GENERAL ANATOMY OF THE NERVOUS SYSTEM 01 



The spinal nerves emerge in pairs which are sym- 

 metrically developed. A single member, right or left, in 

 one of these pairs is found to be united with the cord by 

 two roots. These are best referred to as dorsal and ventral, 

 the former being nearer the skin of the back and the 

 latter at a greater depth. The relations referred to are 

 best understood when presented in a diagram (Fig. 10). 





Fig. 10. The upper figure shows the spinal cord within the arch 

 of a vertebra. A pair of spinal nerves is shown, each nerve having a 

 dorsal and a ventral root. 



In the lower figure the heavy enclosure is the dura mater, the 

 dotted line just within is in the position of the arachnoid, and then 

 comes the space filled by the cerebrospinal fluid. The outline of the 

 cord itself corresponds with the pia mater. 



The same figure will call attention to the protective struc- 

 tures in the vicinity. Of these, the most striking is the bony 

 arch. Within this is a dense membrane, the dura mater. 

 A much thinner membrane is applied to the immediate 

 surface of the cord and is scarcely to be separated from it; 

 this is the pia mater. Between the two membranes there 



