710 Comparative Morphology of Chordates 



spheres bulge forward far beyond the anterior end (lamina termi- 

 nalis) of the median neural tube from which they have grown out 

 (Fig. 522). The "new" commissure, the corpus callosum, extends 

 across the very narrow median external space between the two hemi- 

 spheres. It is so shaped (as best seen in sagittal section: Figs. 522, 528) 

 that a small part of this space is completely enclosed between the 

 corpus and the adjacent external surfaces of the brain. The earlier 

 anatomists recognized four cavities or "ventricles" in the human 

 brain and counted this thin little space beneath the corpus callosum as 

 the "fifth ventricle." But it differs from the other "ventricles" in 

 that they are regions of the lumen of the primary neural tube, while 

 the "fifth ventricle" is a secondarily enclosed bit of space external to 

 the tube. 



Brains of monotremes and marsupials lack the corpus callosum. 



The mammalian olfactory lobes, anteroventral outgrowths of the 

 telencephalon, are well developed in proportion to the extent of the 

 nasal olfactory surfaces. At their most, however, they appear in- 

 significant in contrast to the otherwise enormously enlarged telenceph- 

 alon (Figs. 521B, 522, 524A). Whereas in Anamnia the olfactory 

 mechanism constitutes most of the corpora striata, in reptiles and 

 mammals progressively more of its territory is occupied by association- 

 tracts and centers related to the various centers in the rear parts of 

 the brain. The primary olfactory mechanism may not decrease in 

 absolute value, but it loses its primary sensory independence as it 

 comes to be more closely tied in with all the other sensory mecha- 

 nisms. 



The lateral walls, thalami (Fig. 529), of the diencephalon are 

 occupied by nervous centers whose relations in Anamnia seem to be 

 mainly olfactory (possibly, to a small extent, visual in amphibians), 

 but in Amniota, and becoming most prominent in mammals, important 

 visual centers develop in the thalami, and the various thalamic centers 

 have extensive connections both backward and forward. A consider- 

 able proportion of the fibers of the mammalian optic nerves, instead of 

 going into the visual centers of the mesencephalon, pass to centers in 

 the posterodorsal regions of the thalami, and these thalamic centers 

 are connected with the cerebral cortex. 



The thin non-nervous roof of the diencephalon produces a median 

 dorsal outgrowth, the pineal body, whose position corresponds to 

 that of the pineal eye or pineal organ of Anamnia and reptiles (Fig. 

 142), but its structure has nothing suggestive of an eye and its tissue 

 is apparently glandular, but of unknown function (Figs. 522, 528). 



