PROSENCEPHALON OF SPINAX AS A TYPE OF ELASMOBRANCH FORE-BRAIN. 499 
Summary of Fibre Tracts. 
I have located the following nerve fibre tracts in the prosencephalon of 
Spinax : — 
1. The tractus olfactorius, the course of which in the fore-brain of Spinax agrees 
with that described by Johnston in other Selachian types. 
2. The tractus olfacto-corticalis lateralis (fig. 5), which is not developed to the 
same extent in Spinax as in other types. 
3. The tractus olfacto-corticalis medio-dorsalis (fasciculus marginalis) (fig. 6), the 
distribution of which apparently slightly differs from that in Acanthias. 
4. The tractus cortico-medialis (Botazzi, Kappers, Gajjpp) ; tractus olfacto- 
corticalis septi (Johnston), which docs not extend so far dorsally in Spinax 
as in Scyllium (fig. 7). 
5. The tractus olfacto-corticalis medialis cruciatus (fig. 8), of which there is only 
the slightest indication in Spinax. 
6. The tractus dlfacto-hypothalamicus (fig. 9), the course of which mainly 
corresponds with that described by Johnston in other types. 
7. The tractus taeniae (Edinger, Kappers) ; tractus olfacto-habenularis (Johnston) 
(fig. 11). 
8. The tractus pallii (fig. 10) has been frequently noticed by other authors. 
9. The tractus cortico - habenularis (Johnston) ; tractus olfacto - habenularis 
(Kappers). This tract is not well defined ; it is apparently more developed 
in Scyllium than in Spinax and Acanthias. 
10. Commissura hippocampi (fig. 10). Fibres crossing over to the opposite side 
have not been definitely observed in the brain of Spinax. 
11. The tractus medianus (Kappers and Theunissen) (fig. 8), which is held by 
Johnston to be the homologue of the “ fornix” of Beptilia and Mammalia, 
is not developed dorsally in Spinax as described for Scyllium and Acanthias. 
12. The commissura pallii posterior (figs. 11 and 12). A large bundle of non- 
medullated nerve fibres. 
13. The basal fore-brain bundle, consisting of a median portion (tractus strio- 
thalamicus, Edinger) and a lateral portion, the tractus hypothalamicus 
lateralis (fig. ll). 
14. The corpus callosum. The fore-brain of Spinax does not show a tract and com- 
missure such as has been described and figured for Scyllium. 
With the object of curtailing the letterpress of this paper a list of the literature on 
the Elasmobranch brain is not given, but reference may be made to the bibliography 
at the end of Johnston’s Memoir on the “Telencephalon of Selachians,” already 
cited. Finally, the author would express his regret that, for the same reason, many 
points worthy of discussion and elaboration have had to be omitted. 
TRANS. ROY. SOC. EDIN., YOL. LII, PART II (NO. 18). 
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