344 ANATOMY OF THE RABBIT 



XI I J. THE CENTRAL NERVOUS SYSTEM 

 1. The spinal cord and nerve roots. 



To expose the whole cord or a portion of it from the dorsal 

 surface, the muscles should be removed from the dorsal aspect and 

 both sides of the vertebral arches and the latter should be broken 

 away with bone forceps. The following features may be made oiit 

 when the extent of exposure is sufficient: 



(a) The spinal cord is enclosed in a set of three protective 

 membranes, the meninges, the outermost of which is a 

 relatively thick, tough, fibrous sheet, the dura mater. 

 Unlike that of the brain, the dura mater of the spinal cord 

 is not firmly attached to the inner surface of the surround- 

 ing bone and it is exposed in an intact condition by the 

 removal of the vertebral arches. The epidural space 

 between this membrane and the inner surface of the bone 

 contains a little fat. 



If the dura mater is now cut open, it is found to be 

 separated by a narrow space from a much thinner, vascular 

 membrane, the pia mater, which is closely applied to the 

 surface of the spinal cord. Between the dura mater and 

 the pia mater and attached to both lies a very delicate 

 web of connective tissue, the arachnoidea. 



The arachnoidea and the pia mater together are fre- 

 quently designated leptomeninges, whereas the dura mater 

 is distinguished as the pachymeninx. 



The spaces between these membranes are filled, in the 

 living condition, with cerebrospinal fluid, the same liquid 

 which fills also the cavities within the central nervous 

 organs. 



(b) The spinal cord (medulla spinalis) is a thick, subcylindrical, 

 white cord traversing the vertebral column in the vertebral 

 canal. Its diameter is not uniform, as it exhibits two slight 

 enlargements, one in the cervical, the other in the lumbar 

 region. These enlargements are the regions from which 

 the nerves to the limbs arise and are due to the increased 

 number of nerve elements present on that account. At 

 about the middle of the sacrum, the cord contracts to a 



