THE THORAX It^t 



of the articular processes form segments. This centre is necessarily anterior 

 to the articular processes, and corresponds pretty nearly with the centres of 

 the bodies of the vertebrae. In the lumbar region rotation is impossible, 

 for the following two reasons: (i) the centre of the circle of which the 

 articular processes form segments is posterior to these processes ; and (2) the 

 articular processes are so disposed as to be locked. In the lumbar region 

 circumduction is allowed, which consists in a combination of flexion, extension, 

 and lateral movements. 



Summary of Movements — Cervical Region. — (i) Flexion and extension; 

 and (2) a combination of lateral flexion and rotation. Thoracic Region. — 

 (i) Flexion and extension, but only to a limited extent ; (2) lateral flexion ; 

 and (3) pure rotation. Lumbar Region. — (i) Flexion and extension ; {2) lateral 

 flexion ; and (3) circumduction. 



The articulations of the atlas, axis, and occipital bone will be 

 found described in the section dealing with the head and neck. 



THE ARTICULATIONS OF THE RIBS, COSTAL CARTILAGES, 

 AND STERNUM. 



I. Ribs — Costo-vertebral Articulations. — These are divided into 



costo-central and costo-transverse. 



Costo-central Articulations. — ^These unite the heads of the ribs 

 to the centra or bodies of the thoracic vertebrae, and they are some- 

 times spoken of as the capiitdar joints. They belong to the class 

 diarthrosis, and to the subdivision condylarthrosis. The articular 

 surfaces are the demi-facets on the heads of the ribs and the demi- 

 facets on the sides of the bodies of the thoracic vertebrae, that is 

 to say, the lower demi - facet of the vertebra above and the 

 upper demi-facet of the vertebra below, the intervertebral disc 

 intervening between the two. In the case of the ist, (some- 

 times the loth), the nth, and the 12th vertebrae there is only 

 one facet, and the corresponding intervertebral discs do not enter 

 into the joints, unless in the case of the ist joint, into which the 

 disc between the 7th cervical and ist thoracic vertebrae may 

 enter. 



Ligaments. — ^These are as follows : anterior costo-central, capsular, 

 and interarticular. 



The anterior costo-central ligament consists of strong white fibres 

 which are attached to the anterior margin of the head of the rib. 

 From this point the fibres radiate inwards in three bands, one of 

 which passes upwards to be attached to the body of the vertebra 

 above, a second horizontally inwards to be attached to the inter- 

 vertebral disc, and a third downwards to be attached to the body 

 of the vertebra below. From the radiating disposition of these 

 bands the ligament is known by the name of stellate or radiate. In 

 the case of the ist, loth, nth, and 12th joints, into each of which 

 only one vertebra enters, the ligament is composed of two bands, 

 instead of three. In the ist joint the lower band passes to the 



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