THE PIA MATER 



237 



The pia mater is a very delicate structure dipping between 

 the convolutions of nervous matter and lying in close con- 

 tact with the external surface of the encephalon and cord. 

 It is exceedingly vascular,; indeed its main function is to 

 support vessels belonging to the nervous substance under- 

 neath. Both the arachnoid and the pia mater pass out at 

 the foramen magnum with the dura to cover the cord. 



P IG 73. .Different views of a portion of the spinal cord from the 

 cervical region, with the roots of the nerves. (Slightly enlarged.) 



In A, the anterior surface of the specimen is shown; the anterior nerve-root 

 of its right side is divided; in B, a view of the right side is given; in C, the 

 upper surface is shown; in D, the nerve-roots and ganglion are shown from 

 below, i, the anterior median fissure; 2, posterior median fissure; 3, anterior 

 lateral depression, over which the anterior nerve-roots are seen to spread; 4, 

 posterior lateral groove, int9 which the posterior roots are seen to sink; 5, ante- 

 rior roots passing the ganglion; 5', in A, the anterior root divided; 6, the poste- 

 rior roots, the fibers of which pass into the ganglion 6'; 7, the united or com- 

 pound nerve; 7', the posterior primary branch, seen in A and D to be derived 

 in part from the anterior and in part from the posterior root. (Kirkes after 

 Allen Thomson.) 



