8 NAKAGAIVA. [Vol. IV. 



3. Functional Relation of the Optic Lobe Layers. 



AFF 



EFF 



FROM c ;rebhum 



Diagram sJiozi'ing the supposed 7-e!iitions of the layers of the optic lobes. Each lobe is 

 composed of afterent and efferent portions; but the figure represents (for the 

 sake of clearness) the afferent portion of the one and the efferent portion of the 

 other side. 



The above diagram is intended to represent the conclusions 

 I have reached by the preceding investigations. 



The optic nerves decussating at the chiasma sweep around and 

 enter the lobes in the opposite sides, at the distal and posterior 

 portions {Tit"). These tracts can be traced to layers A and C, 

 as has been described above. Many of the fibres (lit'") of the 

 layer A pass over the tectum opticum and enter the thalami, 

 and thence probably into the cerebrum.^ As there are descend- 

 ing fibres (/;) from the layer B, it is natural to infer that they 

 are those of the optic tract of the layer A turned out of their 

 course by the agency of cells constituting the layer B. Most, 

 if not all, of the fibres of C are turned from their course by the 



^ I have observed in Tropidonotiis the tract going direct (without passing through 

 the tectum opticum) to the thalami (Fig. 12, ///'). In other forms my observation 

 has not been complete. 



