37S John F. Holm 



Mesencephalon. 



With the exception of the Medulla oblongata the Mesencephalon 

 is by far the largest part of the brain and consists of the two di- 

 visions situated between the Ganglia habeniilae and the Medulla 

 oblongata. The relatively large size of the Mesencephalon and Me- 

 dulla is probably a very primitive feature, as in embryos of Bdello- 

 stoma, which is closely related to Myxine, the most extraordinary 

 feature is the great length of the midbrain and Medulla (Bashford- 

 Dean [6]). Two different parts are discernible in the Mesencephalon 

 of Myxine, one situated dorsally of Aquaeductus Sylvi and generally 

 called Tectum opticum in the brains of lower animals on account 

 of the fact that the endings of the nervous opticus are found 

 there ; the other situated ventrally of the Aquaeduct, the Pars basalis 

 mesencephalii. According to Sanders (41) the structure of the 

 Mesencephalon of Myxine strongly resembles that of the »Cerebellum« 

 as he terms the posterior part of the brain. He describes the struc- 

 ture as consisting of bipolar cells occasionally arranged in such a 

 way that their long axis are directed perpendicularly to the outer 

 surface, the cells sometimes placed so as to make rows of cells 

 indistinctly discernible and the whole of the gray matter is bounded 

 externally by a layer of smooth neuroglia. In some places towards 

 the inner side of this layer the fibrils of the network form trans- 

 verse, concentrically arranged bundles. In the Teleostei the anatomy 

 of the Tectum opticum is described as being more complicated, and 

 according to Mayer (26) there are no less than seven diflerent 

 layers to be seen. In a horizontal section through the Tectum opti- 

 cum of Myxine two pairs of well-defined nuclei are seen, each pair 

 corresponding to one division of the Mesencephalon. The anterior 

 pair, or the Nuclei praetectales, are found in the rostral part of 

 the Mesencephalon symmetrically to the middle line close to the Gan- 

 glia habenulae. They are of an oval form, the longer axis of the 

 oval nearly parallel to the axis of the brain. They are rather 

 large and leave only a relatively small layer of smooth neurolgia 

 laterally, but a somewhat broader one caudally, which separates 

 them from the other pair. The cells which constitute these ganglia 

 have the same form and character as those I have described in the 

 Nucleus rotundus but are not of quite the same size. According to 

 Edinger (9), this pair of nuclei are to be constantly found in lower 

 vertebrates, but have not so far been found in the Mammalia. The 



