Herrick, Brain of Certain Reptiles. 125 



cephalon — a fine bundle which passes from the mesal wall of the 

 prosencephalon to the supracommissure. 



3. Diencephalic radix of the olfactory (our tract of the 

 supracommissure). It is described by Edinger (following Bel- 

 lonci) as arising in the tuber and extending to the supracommis- 

 sure. This is constant in all the vertebrates we have examined 

 though its absolute continuity with the olfactory radix is not so 

 clear. 



4. Edinger thinks it very probable (influenced by studies 

 in Selachii) that fibres arise in the lateral part of the mantle 

 which pass caudo-ventrad, passing ectad of the chiasm and cross 

 to the other side behind the latter. After decussation these 

 fibres pass to the roof of the mesencephalon. In other words 

 the course is the same as that of the "mantle bundle." He 

 says " Bei den von mir untersuchten Arten war nur das Stiick 

 des Zuges, welches hinter dem chiasma liegt, gang sicher zu 

 stellen, das frontale ende, der zug aus der Decussatio postoptica 

 iiber den Sehnerv hinweg in das Vorderhirn liess sich aus einzel- 

 nen Befunde vermuten, aber nicht durch solche beweisen." Os- 

 born described a tract from the optic nerve into the hemisphere 

 which the author supposes to be the missing part of the mantle 

 bundle. 



The decussatio postoptica has been identified with the com- 

 missura transversa, but another decussation lying dorsad of it 

 deriving its fibres from the caudad part of the mid-brain, Eding- 

 er considers equivalent to this or v. Gudden's commissure. 

 (There is some confusion due to the fact that on the same plate 

 the three names are used, viz: " Decussatio postoptica," de- 

 cussation of the mantle bundle, and "decussatio transversa," 

 and seem to apply to different tracts.) 



From the diencephalon Edinger derives the following tracts : 

 I, Tractus thalami frontalis springing from the dorsal region of 

 the thalamus and passing cephalo-ventrad and then suddenly 

 caudad (its terminus caudad was not made out). 2. Tractus 

 thalami caudalis, which springs from the ganglion caudale thai- 

 ami and passes in the same direction as the preceeding but, be- 

 cause non-medullated, its fibres cannot be traced beyond the 



