496 
DR J. STUART THOMSON ON THE MORPHOLOGY OF THE 
observation. • The condition in Spinax thus appears to resemble that of Scyllium 
and Raja more than that of Acanthias in this respect. 
Commissura pallii posterior. — This is a large bundle of non-medullated nerve 
fibres, the position of which is indicated in fig. 12. Many authors have held that a 
part of the tractus tsenise (see p. 495) forms a true commissure in various fishes. It 
is apparently a commissure of the pallial area, and is probably homologous with the 
posterior pallial commissure in amphibians and reptiles. 
Tractus medianus (Kappers and Theunissen) ; median-bundle (Edinger) ; fasci- 
culus cortico-medialis and median bundle (Botazzi).— This tract has been the subject 
of much difference of opinion, and I still have some doubts, despite the certainty 
entertained by Johnston, as to this tract really being the homologue of the “ fornix ” 
of Reptilia and Mammalia, but it is possible that Spinax does not form such a good 
subject as Scyllium for tracing the course of this tract. Kappers and Theunissen 
have apparently been led into an error regarding the course of the tractus medianus 
by a wrong identification of certain fibres, which I have not space enough to enter 
into in this paper. Johnston writes (p. 23) : “ The tractus medianus undoubtedly 
arises in the roof, descends through the septum and runs to the hypothalamus. 
This is the description of the fornix in reptiles and mammals, and I think there can 
be no doubt of the homology.” I am certain that I am dealing in Spinax with the 
same tract as that described and figured by Johnston in Scyllium and Acanthias 
under the term “fornix,” for I largely agree with his description of the ventral part 
of its course and its relations to the so-called tractus cortico-medialis septi (fig. 8). 
The part with which my investigations on Spinax does not correspond is in regard to 
the more dorsal part of its course. He writes of fibres which collect from the dorsal 
part of the primordium hippocampi, pass meso-ventrally, and form two dense bundles 
medial to the inner walls of the lateral ventricles and dorsal to the zona limitans 
medialis. They cross the zona limitans, pass caudally over the anterior commissure, 
and enter the medial fore-brain bundle. The part of the bundle which they enter 
runs to the hypothalamus. I have, however, been unable to detect in the fore-brain 
of Spinax fibres of this tract so far dorsal as he has described and figured in other 
types. The tractus medianus and the tractus cortico-medialis are for part of their 
courses situated so close together in the “ septum ” that it is almost impossible to 
say that they are separate tracts. When, however, one examines sections further 
posteriorly, it is clear that these two tracts are distinct, and arise ventrally from 
different parts. The tractus medianus lies nearer the median line than the tractus 
cortico-medialis, and I have definitely traced fibres of the former tract running to 
the medial fore-brain bundle ; further, when traced posteriorly after the fibres of the 
tractus cortico-medialis have disappeared, those of the tractus medianus may still be 
seen. The ventral ends of the fibres of the tractus cortico-medialis pass to the ental 
medial end of the tuberculum olfactorium. The fibres of the tractus medianus, as 
seen in transverse section, run in a different direction from those of the tractus 
