The finer Anatomy of the Nervous System of Myxine glutinosa. 377 



and ventrally and enter the basal lateral region of the brain. It 

 has been impossible to discover whether other fibres ramify amongst 

 the diffusely distributed nerve cells of this region or if they spring 

 from further caudally situated cells. The other nuclei of the Thala- 

 mus are difficult to honiologise with those of higher animals, but 

 one small and distinct one situated close to the rostral end of the 

 Ganglia habenulae is in a position analogous to the Nucleus anterior 

 in other animals and may possibly be homologous with the same. 



I have not been able to find any distinct connecting tracts 

 springing from this ganglion in Myxine as is the case in higher 

 animals, where it is the origin of the fascicle of Viq. d'Azyk or the 

 Tractus thalamo-mamillaris. From the Tractum opticum the cell nuclei 

 of the Thalaraencephalon receive nerve fibres which run in a rostral 

 and somewhat lateral direction, but they are not arranged in a 

 distinct tract as is the case in higher animals, where the Tractus 

 tecto-thalamicus connects the Tectum opticum with the Nucleus 

 rotundus. 



The Nervus opticus in Myxine exists in an exceedingly 

 rudimentary state; in some specimens of the animal I have been 

 unable to find any trace at all. Johannes Müller (27) considered 

 its existence doubtful, having only once found a single thread spring 

 from the base of the brain, but in Bdellostomae he could easily de- 

 monstrate its presence. W. MtJLLEii (28) has clearly demonstrated 

 its existence, believing he had found a decussation of the fibres in 

 the base of the brain and Retzius (38) agrees with this view. 

 Sanders (41) describes the nerve as consisting of connective tissue 

 containing a few nerve fibres, and at the point where they join the 

 brain there is a sort of confused vortex of fibres which looks as if 

 it might possibly be the remains of a chiasma. 



According to my own researches the description given by 

 Sanders (41) seems to be the most correct. The nerve on entering 

 the brain seems to split up brushlike amongst an indefinite cluster 

 of small nerve cells situated round the nerve entrance; the fibres 

 from the two nerves do not seem to decussate, even if single fibres 

 cross each other. The decussation W. Müller describes, and of 

 which the fibres should continue laterally and caudally, is evidently 

 nothing else than the Commissura postoptica which I have described 

 above. I doubt that the optic tract continues further than to the 

 collection of nerve cells mentioned above and I have never been 

 able to follow any fibres further. 



