332 NERVOUS SYSTEM. 



level, one before and the other behind, and each root consists in 

 several fasciculi of nervous matter. The front root arises from 

 the anterior chord of the medulla spinalis, and the other from 

 the posterior chord. The posterior root is larger than the an- 

 terior, but has fewer fasciculi in its composition, and is not so 

 filamentous. The two roots are kept asunder by the Ligamen- 

 tum Denticulatum. The fasciculi of each are slightly connected 

 by a loose delicate cellular substance, and as they are about 

 penetrating the dura mater, 3ach fasciculus collects into a sin- 

 gle chord, which passes the dura mater through its appropriate 

 foramen. In this way the anterior and posterior roots are kept 

 distinct till they have got to the outside of the membrane men- 

 tioned; but the foramina, through which they pass, border close- 

 ly upon one another. The posterior root, then forms a ganglion 

 of a round or oval shape ; from w.hose external extremity there 

 proceeds a single nervous trunk, which is joined immediately 

 at its commencement by the anterior root. 



With the exception of the ganglions of the sacrum, which 

 are in the spinal cavity of that bone, these bodies are placed in 

 the intervertebral foramina. The size of the ganglion is not 

 proportionate to that of the nerve from which it proceeds ; for 

 some of the dorsal ganglions are the largest, while those of the 

 sacrum are smaller than any others. 



The two nerves of the same pair, though generally symme- 

 trical, or precisely resembling, are not invariably so; some- 

 times one is placed higher than another, and the number of the 

 fasciculi may be greater or smaller. ^ The roots of the nerves 

 are much nearer, or cluster more at the extremities of the spi- 

 nal marrow, than its middle. 



The Cervical Pairs of Nerves are nearly horizontal in their 

 course from the medulla spinalis to the foramina in the dura 

 mater. The first one, or the sub-occipital, is strictly so; the 

 others incline very gradually more and more downwards. They 

 have, therefore, but a very short passage before they reach the 

 interverlebral foramina. Their roots are so pyramidal, that the 

 bases nearly touch each other, and, for the most part, are con- 

 nected by an anastomosing filament, which goes from the upper 

 margin of the nerve below to the lower margin of the nerve above. 

 These anastomoses are found connecting the upper with the 



