THE CENTRAL NERVOUS SYSTEM. 271 
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. 
2. Asmall portion of the spinal cord may be excised and exam- 
ined (preferably under water) for the following: (see p. 38, Fig. 19). 
(a) The division of the cord into lateral halves by two median 
depressions, the anterior median fissure (fissura mediana 
anterior) and the posterior median sulcus (sulcus me- 
dianus_ posterior). 
(b) The division of each half into three columns by shallow 
grooves, the anterior and posterior lateral sulci. The 
grooves are marked by the attachments of the anterior and 
posterior nerve roots. The three columns of each half of 
the cord are the anterior, lateral, and posterior funiculi. 
(c) On the cut surface the white substance (substantia alba) is 
seen to form a peripheral investment enclosing the grey sub- 
stance (substantia grisea) of the centre of the cord. The 
grey portion is somewhat H-shaped in secticn, each half 
being composed of anterior larger and posterior smaller 
masses, known in section as the horns of the grey matter, or 
as complete structures, the anterior and posterior grey 
columns. In the median plane, but nearer the dorsal than 
the ventral surface, is the minute central canal (canalis 
centralis), the cavity of the spinal cord. 
3. The brain may be exposed by breaking away the supra- 
orbital processes of the frontal bone and then removing the roof of 
the skull with bone forceps. In order to clear the brain and the 
roots of the cerebral nerves, it is necessary to remove the entire 
