314 THE AKTICULATIONS OK JOINTS. 



The head of every typical rib is wedge-shaped, and presents two articular 

 facets, an upper and a lower, separated from each other by an antero-posterior 

 ridge which abuts against an intervertebral fibro- cartilage, while the articular facets 

 articulate with similar surfaces on the contiguous margins of the two vertebrae 

 adjoining tfye fibro-cartilage. These surfaces form a wedge-shaped depression or 

 cup, the bottom of which is more elastic than the sides, and thus an arrangement 

 is provided which tends to reduce the shock of blows upon the walls of the chest. 



Each of these articulations is provided with an articular capsule which 

 surrounds and encloses the joint, and is attached to contiguous non-articular 

 margins on the head of the rib and the two vertebral bodies. On its anterior or 

 ventral aspect the capsule presents three radiating fasciculi which collectively form 

 the lig. capituli costse radiatum (radiate ligament of the head of the rib (O.T. stellate)) 

 (Fig. 293). These fasciculi radiate from a centre on the anterior surface of the head 

 of the rib, so that the middle _ fasciculus becomes attached to the intervertebral 

 fibro-cartilage while the upper and lower fasciculi proceed to the adjacent margins 

 of the two vertebrae between which the fibro-cartilage is situated, and with which 

 the rib articulates. To a slight extent these radiating fasciculi pass under cover of 

 the lateral margin of the anterior longitudinal ligament of the vertebral bodies. 

 In those joints in which the head of the rib does not articulate with an inter- 

 vertebral fibro-cartilage the central fasciculus of the radiate ligament is wanting, 

 but the other two retain the same general arrangement. 



Lig. Capituli Costse Interarticulare. The interarticular ligament of the head 

 of the rib consists of short transverse fibres within the capsule. These are attached, 

 on the one hand, to the ridge which intervenes between the two facets on the head 

 of the rib, and on the other to the lateral aspect of the intervertebral fibro-cartilage. 

 This ligament is not a disc or meniscus, but merely an interarticular ligament, of 

 width sufficient to divide the joint cavity into an upper and a lower compartment. 

 It is absent from those joints which do not articulate with an intervertebral fibro- 

 cartilage, i.e. from those ribs which articulate with the body of only one vertebra. 



The interarticular ligament is supposed to represent the lateral end of a ligament which, 

 under the name of the lig. conjugale costarum, connects the heads of the ribs of certain mammals 

 across the posterior aspect of the intervertebral fibro-cartilage, and, in the human subject, until 

 the seventh month of foetal life, connects the posterior aspects of the necks of a pair of ribs with 

 each other across the median plane. 



A stratum synoviale lines each joint cavity, and therefore, in all cases where the 

 joint is divided into two compartments, each one has its own synovial lining. 



ARTICULATIONES COSTOTRANSVERSARLE. 



In the costo-transverse joints the tubercle of each typical rib articulates with 

 the transverse process of the lower of the two thoracic vertebrae with which the 

 head of the rib is associated. Near the tip of the transverse process there is 

 an articular facet, on its anterior aspect, for articulation with the corresponding 

 facet on the medial articular part of the rib tubercle. The joint so formed is 

 an arthrodial diarthrosis. 



The joint cavity is surrounded by a comparatively feeble capsula articularis, 

 which is attached immediately beyond the margins of the articular facets, and in 

 which no special bands can be distinguished. 



A simple stratum synoviale lines the capsule in all cases where the latter ii 

 present. 



The following accessory ligaments, in connexion with this joint, strengthen an( 

 support the articulation : 



Ligamentum Costotransversarium Anterius. The anterior costo-transvt 

 ligament (O.T. superior) (Fig. 293) consists of strong bands of fibres which ai 

 attached to the superior border of the neck of the rib, extending from the h( 

 laterally to the non-articular part of the tubercle. All these fibres may be tract 

 upwards. Those situated nearest to the head of the rib proceed obliquely upw* 

 and laterally, to be attached to the transverse process immediately above, but witl 



