THE NERVOUS SYSTEM. 391 



zeig.," 1850, No. 43); as has Prof. Domrich, in the cortical substance 

 of the cerebellum, according to a communication to me by letter. R. 

 Wagner again (" Grb'tt. Nachr.," Oct. 1851), has, recently, also found in 

 the electric lobes of the Ray, that from the many-rayed ganglion-globules 

 or nerve-cells, one or more, rarely two, unbranched processes are con- 

 tinued into dark-bordered fibres. He now explains this transition, in 

 the same way as before, saying that the processes were continued as 

 axis-cylinders into the dark-bordered tubes, in which Leydig, who has 

 observed the same transition in the cerebellum of the "Hammer-headed 

 Shark," agrees with him, as does Stannius, in the case of Petromyzon. 

 Nevertheless, it is still not quite evident to me, that any condition should 

 exist in this case, different from that which obtains in the ganglia, where 

 the processes of the nerve-cells are not simply axis-cylinders, but also 

 have a coat, which investing the medullary matter of the nerve, is con- 

 tinuous with the sheath of the dark-bordered tubes ; although, seeing 

 that the presence of tunics on the nerve-corpuscles of the central organs, 

 and their processes, in general, is still a disputed point, I am prepared 

 to admit that the fact may be otherwise. These researches have opened 

 the way, and I have no doubt, as I have already said in rny Microsco- 

 pical Anatomy, that in time we shall succeed in demonstrating the origin 

 of dark-bordered tubes in many other situations in the central organs, 

 in man, and other animals. On the other hand, however, supported by 

 repeated investigation of the human brain, I must assert, that it is in 

 the highest degree probable that in many places it will be altogether 

 impossible to demonstrate the origin of fibres from nerve-cells, because 

 very many nerve-tubes, particularly those of the cortical substance of 

 the cerebellum and cerebrum, ultimately become so pale and slender, as 

 not to allow of their being distinguished from the processes of nerve- 

 cells. Whether the loops which distinctly exist in the convolutions of 

 the cerebrum, and which I have also seen in the corpora striata, are ter- 

 minations, or whether free prolongations of nerve-tubes exist, we know 

 not, and the less so because it cannot even be asserted that certain fibres 

 really so terminate. It may fairly be assumed that the fibres of the 

 corpus callosum and the commissural fibres in general, commence in the 

 one hemisphere in connection with cells and terminate in the other, and 

 that the fibres which proceed from the surface of the convolutions to the 

 optic thalami and corpora striata terminate in the latter, but to assert, 

 that it is so, is impossible, notwithstanding the visible loops, for it may 

 be that these latter are not terminations at all, and that the fibres in 

 question are all in the one place and the other in connection with nerve- 

 cells. That nerve-fibres should originate independently of any connec- 

 tion with cells would be contrary to all analogy, but in such an obscure 

 subject we must always be prepared for much that is new, and be careful 

 not wholly to reject any possibility, simply from d priori considerations. 



