184 Anatomy of the Rabbit. 



XIII.. 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 on both sides of the vertebral arches and 

 the latter broken iaway with the bone forceps. The following features 

 may be made out according to the extent of exposure : 



(a) The spinal cord (medulla spinalis) is a thick cylindrical white 

 cord traversing the vertebral column in the vertebral canal. 

 It is of almost uniform diameter, but exhibits two slight 

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

 region. At about the middle of the sacrum it contracts to 

 a slender filament, the filum terminale, which may be traced 

 backward to the middle of the tail. 



(b) The enclosing membranes or meninges of the cord are three 

 in number, of which two are readily identified. Lining the 

 internal surface of the bone is a thick fibrous investment, 

 the dura mater; on the surface of the nervous matter a 

 thin vascular membrane, the pia mater. 



The dura and pia are connected by a loose web of connective 

 tissue, the arachnoidea. 



(c) The regional distribution of the nerve roots — eight cervical, 

 twelve dorsal, seven lumbar, four sacral, and six caudal. 



The cervical nerves are numbered from the vertebrae lying behind 

 the intervertebral foramina from which they proceed, the remaining 

 nerves from the vertebrae lying in front of the intervertebral foramina. 

 The nerve transmitted by the intervertebral foramen between the 

 seventh cervical and first thoracic vertebrae is described as the eighth 

 cervical. 



(d)j|The origin and primary divisions of the nerve roots may be 

 worked out by removing carefully the lateral portions of the 

 arches of one or two vertebrae. The posterior, or sensory 

 root (radix posterior), arises from the dorsolateral surface of 

 cord, and expands immediately into a spinal ganglion (g. 

 spinale). The more slender anterior, or motor root (radix 

 anterior), arises by a number of filaments from the ventro- 

 lateral surface. The combined roots give origin to the 

 posterior, anterior, and communicating rami. The posterior 

 ramus is an inconspicuous branch (except in the first two 

 cervicals) passing to the dorsal musculature and skin. The 

 anterior ramus is the chief portion of the spinal nerve, the 

 successive rami appearing as the components of the cervical 

 and lumbosacral plexuses or as individual spinal nerves. 

 The ramus communicans is a slender filament which passes 

 ventrad to join the sympathetic trunk. 



