THE SPINAL CORD 



825 



cephalacl with the medulla oblongata. Tts length is 45 cm. (44 to 50 cm.) or 

 ' to twenty in oh PS in the male ancf 48.5 cm. (39 5 to 47 rm ) nr sixteen 



to nineteen inches in the female. In the course of fetal development the spinal 

 cord occupies the entire length of the vertebral canal up to the third month, but 

 after this period it gradually recedes cephalad owing to the more rapid growth 

 of the vertebral column, so that at birth the caudal end of the spinal cord has 

 risen to the level of the third lumbar vertebra. 



The spinal cord does not entirely fill the vertebral canal. A wide space or 

 rather a concentric series of spaces intervene between its surface and the walls 

 of the canal, affording a marked freedom of movement of the vertebral column 

 without exerting undue tension upon the spinal cord. These spaces, three in 

 number, which concentrically surround the cord, are demarcated by the three 

 protective membranes (1) pia, (2) arachnoid, and (3) dura which are continuous 

 with the like meninges of the brain. The arrangement of the spaces and the 

 membranes may be shown thus: 



Subarachnoid 



space 



and 



subarachnoid 



reticulum. 



(Cerebrospinal 



fluid.) 



& 



=-i 71 



The pia closely invests the entire surface of the spinal cord and sends septal 

 ingrowths into its substance as well as a fold occupying the anterior median fissure. 

 A leaf-like, serrated fold of pia, the ligamentum denticulatum, passes from each 

 lateral border to the inner surface of the dural sheath and helps to support the 

 cord within the subarachnoid space. The arachnoid and the pia are not sepa- 

 rable in gross dissection as they merge insensibly, though usually described as 

 distinct membranes. The arachnoid is in reality an exceedingly delicate and 

 transparent web-like reticulum whose meshes constitute a relatively wide cavity 

 filled with cerebrospinal fluid. The dura constitutes the outermost and thickest 

 sheath, while the narrow interval between the dura and the vertebral canal is 

 filled by a fine venous plexus, together with soft, areolofatty tissue. The tubular 

 sheath of dura ends in a pointed cul-de-sac at the level of the lower border of the 

 second sacral segment. Of the three spaces which surround the cord, only the 

 two innermost contain fluid, and that of a serous character; the amount in the sub- 

 dural space is very small, just sufficient to moisten the contiguous endothelial 

 surfaces of the dura and arachnoid; that in the subarachnoid space is consider- 

 able. (For detailed description see section on the Meninges.) 



Weight . The weight of the spinal nnrd r exclusive nf all nprvp roots 



slihtl 



y 



Including the 



Tierveroots as ordinarily cut in postmortem procedure, the weight averages 45 



