THE COMMISSURES 291 



roof is a membranous chorioid plexus except in some fishes, in which 

 the large acousticolateral areas fuse over the ventricle, with some 

 decussating fibers. In the region of the calamus there is a dorsal 

 crossing of visceral sensory fibers (com. infima of Haller) and of 

 somatic sensory fibers between the funicular nuclei. This dorsal com- 

 missure, reduced in size, is present through the entire length of the 

 spinal cord. 



The ventral series of commissures is concentrated in the anterior 

 commissure of the telencephalon, the chiasma ridge of the diencepha- 

 lon, the so-called ansulate commissure of the midbrain; and pos- 

 teriorly of this it extends continuously as the ventral commissure of 

 the rhombencephalon and spinal cord. The ventral decussations of 

 Necturus from the tuberculum posterius to the spinal cord were 

 described in 1930 ('30, p. 89) and the other commissures in papers 

 before and after that date. 



The arrangement of the telencephalic commissures of the Am- 

 phibia differs radically from that of all animals higher in the scale. 

 As seen in median section (fig. 2), the anterior and hippocampal com- 

 missures do not cross in or above the lamina terminalis but in a high 

 anterior commissure ridge which projects upward from the floor 

 posteriorly of the interventricular foramina. This ridge is separated 

 from the lamina terminalis by the wide precommissural recess, or 

 aula, which is the vestibule of the interventricular foramina (fig. IB). 

 Both pallial and subpallial parts of the hemispheres contribute fibers 

 to the commissures in this ridge, and many fibers from the olfactory 

 and hippocampal areas also cross in the habenular commissure. 



The peculiar arrangement of the anterior and hippocampal com- 

 missures, the lamina terminahs, and the related chorioid plexuses of 

 Ambly stoma has been described and illustrated ('35). It is explained 

 by the topography at the di-telencephalic junction and the presence 

 of unusually wide interventricular foramina. The explanation of this 

 topography, in turn, is to be^ sought in the phylogenetic ancestors of 

 existing Amphibia. In Protopterus as described by Rudebeck ('45), 

 pallial formation is confined to the lateral wall of the hemisphere, and 

 the hippocampal commissure passes down behind the foramen to 

 cross in close association with the anterior commissure in essentially 

 the same way as in amphibians. In view of the close similarity of 

 development of the brains of amphibians and lungfishes, it is prob- 

 able that the ancestral amphibian resembled Protopterus in this 

 region. 



