THORACIC ARTICULATIONS 401 



MOVEMENTS OF THE VERTEBRiE IN GENERAL 

 AND OF CERTAIN OF THEIR JOINTS IN PAR- 

 TICULAR 



The amount of motion between any two vertebra? is extremely limited, 

 with the exception of the atlo-axoid articulation, in which the degree of 

 rotation is considerable. But when the spine is viewed as a whole, these 

 slight individual movements multiplied together are sufficient to allow of 

 flexion and extension, as well as of inclination to either side. In the region 

 of the back the joints are rigid, in the loins less so, but in the neck and tail 

 great liberty is allowed. Flexion and extension, as well as lateral motion, 

 are dependent entirely upon the elasticity of the intervertebral substance, 

 which allows of one part being compressed while the other is extended. 

 Thus, when the loins are arched upwards the lower edge of this substance 

 is compressed, while the upper part assumes a moi'e expanded condition, and 

 at the same time the spinous processes are separated more v/idely, and their 

 ligaments are stretched. The reverse of this takes place when these bones 

 are arched downwards, while in lateral inclination the sides are compressed 

 and expanded in a corresponding manner. Very slight rotation of the 

 whole spine, or more proj^erly twisting, is permitted by the elastic nature of 

 the intervertebral substance ; but in the atlo-axoid articulation a perfect 

 I'otation occurs around the centre of the odontoid pi^ocess, allowing the 

 muzzle to be turned in either direction, which could not be done without an 

 arrangement of this nature. The capsular ligaments and the supei'ior and 

 infei'ior atlo-axoid ligaments are necessarily lax to allow of this motion. 

 Lastly, the great ligament of the neck serves to support the weight of the 

 head, which would be too great for the muscles of the neck, in consequence 

 of the length of leverage which is presented. 



THORACIC ARTICULATIONS 



The articulations which unite the bones composing the thorax may be 

 divided first into costo- vertebral articulations, or those which unite the ribs 

 with the spine ; secondly, chondro- costal, or those which unite the ribs with 

 their cartilages ; thirdly, the chondro-sternal, or those which unite the 

 cartilages with the sternum ; fourthly, the articulations of the cartilages 

 cmong themselves. 



Each rib (with the exception of the first and last) is connected with the 

 bodies of two vertebrte, the three bones, together with the intei'vertebral 

 substance, forming two joints which are sepai\ated from each other by a 

 band of fibres passing from the head of the rib to the intervertebral sub- 

 stance. Besides these there is also an articulation between the rib and the 

 transverse process of the vertebra behind it. 



The superior costo-vertehral ligament connects the head of each rib to the 

 sides of the bodies of the vertebrae, and is divided into three bundles, of 

 MJiich one bundle (the middle) passes to the corresponding intervertebral 

 fibro-cartilage, whilst the anterior passes to the body of the vei'tebra before, 

 the posterior to the body of the vertebra behind. This ligament is called 



