The finer Anatomy of the Nervous System of Myxine glutinosa. 381 



canal of the cord and enter the base of the Mesencephalon, then 

 turn straight upwards, where they spread in a fanlike way and lose 

 themselves amongst the cellnuclei of the Tectum opticum. 



Besides the two decussations already mentioned as existing in 

 the Mesencephalon of Myxine I have recognised three commissures. 



The Commissura posterior is situated close behind the Gan- 

 glia habenulae and is rather voluminous but not distinctly defined. 

 Mayser (26) in Teleosts and Haller (17) in Mustelus describes this 

 commissure as consisting of two kinds of fibres, one part of the 

 commissure having thick ones, the other part fine. In Myxine, 

 however, only one kind of fibres are to be seen and they seem to 

 connect both sides of the anterior Mesencephalon; if any fibres are 

 collected from the longitudinal tracts as is the case in Mustelus I 

 am unable to say, as I could not obtain any successful impregna- 

 tions of this part of the brain with the Golgi method. 



The Commissura ansulata or as it perhaps more correctly 

 ought to be called Decussatio ansulata, is situated about half 

 way between the Aquaeductus Sylvi and the base of the Mesence- 

 phalon in the way of the Fasciculus retroflexus. It seems to con- 

 sist of two different kinds of fibres, one coming from the Oblongata 

 and its lateral tracts, which on arriving beside the commissure bends 

 in right angle and decussates, passing towards the dorsolateral part 

 of the Tectum opticum; the other kind being real commissure fibres, 

 connecting the two lateral parts of the Mesencephalon. Mayser (26), 

 who describes this commissure in the Teleostei, states that in these 

 fishes its fibres are chiefly derived from two sources, the lower 

 Pyramides and the intermediary system, which should be situated 

 between the first named and the Lemniscus of Reil. IIaller (17) 

 considers the Lemniscus of Reil the non- decussating, the inter- 

 mediary fascicle the decussating, portion of one longitudinal system. 



The Lamina commissuralis Mesencephali is a true commissure. 

 It is very distinct in Myxine and is found just over a large part of 

 the Aquaeduct, where it constitutes a kind of slightly arched roof. 

 In Golgi preparations it is easy to demonstrate that the fibres of this 

 commissure arise from the ganglian cells on both sides of the Aquae- 

 duct, each ganglion sending its processes into the ganglion of the 

 opposite side. » 



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