Observations o?i the Striicture of the Brain. 185 



the brain. The cortical substance seems, according to Ehren- 

 berg's observations, to differ from the medullary or white sub- 

 stance chiefly in the want of the straight cylindrical fibres, and 

 in the articulated fibres being contained in a densel* net-work 

 of bloodvessels, and being covered by a layer of free granules 

 larger than the dilated parts of the knotted fibres. 



In the brain, the fibres run for the most part parallel to one 

 another ; they are sometimes seen to cross, and, in a few instan- 

 ces, Professor Ehrenberg states that he has observed two fibres 

 uniting into one, but never any distinct anastomosis. 



The larger straight Cylindrical fibres, he states, are manifestly 

 tubular, because it is possible to see the inner paries of the tul)e, 

 and on dividing some of these fibres and gently pressing them 

 between plates of glass, a granular medullary matter was made 

 to issue from them. In the Knotted or Articulated fibres he 

 never was able to discover a distinctly tubular appearance, nor 

 could any matter be pressed from their interior ; but notwith- 

 standing this, Ehrenberg considers these also as tubular. 



Professor Ehrenberg has observed a remarkable difference in 

 the minute structure of some of the Nerves of special sensation, 

 the great sympathetic nerve, and the compound spinal nerves. 

 He finds that the olfactory, the optic, and the auditory nerves, as 

 well as the branches of the great sympathetic, are entirely com- 

 posed of knotted or articulated fibres, similar in size and appear- 

 ance to those forming the great bulk of the nervous matter 

 in the cerebrum ; while the nerves of motion and the regular 

 spinal nerves, are entirely composed of the straight cylindrical 

 tubular fibres. 



The cylindrical tubular fibres of the spinal nerves and of the 

 nerves of motion coming from the brain, are considered by Pro- 

 fessor Ehrenberg as prolongations of some of the articulated 

 fibres of the brain itself, for he has observed at the origin of a 

 nerve of motion, that the articulated fibres gradually lose their 

 knotted appearance as they pass into the root of the nerve, and 

 increasing slightly in diameter, become the straight tubular cy- 

 lindrical fibres proper to nerves of this description. 



The net-work of the retina affords an excellent opportunity 

 of viewing the articulated cerebral fibres, but in order that these 

 may be well seen, there must be removed from their surface, a 



