150 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



In the lamina terminalis, a little below the interventricular foramen, 

 an anterior commissure, connecting the two hemispheres; a poster- 

 ior commissure in the roof at the junction of di- and mesencephalon; 

 and a superior or habenular commissure associated with the habenular 

 ganglia and lying between the epiphysial structures and the velum 

 transversum. In the amphibia, with the differentiation of the hip- 

 pocampal region, a dorsal or hippocampal commissure appears in 

 the lamina terminalis, just dorsal to the anterior commissure, connecting 

 the hippocampi of the two sides. This persists, with slight modifica- 

 tions, through the sauropsida and monotremes, but in the higher 

 mammals it is subdivided into the hippocampal commissure proper and 

 a more anterior portion, the corpus callosum. This corpus callosum 

 is only in part the result of the division, but is more largely formed by 

 new fibres, anterior to the hippocampal portion, connecting the neopal- 

 lium of the two sides. The result is a broad band (the largest com- 

 missure in the brain of man) which invades the intercerebral fissure 

 from behind. In the lower vertebrates a few fibres pass downward from 

 either side of the cerebellum beneath the fibre tracts of the medullary 

 region and so to the other side of the cerebellum. In the mammals 

 these are greatly increased in number, forming a marked projection on 

 the lower surface, the pons (Varolii), the prominence of which is in- 

 creased by the great development of ' nuclei ' in the medullary floor. 



The longitudinal tracts are more numerous and more complex. 

 As will be recalled, there are dorsal, lateral and ventral columns in 

 the spinal cord. These extend into the medulla oblongata and there 

 pursue different courses. 



Some of the fibres of the dorsal columns end in connection with the nuclei of 

 the medulla (p. 144), while others unite with fibres from the lateral column and with 

 some from the oliva to form an enlargement, the corpus restiforme, and then 

 bend upward (posterior peduncle) to enter the cerebellum. Other fibres from 

 the lateral column, together with some from the dentate nucleus, enter the cere- 

 bellum farther in front as the anterior peduncle, those from the dentate nucleus 

 pass forward to the roof of the mid-brain, some terminating in the optic lobes, 

 others continuing to the cerebrum. In this forward course, after leaving the cere- 

 bellum, the fibres cross (decussate), those from the right side passing to the left 

 side of the brain farther forward and vice versa. In the dorsal region of the medulla 

 there is a short tractus solitarius (fasciculus communis) derived from fibres 

 from the seventh to tenth nerves and extending no farther forward than the seventh. 



In the higher vertebrates there are the crossed and the direct pyramidal tracts 

 on the ventral side of the medulla, the direct being continuations of part of the ven- 

 tral columns, the crossed of the deeper lateral columns. In the medulla these en- 



