198 THE ARTICULATIONS 



^^'^,•re it not for theso ])roct'Sses, tlie column, instead of being a stationary one, 

 endowed with the eajiaeitv of movement l)y muscular agency, would be a tottering 

 one, requiring nniscles to steady it. Tlie influence of tlie articular processes in 

 limiting the direction of inclination will ai)pear from a study of the movements in 

 tlie three regions of the spine. 



In the neck, the obliciuity of the processes permits all movements, but is especi- 

 ally favourable for extension. Flexion is less free here than in the lumbar region, 

 while extension is freer. Lateral flexion is more free in the neck than in any other 

 region. Rotatory movements are also free in the neck, especially in the lower part. 

 There is but slight movement of any sort between the axis and third cervical ver- 

 tebra, owing to" the shallow intervertebral disc and the great prolongation of the 

 anterior lip of tlie inferior surface of the body of the axis, which checks forward 

 flexion considerably. 



In the thoracic region, especially near its middle, antero-posterior flexion and 

 extension are very slight; and, as the concavity of the curve here is forwards, the 

 flat and nearly vertical surfaces of the articular processes prevent anything like 

 sliding in a curvilinear manner of the one set of processes over the sharp upper 

 edges of the other, Avhich would be necessary for forward flexion. A fair amount 

 of lateral flexion would be permitted but for the impediment offered by the rilis; 

 while the slightly outward inclination of the superior process, and inward inclina- 

 tion of the inferior, allow a little rotation, which is freer in the upper than in the 

 lower part of the thoracic region. 



In the lumbar region, extension and flexion are very free, especially between 

 the third and fourth, and fourth and fifth vertebra?, where the lumbar curve is 

 sharpest; lateral inclination is also very free between these same vertebra?. It has 

 been stated that the shape and position of the articular processes of the lumbar and 

 the lower two or three dorsal are such as to prevent any rotation in these regions; 

 but, owing to the fact that the inferior articular processes are not tightly embraced 

 by the superior, so that the two sets of articular processes are not in contact on 

 both sides of the l)odies at the same time, there is alwa^'S some space in Avhich 

 horizontal motion can occur round an axis drawn through the central part of the 

 bodies and intervertebral discs. Thus, the motions are most free in those regions 

 of the column which have a convex curve forwards, due to the shape of the inter- 

 vertebral discs, Avhere there are no bony walls surrounding solid viscera, where 

 the spinal canal is largest and its contents are less firmly attached, and where the 

 pedicles and articular processes are more nearly on a transverse level with the 

 posterior surface of the bodies of the vertebra?. 



Nor must the uses of the ligamenta subflava be forgotten: these useful little 

 structures (1) complete the roofing-in of the spinal canal, and yet at the same 

 time permit an ever-changing variation in the width of the interlaminar spaces 

 in flexion and extension; (2) they also restore the articulating surfaces to their 

 normal position with regard to each other after movements of the colunm; 

 (3) and by forming the inner portion of each capsular ligament they take the 

 place of muscle, in preventing it from being nipped between the articular surfaces 

 duriner movement. 



2. THE SACRO-VERTEBKAL ARTICULATIONS, OR THE ARTICULA- 

 TIONS OF THE PELVIS WITH THE SPINE 



(" ) Class. — . 1 iiqili iarthrosift. 



(Ii ) Class. — Diartltro.si.'^. Subdivision. — Arthrodta. 



As in the intervertebral articulations, so in the union of the first portion of the 

 sacrum with the last lumbar vertebra, there are two sets of joints — viz. (a) an 

 amphiarthrodial one, between the bodies and intervertebral disc; and (6) a ])air 

 of arthrodial joints, between the articular processes. The union is efli'ected by the 

 following ligaments, which are connnon to the sjanal column: — (i) anterior^ and 

 (ii) iiosterior, connnon; (iii) lateral or short vertebral; (iv) capsular; (v) liga- 

 mentum subflavum; (vi) sujira- and (vii) interspinous ligaments. Two special 



