ANATOMY. PHYSIOLOGY, OF NERVOUS SYSTEM. 413 



cord. In fact, the dura-mater is loosely attached around 

 the spinal cord and does not serve as a periosteum for the 



Fig. 136. SECTION OF THE SPINAL CORD SHOWING TKE ARRANGEMENT OF ITS INVEST- 

 ING MEMBRANES. (After Key and Retzius.) 



a, dura mater; b, arachnoid; c, posterior septum; d, e,f, subarachnoid tissue; A, an- 

 terior root fibers cut; A, I, subarachnoid space. ^ ^ ' 



vertebrae, these bones having a periosteum of their own. 

 In the meshes of the arachnoid membrane there is usually 

 contained a small quantity of lymph-like liquid called the 

 cerebro-spinal liquid. It is not probable that this has any 

 specific function. 



The Spinal Cord. 



The spinal cord is a cylinder of nervous matter enclosed 

 in the neural arches. Its average length is about seven- 

 teen inches. It does not, therefore, reach from the cer- 

 vical region entirely through the lumbar. In fact, the 

 neural space in the lower lumbar region is occupied by a 

 number of nerve fibers from the spinal cord, while the cord 

 itself is here reduced to a slender filament called the filum 

 terminate which runs to the end of the neural canal in the 

 sacrum. 



The cord is not quite round in cross-section, it being a 

 little wider from side to side than from before backwards. 

 Its average diameter is about three-fourths of an inch. It 



