COSTO-VERTEBRAL ARTICULATIONS 



241 



symphyseal cartilage becomes softer and more vascular, so as to permit the temporary enlarge- 

 ment of the pelvis; but it must be remembered that the fibres of the oblique muscles decussate 

 and thus, during labour, while they force the head of the fcetus down, they strengthen the joint 

 by bracing the bones more tightly together. 



Relations. — The interpubic joint is in relation above with the linea alba. Behind with 

 the prostate and the anterior border of the bladder. In front with the suspensory hgament 

 of the penis or clitoris and below with the dorsal vein of the penis or cUtoris and the upper 

 border of the urogenital trigone (triangular ligament). 



4. THE COSTO-VERTEBRAL ARTICULATIONS 

 These consist of two sets, viz.: — 



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

 with the vertebrae. 



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

 first ten ribs) 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 with the second thoracic 

 vertebra, and so on. 



Fig. 279. — The Capsul.\r Ligamexts of the Costo-vertebral Joints. 



Spinal process of seventh 

 cervical vertebra 



Capsular ligament of the first costo 

 transverse joint 



14 Capsular ligament of first 



capitular joint 



First rib 



(a) The Capitular (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, with the intervertebral disc between them, enter. In the case of 

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

 articulating with a single vertebra. 



The ligaments are : — 



Articular capsule. 



Interarticular. 



Radiate. 



The articular capsule (fig. 279) consists of short, strong, woolly fibres, com- 

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

 substances, a little beyond their articular margins. 



At its upper part it reaches through the intervertebral foramen toward the back of the 

 bodies of the vertebrae, being strengthened here by fibres which at intervals connect the anterior 

 with the posterior longitudinal ligaments. The lower fibres extend downward nearly to the 

 demi-facet (costal pit) of the rib below; behind, it is continuous with the neck ligament, and 

 in front is overlaid by the radiate. 



The interarticular ligament (fig. 280) consists of short, strong fibres, closely 

 interwoven with the outermost ring of the intervertebral disc, and attached to 

 the transverse ridge separating the articular facets on the head of the rib. It 

 completely divides the articulations into two parts, but does not brace the rib 

 tightly to the spine, being loose enough to allow a moderate amount of rotation 



16 



