of the Central NervoHfi Sj/sfem in Ascidia &C. 221 



substance into the funiculus dorsalis and funiculus ventralis 

 and \\\e, funiculus lateralis^ which Ahlborn also has employed 

 in Petromyzon^ I do not find to be really quite characteristic ; 

 but as they present several advantages for descriptive pur- 

 poses, I will nevertheless retain them. The above-mentioned 

 radial fibrillffi occur in the whole of the white substance, but 

 they appear in the greatest number in its dorsal part, in the 

 funiculus dorsalis and the dorsal parts o^ i\\Q funiculus lateralis^ 

 where their radial arrangement is also most easily observed. 

 A grey granular mass, which Ahlborn * describes in Petro- 

 myzon as existing between the pia mater and the white sub- 

 stance, I have been absolutely unable to observe in Myxine; on 

 the contrary, I have always been able distinctly to demonstrate 

 the connexion of the radial fibres with the pia mater ; and I 

 assume that the same must also be the case in Petromyzon and 

 that the grey mass which Ahlborn saw in his osmic-acid pre- 

 parations was probably an artificial product, as he himself 

 appears also to suppose. Between the radial fibres a network 

 of fine fibrillse is interwoven, which, in great part at any rate, 

 must be regarded as of the same nature as the former, and 

 therefore as belonging to the connective substances. Besides 

 these parts belonging to the connective substances, we have 

 also in the white substance the true nervous constituents. Of 

 these what especially catch the eye in a transverse section of 

 a spinal cord are the coarse longitudinal fibres, which occur 

 especially in the ventral part, in the funiculus ventralis, and 

 partly in the funicidus lateralis. These, which are the so- 

 called " Miillerian fibres," were first described by Johannes 

 Miiller; there is, however, no other difference than size 

 between them and the finer longitudinal fibres, and all possible 

 transition-stages occur, from the very coarsest to the very 

 finest. The coarsest longitudinal fibres are situated, together 

 with others of smaller calibre also, in the funiculus ventralis 

 on both sides of the ventral longitudinal furrow ; they appear 

 to be present in greater number than in Petromyzon, in which 

 Ahlborn notes about eight coarser fibres ; to give any definite 

 number seems to me, however, to be quite arbitrary, seeing 

 that, as already stated, there are all possible transitions, and 

 therefore it is impossible to lay down any limits. Numerous 

 longitudinal fibres likewise occur in the funiculus lateralis, 

 especially in its more ventral parts ; there are, however, only 

 a few very coarse ones, and the calibre of the fibres diminishes 



* Loc. cit, p. 245, note. 



