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. A small 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 section, 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 



