OF DENISON UNIVERSITY. 45 



/, descending fornix tract shortly after leaving the fornix body; op, optic tract; 

 M t Meynert's bundle; III, peduncular nidulus of the third nerve. 



Figs. 4, 5, 6, 7, 8. A series of transverse sections through the brain of 

 Geomys bursarius, drawn to a uniform scale, which is found under Fig. 5. 



Fig. 4. Transverse section through the olfactory lobe; rxl, fibres of the 

 radix lateralis (really the combined superficial olfactory tracts, gr, laminated 

 granular layer; g, ganglionic layer; n, neuroglia, or gelatinus layer. 



Fig. 5. Transverse section through the olfactory cms; c, cortex of the olfac- 

 tory crus; pc, precommissural tract of the olfactory; rxl, radix lateralis. 



Fig. 6. Transverse section at the point of union of the olfactory crus with 

 the hemisphere; rxl, radix lateralis; pc, precommissural tract; rxm. radix 

 mesalis, or medial olfactory root. 



Fig. 7. Transverse section through the hemisphere cephalad of the cal- 

 losum; rxl, radix lateralis; pc, precommissural tract; rxm. radix mesalis. 



Fig. 8. Transverse section just cephalad of the anterior commissure ; rxl, 

 radix lateralis; pc, precommissural tract; f, fornix body; c, callosum. 



Figs. 9, 10, n, 12. A series of horizontal longitudinal sections through the 

 brain of Geomys bursarius, illustrating tbe commissural apparatus of the 

 habenae and optic lobes, all drawn to the same scale as Fig. 2. 



Fig. 9. Horizontal section through the thalamus and corpa quadrigemina 

 at the level of the posterior commissure, which lies in the centre of the figure; 

 /, taenia thalami beginning to descend from the habena; op. optic tract. 



Fig. 10. Horizontal section through the entire brain immediately dorsad of 

 Fig. 9; fibres of the superior commissure arch caudo-dorsad from the habenae to- 

 ward the meson; at the cephalic terminus of the corpora quadrigemina are 

 fibres of the commissure of the optic lobes, just cephalad of which is a small tri- 

 angular depression which marks the boundary between the optic lobes and the 

 thalamus. 



Fig. II. Horizontal section through the thalami and corpa quadrigemina 

 immediately dorsad of Fig. 10; sc, supracommissure; op, optic tracts; a few scat- 

 tered fibres of the commissure of the optic lobes appear in the center of the 

 figure. 



Fig. \i. Horizontal section through the corpora quadrigemina taken near 

 their dorsal limit; the dorso-median fissure has separated the cephalic members; 

 the caudal members are connected by a strong bundleof fibres of the commissure 

 of the optic lobes. 



Fig. 13. Horizontal longitudinal soction through the brain of Erethizon 

 dorsatus dorsad of the last, slightly inclined ventro-cephalad; ////, taenia thalami; 

 op, optic tracts. 



Fig. 14. Transverse section through the brain of Erethizon dorsatus at the 

 region of the posterior commissure, which lies immediately above the third 

 ventricle; M, Meynert's bundle; /, pyramidal tract 



Fig. 15. Perpendicular longitudinal section through the brain of Erethi- 

 zon dosatus taken slightly laterad of the meson; sc, supracommissure; M. Mey- 

 nert's bundle; III, exit of the third nerve with the peduncular nidulus. 



