270 Anatomy of the Rabbit. 



(c) The abductor caudae anterior. Origin: Ischial spine. 

 Insertion: Lateral surface of the sacrum and the transverse pro- 

 cesses of the caudal vertebrae. 



(d) The flexor caudae. Origin: Ventral surface of the sa- 

 crum and anterior caudal vertebrae. Insertion: Ventral surfaces 

 of succeeding vertebrae. 



These muscles are also known as sacro-coccygei, dorsalis, lateralis, 

 and ventralis (a, b, d,) and coccygeus (c). 



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 verte- 

 bral arches and the latter broken away 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 behin d 

 the intervertebral foramina from which they proceed, the remain ing 

 n erves from the vertebrae lying in front of the intervertebral foramin a. 

 The nerve transmitted by the intervertebral foramen between the 

 seventh cervical and first thoracic vertebrae is described as the eighth 

 cervical. 



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

 worked out by removing carefully the lateral portions of the 



