208 THE ARTICULATIOXS 



in i)art, from tlie internal piulir and in part from the ilio-hypogastric and ilio- 

 inguinal. 



The movements amount only to a slight yielding of the cartilage; neither 

 mu.^eular force nor extrinj?ic forces produce any appreciable movement in the 

 ordinary condition. Occasionally, as the result of child-bearing, the joint becomes 

 unnaturally loose, and then walking and standing are painfully unsteady. It is 

 known that, during pregnancy and parturition, the symphysial cartilage becomes 

 softer and more vascular, so as to permit the temporary enlargement of the pelvis; 

 but it nuist be remeni1)ered that the fibres of the oblique muscles decussate, and 

 thus, during labour, Avhile they force the head of the foetus down, they strengthen 

 the joint l)y bracing the bones more tightly together. 



4. THE ARTICULATIONS OF THE RIBS WITH THE VERTEBRA 



These consist of tAvo sets, xiz. : — 



(a) The costo-central : i.e. the articulation of the head of the rib with the 

 vertebra?. 



(6) The costo-transverse, or the articulation of the tubercle (of each of the 

 first ten ril)s) with the transverse process of the lower of the two vertebrae, with 

 which the head of the rib articulates: i.e. the one bearing its own number, as the 

 first rib with the first thoracic vertebra, the second rib Avith the second thoracic 

 vertebra, and so on. 



(a) The Costo-central Articulation 

 Class. — Diarthrosis. Subdivision. — Condylarthrosis. 



It is a very perfect joint, into the formation of which the head of the rib and 

 two vertebrae, Avith the interA'erteliral disc betAveen them, enter. In the case of the 

 first, tenth, eleventh, and twelfth ribs, it is formed by the head of the rib articulating 

 Avith a single A'crtebra. 



The ligaments are: — 



Capsular. Interarticular. 



Stellate or anterior costo-central. 



The capsular ligament (fig. 214) consists of short, strong, Avoolly fibres, com- 

 pletely surrounding the joint, Avhich are attached to the bones and intervertebral 

 substances, a little beyond their articular margins. At its u]iper part it reaches 

 through the intervertebral foramen toAvards the back of the ]>odies of the vertebrae, 

 being strengthened here by fibres Avhich at intervals connect the anterior Avith the 

 posterior common ligaments. The lower fibres extend doAvuAvards nearly to the 

 demi-facet of the rib beloAv; behind, it is continuous Avith the middle costo- 

 transverse ligament, and in front is overlaid by the stellate. 



The interarticular ligament (fig. 215) consists of short, strong fibres, closely 

 interAVOven A\ith the outermost ring of the intervertel iral disc, and attached to the 

 ti-ansvcrse ridge separating the articular facets on the head of the rib. It completely 

 divides the articulations into tAvo parts, but does not brace the rib tightly to the 

 spine, being loose enough to alloAV a moderate amount of rotation on its OAvn axis. 

 There is no interarticular ligament in the costo-vertebral joints of the first, tenth, 

 elcA'cntb, and tAvelftli ribs. 



The anterior costo-central or stellate ligament (figs. 215 and 216) is the 

 most striking of all, and consists of bright, pearly-Avhite fibres attached to the 

 anterior surface, and upper and loAver borders of the neck of the rib, a little Avay 

 beyond the articular facet ; from this they radiate upAvards, forAvards, and doAvn- 

 Avards, so as to form a continuous layer of distinct and shari)ly defined fibres. The 

 middle fibres run straight forAvard'to be attached to the iiitevertebral disc; the 

 upper ascend to the loAver lialf of the lateral surface of the vertebra above, and the 

 loAver descend to the ui»i)er half of the vertebra beloAv. 



