and tenth costal cartilages have each an articulation 

 by a true joint on the projections above described with the one above it. There is 

 a connection between the fifth and sixth cartilages ; usually on the right, very 

 frequently on the left.^ This is, as a rule, also a true joint, but the cartilages may 

 be merely bound together by bands of fibres. The joint on the right side is almost 

 always a true one. The ends of the eighth, ninth, and tenth cartilages are joined 

 by fibrous tissue to the cartilage above. 



The costo-xiphoid ligament is a band extending from either side of the base 

 of the ensiform to the lower border and, perhaps, the front of the seventh cartilage 

 near its end. 



THE COSTO-VERTEBRAL ARTICULATIONS. 



The joints between the ribs and the spine are in two series : an inner, or 

 Costo- Central, between the heads of the ribs and the bodies of the vertebrae ; an 

 outer, or Costo- Transverse, between the tubercles and the trayisverse processes. 



The Costo-Central Joints. — The head of the rib is received in a hollow 

 articular fossa formed by a part of two bodies and the disk between them. Although 

 as a whole concave, it may in a typical case be further analyzed. The lower half of 

 the socket is convex from above downward, fitting into the hollow at the lower part 

 of the joint of the rib ; the upper part is about plane, looking downward and out- 

 ward, with the upper border considerably overhanging the joint. These two facets 

 have each a synovial capsule and are separated by an interarticular ligament,^ a 

 band running from the ridge on the head of the rib to the posterior part of the inter- 

 vertebral disk. In the foetus before term it extends across the back of the disk to 

 the head of the opposite rib. 



The front of the capsules is strengthened by the anterior costo-vertebral ligameyit,*' 

 which is a series of radiating fibres from the head to both vertebrae and the interven- 

 ing disk, not clearly separable into three bands. These stellate ligaments (Fig. 184) 

 are least developed in the upper part of the thorax. The strongest collection of 

 fibres is to the lower vertebra. The joint of the first and last two ribs is not sub- 

 divided ; that of the tenth is uncertain. Strong fibres pass from the head of the 

 first rib to the seventh cervical vertebra. Few or no fibres from the last rib reach 

 the body of the eleventh thoracic. The lower fibres are made tense when the rib is 

 raised and the upper when it is depressed. 



The Costo-Transverse Joints. — The articular surfaces of the tubercles, 



' Musgrove : Journal of Anatomy and Physiolog}% vol. xxvii., 1893. ^ Fawcett : Anat. 

 Anzeiger, Bd. xv. Bardeleben : ibid. 



^ Llg. capitui) costae Interartlculare. ^ Lig. capifuli costae radiatum. 



