THE NERVOUS SYSTEM. 1 15 



missural fibres, which may indeed be divided into an anterior 

 and posterior set, the latter constituting a commissure between 

 the two points of origin of the optic nerves, whilst the anterior 

 could only unite the two retina. The existence of the first- 

 named fibres is certain, although they are much more scanty 

 than the decussating elements; but that of the others also can 

 hardly be denied. Speaking physiologically, a commissure of 

 the optic thalami and corpora quadrigemina may perhaps be ex- 

 plained, but a commissure also of the retina does not appear to be 

 altogether impossible, because we know that the retina contains 

 grey substance, and in it, nerve-cells with branched processes. 



[With respect to the origin of the nerve-fibres in the brain 

 and higher central organs, in general, it is several years since I 

 first observed the origin of dark-bordered fine fibres from the 

 processes of the nerve-cells in the spinal cord of the Frog 

 ('Zeitsch. f. wiss. Zool./ vol. I, p. 144, tab. xi, fig. 7). In man 

 I have not as yet been so fortunate as to perceive anything of 

 the sort with certainty, though I do not myself doubt that 

 similar conditions obtain in this case also. In fact, R. Wagner 

 and Leuckart think they have seen, in man, the processes of 

 the many-rayed cells in the substantia ferruginea, passing into 

 nerve-tubes (' Gott. Anzeig./ 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 ( f Gott. 

 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 

 continued 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 continuous with the sheath of the dark- 

 bordered tubes j although, seeing that the presence of tunics on 



