10 



Anatomy of Skeleton 



part between this process and the spine, termed the lamina, and a portion in front 



between the process and the body, which constitutes the pedicle. These two parts 



are under somewhat different con- 

 ditions : the pedicles are thick 

 and somewhat rounded bony bars 

 transmitting to the body the 

 weight of the body-wall carried 

 by the rib to the transverse pro- 



Surroundtng 



drftculation 



cess, whereas the laminae are not 

 concerned in this, but are flat- 

 tened and sloped to cover in the 

 spinal canal. It can be seen, 

 however, that thickened portions 

 extend in the laminae between the 

 spines and the pedicles, better 

 marked as we proceed down the 

 column ; these bear the strain of 

 muscles acting on the spines. 

 The arch articulates by means of 



FIG. 6. Two dorsal vertebrae articulated to show the disc in , , , , , , , , 



position and the formation of the intervertebral foramen. articular processes With the arches 



above and below it, so that it 



carries two pairs of such processes, superior and inferior. They are placed in a general 

 way above and below the base of the transverse processes that is, the upper ones 

 are always at the junction of the processes and pedicles, but the lower are usually 

 further back and even on the lower borders of the laminae. 



Turning now to the pedicles, we observe that in vertical depth they are not equal 

 to the bodies to which they are attached. In 

 this way " notches " lie above and below the 

 pedicles, and, because the pedicle is attached 

 nearer the top than the bottom of the body, the 

 inferior intervertebral notch is much the deeper 

 of the two. The notches are bounded behind by 

 the articular processes, and when the vertebrae 

 are in apposition it is seen (Fig. 6) that the upper 

 and lower notches of contiguous vertebrae are 

 combined to form an intervertebral foramen, which 

 is bounded above and below by pedicles, behind 

 by the joined articular processes, and in front by 

 the intervertebral disc and the lower part of the 

 body above it. The spinal nerves escape through 

 the intervertebral foramina, and small branches 

 of the segmental arteries enter the canal through 

 them. 



The body, being the weight-carrying part of 

 the vertebra, is built accordingly. A certain 

 amount of elasticity in the column is obtained 

 by the formation of the body almost entirely from cancellous tissue, and the strongest 

 lamellae of this run in the direction of pressure from above downwards, being held 

 together by weaker horizontal lamellae (Fig. 7). 



The intervertebral discs add greatly to the elasticity of the column, at the same 



FIG. 7. A scheme to show the direc- 

 tion of the lamellae of cancellous 

 bone in the body of a vertebra ; 

 the main direction is vertical, in 

 the line of pressure as indicated by 

 the arrow, but weaker lamellae 

 bind these together transversely. 



