Pemhektox, Structure of the Optic Thalami. 139 



peduncular fibres that descend from the thalamus and pass 

 partly through the base of the optic tract, also connecting 

 with the inferior lobes; (4) the stratum zanole, which does 

 not come from the optic nerve, but from the outer layer 

 of the anterior part of the tectum. Some fibres of the 

 stratum zonale assist in the formation of the inferior commis- 

 sure, and intertwine themselves in a complex manner with 

 the optic and with the thalamus fibres that originate partly 

 in the inferior commissure. 



In the amphibians the optic tracts are principally confined 

 to the median portions of the tectum opticum. According 

 to Osborn [■'" p. 81, 83] they can be traced as far as the pos- 

 terior portion of the lobes. A second tract arises from a 

 mass of cells imbedded in the thalamus; a third enters the 

 hemisphere directly. These are the main sources of origin. 

 The course of the optic tract in reptiles, as described by 

 Rabl-Riickard is quite similar to the course of the same tract 

 in amphibians. 



Gadow says ['', p. 378] that in the birds, the coverings of 

 the optic lobes join on either side to form the optic tract, 

 some of the fibres passing by the thalamencephalon and 

 others entering it. After examining some of my own 

 sections of the pigeon's brain, it seems to me that whatever 

 fibres terminate in the thalamus are those coming from the 

 chiasma; I discovered none from the corpora bigemina that 

 terminated there. In birds, according to Bellonci ['', p. 17], 

 the optic tract divides into two portions, an upper anterior 

 one, and a lower posterior one. The division is not an entire 

 separation, a layer of nerve-fibres lying between them. Of 

 course they correspond to the optic nerve-roots of the fishes, 

 amphibians and reptiles. The anterior one covers a swelling 

 that no doubt corresponds to the geniculate body of the 

 mammals. The grey substance of the thalamus is traversed 

 by optic fibres as follows: i. The lower ones, which, having 

 separated from the chiasma, and being joined in small 

 bundles, penetrate the lower grey substance of the third 



