230 



NERVOUS SYSTEM OF VERTEBRATES. 



become more important relative to its primary connections than 

 in lower fishes. The visceral nuclei (Figs. 114, 115) are situated 

 somewhat farther ventrally and are both actually and relatively 

 much larger than hi selachians owing to the greater development 

 of the gustatory organs. The position of the commissure between 

 the gustatory nuclei is very different from that in selachians. 

 Instead of lying in the roof it lies deeply imbedded in a massive 

 median structure the valvula cerebelli which largely fills up 

 the ventricle of the cerebellar and mesencephalic segments. The 

 position of the commissure is shown in Figure 91, which represents 



Valvula 



Inferior comrriss. of cerebellum 



Tectum mesenc. 



Tr. lobo-cerebel. 

 Radix mesenc. V 



Tr. tecto-cerebel. 



Tr. tecto-bulbaris 



Tertiary gust. tr. 



IV. 



Tr. lobo-bulbaris 



Tr. mamm.-bul- 



baris 



Brachium conjunct. Tr. haben.-peduncularis 



FIG. 116. Transverse section through the midbrain of the sturgeon. 



a sagittal section of the brain of a teleost a few days after hatching 

 (see also Figs. 115 and 116). The form of the adult cerebellum 

 of teleosts is essentially the same as this. The place of crossing 

 of the commissure and of the IV nerve is homologous with the 

 velum in selachians and it is clear that the valvula has been formed 

 by an infolding of the roof in the region of the velum. Since the 

 structure of the valvula is essentially that of the cerebellum it may 

 be assumed that the typical layers of the cerebellum have spread 

 forward into the valvula. The infolding of the roof, however, 

 came about undoubtedly on account of the great size of the second- 



