332 John F. Holm 



Medulla Oblongata and the Metameric Cranial Nerves. 



The Medulla oblongata is as important a part of the brain of 

 Myxine as the Mesencephalon and in an embryological state is pro- 

 bably even more conspicuous than the latter, to judge by B. Dean's (5) 

 figures of Bdellostoma embryos, where, in phases of the development, 

 the volume of the Medulla oblongata alone seems to exceed that of 

 the rest of the brain. Sanders (41) has given an account of the 

 anatomy of the Medulla oblongata of Myxine, in which he finds two 

 pairs of ganglia, one large-sized pair situated close to the middle 

 line and which he terms »the central ganglia«, the others situated 

 near the outer edge of the Medulla oblongata, towards the ventral 

 side. He believes that these last mentioned ganglia, which he terms 

 >the latero-ventral ganglia« extend all along each side of the lateral 

 margin of the Medulla oblongata, in one continuous column of cells 

 of various thickness, from the entrance of the Trigeminus in front 

 to that of the Vagus behind. In connection with the central ganglia 

 he finds a large fountain-shaped decussation of fibres and he supposes 

 that this decussation connects the ventral cells of the central gang- 

 lion of one side with the dorso-lateral region of the other. Of cranial 

 nerves he finds, besides the first and second nerves, the fifth, seventh, 

 eighth and tenth nerves present and a little posterior of these he 

 found, in the transition part of the Medulla oblongata, another nerve, 

 which he thinks may possibly be homologous with the root in Petro- 

 myzon which Schlemm and D' Alton (42) considered to be the 

 hypoclossal, but which he thinks more likely to belong to the Vagus 

 group. 



G. Retzius (38) found the same nerves as Sanders, but considered 

 the post-vagal one to be a spinal nerve. In Myxine and Bdellostoma 

 FüRBRiNGER (12) has made some observations on the Vagus and 

 post-vagal nerves, which latter he calls »Spino-occipital nerves«. I 

 will take his views into consideration when dealing specially with 

 the Vagus. 



During the gradual transition of the spinal cord into Medulla 

 oblongata, the characteristic roof-like form of the cord changes into 

 a nearly circular shape (Fig. 23). The gray matter, which is situated 

 as two laminae, one each side of the central canal and joins round 

 this in the middle line, ventrally nearly reaches the surface of the 

 cord. It becomes less extended the further up the Oblongata the 

 sections are cut and its form becomes more like the usual one in 



