THE SPINAL COLUMN AND BACK. 



143 



of the lower two at the disc between the 5th lumbar and 1st 

 sacral vertebrae. The apices of the two middle pyramids meet 

 at the disc between the 4th and 5 th dorsal vertebrae, which 

 have therefore the narrowest bodies of the vertebral series. 

 The narrowing in the upper dorsal region is due to the 

 fact that the weight of the upper half of the trunk is 

 partly borne by, and transmitted to, the lower dorsal region by 

 the sternum and ribs which thus relieve the spine to some 

 extent (Fig. 113). At the sacrum the weight is transferred to 

 the pelvis and lower limbs, hence the rapid diminution of the 

 sacrum and coccyx. A well-marked thickening or bar in each 

 ilium runs from the auricular surface to the acetabulum and 

 transmits the weight to the femora. 



The Curves of the Spinal Column. — There is only one curve — 

 an anterior concavity — until the 3rd month (Fig. 114 A). About 



-dorso-lumbar 

 curve 



M- cervical 



dorso-lumbar 

 curve 



f&jsacro-vert. 

 angle 



' — dorsal curve 



ervw. curve 



sacro-vert. angle 



B-cervw. curve 



- dorsal curve 



W, lumbar curve 



-sacral curve 



ABO D 



Fig. 114.— Diagram of the Curves of the Spinal Column. 



A. At the 6th week of foetal life. B. At the 4th month of foetal life. C. Curves 

 present at Birth. D. Curves present in the Adult. 



the beginning of the 4th month the sacro-vertebral angle forms 

 between the lumbar and sacral regions (114 £). At birth the 

 cervical and sacral curves have appeared, but the sacral not to a 

 pronounced extent (Fig. 114 C). The lumbar curve appears as 

 the child learns to walk. It is produced to allow the body 

 to be brought vertically over the lower extremities. The sacral 



