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. 
Ke The spinal cord and nerve roots. 
o 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) 
(b) 
(c) 
(d) 
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. 
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. 
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. 
The origin and primary divisions of the nerve roots may be 
worked out by removing carefully the lateral portions of the 
