The finer Anatomy of tlie Nervous System of Myxine glutinosa. 379 



other pair, or the Corpora quadrigeraiDa posteriore, are situated in 

 the caudal division of the Mesencephalon. They are a little smaller 

 than the other pair, but have the same form, are in their most 

 dorsal part separated one from the other, but a little deeper they 

 join each other in the middle line. 



Towards the level of the Aquaeduct they commence getting less 

 defined and are connected with the Nuclei praetectales through rows 

 of cells, mostly along the median line. In the basal part of the 

 Mesencephalon I have only found two distinct ganglia or cellnuclei, 

 one of which, the Ganglion interpedunculare, is situated rostrally, 

 close to the basal surface of the brain. It consists of small nerve- 

 cells, some of triangular form, resembling the motorcells of the Me- 

 dulla oblongata but much smaller, and, like them, they stain rather 

 black with Iron-haematoxylin. The other ganglion, or »Anterior 

 ganglion« as called by Sanders, is situated just below, or on the 

 level of, the Aquaeductus Sylvi, and consists of relatively large cells 

 about half the size of the giant cells in Petromyzon, with which 

 they are probably homologous. Their processes run caudally, as 

 is the case in Ammocoetes (Mayer) and the processes join the Trac- 

 tus tecto-spinales as the MuUerian fibres. Mayer (25) states that 

 the giant cells which are found in the Mesencephalon and in the 

 region of the cranial nerves in the Medulla oblongata of Petromyzon 

 all have connection with the spinal cord and substitute functionally 

 the ganglian groups in higher animals. 



The Fasciculus retroflexus, or perhaps more correctly, the 

 Fasciculi retroflexi, connect the Ganglia habenulae with the base 

 of the Mesencephalon. Sanders (41) describes them in Myxine as 

 two bundles of fibres which pass downwards and backwards from 

 the inferior side of the ganglion and terminate in a small accumu- 

 lation of smooth neuroglia with nuclei scattered through it; in Myxine 

 they did not seem to stop at the Ganglion interpedunculare but he 

 could find them in sections cut more posteriorly. In Petromyzon 

 Ahlborn (2) found the two fasciculi asymmetrical, the right one con- 

 siderably stronger than the left; they pass towards the Ganglion 

 interpedunculare, but Ahlborn has never seen any fibres enter this 

 ganglion and does not believe it probable that such is the case. 



Mayser (26), in his work on the brain of the Teleostei, con- 

 siders it unnecessary to divide the fascicle into two. The fascicle 

 pierces the Commissura ansulata on its way, a small medial part 

 of the fibres decussate before they lose themselves in the rostral 



Morpliolob'. Jahrbuch. 29. 26 



