192 HEAD AND NECK 



It takes a recurrent course through the intervertebral foramen to end in 

 the bones and periosteum of the vertebral canal. 



Ganglia Spinalia. These ganglia are oval swellings de- 

 veloped upon the posterior nerve-roots, just before they unite 

 with the anterior roots to form the spinal nerve-trunks. They 

 are found upon the posterior roots of all the nerves, except, 

 occasionally, upon those of the sub-occipital and the coccygeal 

 nerves. 



The spinal ganglia are formed upon the posterior nerve- 

 roots as they lie in the intervertebral foramina, except in the 

 cases of the first two cervical and the sacral and coccygeal 

 nerves. The ganglia of the first two cervical nerves lie upon 

 the posterior arch of the first and the vertebral arch of the 

 second cervical vertebrae respectively ; the ganglia of the 

 sacral nerves are placed within the sacral canal, but out- 

 side the tube of dura mater. The ganglion on the posterior 

 root of the coccygeal nerve is inside the tube of dura mater. 



Spinal Nerve -Trunks. These are formed by the union 

 of the anterior and posterior nerve-roots immediately beyond 

 the spinal ganglia. This union takes place in the case of the 

 coccygeal and sacral nerves in the sacral canal; in the 

 lumbar, thoracic, and lower six cervical nerves, in the inter- 

 vertebral foramina ; and in the case of the first two cervical 

 nerves, on the arches of the atlas and epistropheus. 



The nerve-trunk is exceedingly short; indeed, it divides 

 almost immediately into its anterior and posterior branches. In 

 the case of the sacral and coccygeal nerves, this subdivision 

 takes place in the sacral canal, and the spinal nerve-trunks 

 of these nerves are distinctly longer than in the case of the 

 nerves which occupy a higher level. 



The distribution of the posterior branches has already 

 been examined (p. 173). 



Dissection. At this stage the dissector may adopt one of two methods 

 in the further treatment of the medulla spinalis and the nerves which spring 

 from it. If the medulla spinalis is fresh and in such a condition that it may be 

 successfully hardened, it is best to transfer it at once to the preservative fluid. 

 If, on the other hand, it is soft and not fit for proper preservation, it should 

 be removed with all its membranes and nerve-roots, and placed in a cork- 

 lined tray filled with water. There is no method by which the arachnoid, 

 the pia mater, the ligamenta denticulata, and the nerve-roots can be so 

 well studied as this. 



In removing the medulla spinalis, the spinal nerves should be divided as 

 they lie in the intervertebral foramina, and in such a manner that as 

 long a piece as possible of each nerve remains attached to the dura 



