ORIGIN OF THE NERVES. 383 



the fissure along the attachment of the posterior nerve-roots. At its ex- 

 tremity, where it is slightly enlarged, it is incased with a rather transpa- 

 rent stratum of small nerve cells, which has been named sabstantia gelati- 

 nosa (fig. 126, 3 ). 



The anterior cornu (fig. 126, 2 ) is shorter and thicker than the other, 

 and projects towards the anterior roots without reaching the surface. Its 

 end lias an irregular or zigzag outline. 



Mr. Clarke describes two special collections of cells, one on the outer 

 and the other on the inner side of the gray crescent, which extend through 

 the cord below the cervical swelling, and with which the roots of the 

 nerves are connected. 1 



The inner group, or the posterior vesicular column (fig. 126, 8 ) is close 

 behind the transverse commissure ( & ) ; whilst the outer group, or the inter- 

 Fig. 126. 



1. Posterior cornu (caput cornu, Clarke) of the crescent. 



2. Anterior cornu. 



3. Casing of the substautia gelatinosa. 



4. Central canal of the cord. 

 6. Transverse commissure. 



6. Tractus iutermedio-lateralis. 



7. Cervix cornu of Clarke, reaching from the anterior points of 



the substantia gelatinosa to the level of the canal. 



5. Posterior vesicular column 



A REPRESENTATION OF THE GRAY SUBSTANCE TN THE INTERIOR OF THE SPINAL CORD, NEAR THB 

 MIDDLE OF THB DORSAL KEUION (Clarke). 



mediate tract ( 8 ), is placed outside the crescent, about midway from front 

 to back. The first is best marked in the dorsal enlargement, and the last 

 in the upper part of the dorsal region. Above the cervical swelling of the 

 cord only rudiments of those tracts remain ; and the cells in a line with 

 the intermediate tract are traversed by the roots of the spinal accessory 

 nerve. 



The white substance of the cord is composed chiefly of nerve fibres dis- 

 posed longitudinally in bundles, so as to give passage to intermediate 

 vessels. 



A framework of very fine areolar tissue extends through the substance 

 of the spinal cord, supporting the fibres and cells : this is continuous with 

 the pia mater on the surface. 



Modifications of the gray and white substance. The white substance 

 exceeds the gray in quantity in the neck and back ; but it is less abundant 

 in proportion to the gray matter in the lumbar region. 



The cornua of the gray crescents decrease in length from above down, 

 especially the posterior, and towards the end of the cord they blend in one 

 indented or cruciform mass. 



ORIGIN OF THE NERVES. The deep origin of the spinal nerves is un- 

 certain, like that of the cranial nerves, but the fibrils in each root enter 

 the gray matter of the cord (fig. 125). 



The anterior root (k) traverses the longitudinal fibres of the antero- 

 lateral column in distinct bundles ; and entering the anterior cornu of the 



1 Further researches on the gray substance of the spinal cord. By J. Lockhart 

 Clarke, F.R.S., Philosoph. Trans, of the Royal Society for 1859. Part i. 



