THE HUMAN SPINAL COBD 



399 



dorso-vcntrally, and has an average diameter of three 

 quarters of an inch, with a lengt.h of about seventeen 

 inches. It is largest in the neck region from the third to 

 the first dorsal vertebrse, and there is a second expansion 



Ptg. 187 — Cross section of spinal cord; 1, anterior fissure; 2, posterior fissure, 3, 

 central canal; 4, 5, commissures; 9, posterior root; 10, anterior root; a, e, gray 

 matter. The outer layer is the pia mater, covering the white matter and send- 

 ing into it blood vessels. 



in the last dorsal vertebra. Externally, it is covered 

 with three membranes hke the brain: the dura mater, 

 arachnoid, and pia mater. 

 The dura mater is not in close contact with the bones, 



