Herrick, Morphology of Nervous System. 17-^ 



(4) is a tract of nerve fibres passing to the " crura of the 

 optic lobes" or ental optic tract. The outer part of (3) 

 might be divided in the same way. For a description of the 

 minute structure of the tectum in teleosts, see Sanders.(') 

 Bloodvessels are very numerous, indicating a high degree of 

 activity in this region. The necessary regenerative or nutri- 

 tive cells seem to be collected in the tori. 



From the base of each optic ventricle there is a projec- 

 tion w^hich has been called torus semicircularis. Sanders 

 says:(-) " The tori semicirculares may be considered as the 

 anterior termination of the medulla oblongata; they are 

 tuberosities of a more or less semicircular shape, which pro- 

 ject into the floor of the ventricle of the optic lobe; they are 

 principally composed of grey matter, through which the 

 bundles of the crura lobi optici pass on their way to the in- 

 ternal surface of the tectum." To this it is to be said 

 that these lobes must be regarded as an inherent part of the 

 mesencephalon, being provided with a slightly modified ven- 

 tral accumulation of the cell -layer constituting the fourth of 

 the above-mentioned layers of the tectum. The structures 

 are those already familiar in reptiles, and called (rather un- 

 fortunately) by Rabl Riickhard the coUiculi.i^'') 



The roof of the aqueduct is thickened and thrust far for- 

 ward under the optic lobes, or better, the optic lobes are 

 thrust so far caudad that they over-arch the valve of Vieus- 

 sens or valvida ccrebelli. The presence in this body of the 

 decussation of the fourth nerve is sufficient proof of the cor- 

 rectness of the homology. 



In the sturgeon the relations are greatly disguised by the 

 forward protrusion of the great medio-cephalic lobe of the 

 cerebellum within the common ventricle of the optic lobes. 



1 L. c, p. 751. 



2 L. c, p. 754. 



3 Sanders, op. cit , p. 768, "the torus semicircularis I would, with some doubt, refer 

 to the corpus geniculatum externum." Fritsch regarded the deeper part of this body as 

 the corpus quadrigeminum and the superficial part as the thalamus. 



