704 Comparative Morphology of Chordates 



tefistic of the brains of all other vertebrates are well differentiated. 

 Each is the seat of important nervous mechanisms. The telencephalon 

 is concerned mainly with the olfactory sense. The visual centers are in 

 the mesencephalon. The integumentary senses, taste and other visceral 

 senses, equilibrium, and (if present at all) hearing have their primary 

 centers in the myelencephalon. In the metencephalon the cerebellum 

 is an important coordinating mechanism. In the telencephalon the 

 differentiation of nervous structures takes place mainly in the ventral 

 and lateral walls, forming the corpora striata. In teleosts the dorsal 

 wall (pallium: Fig. 324) is quite devoid of nervous elements, and in 

 other fishes the pallium is only moderately thickened by the presence 

 of nervous elements. In amphibians, reptiles, and mammals, the nerv- 

 ous mechanisms of the pallium progressively increase in number and 

 importance until, in many mammals, the pallial tissue not only out- 

 weighs the corpora striata (the ventral masses of the hemispheres), but 

 may even exceed all the rest of the brain. Functionally, the pallial 

 mechanism dominates all the rest of the brain. It seems likely that in 

 the most primitive fishes the pallium was entirely non-nervous, as it is 

 in modern cyclostomes. The nervous pallium, therefore, and especially 

 in its highly developed state in mammals, can be conceived as a "new" 

 acquisition of the vertebrate brain. Meanwhile, the corpora striata 

 and all regions posterior to them have undergone relatively little 

 change. Collectively, they constitute the persisting original or "old" 

 brain. Kdinger proposed to call it the '■'palaeencephalon," while the 

 elaborated pallium, together with some other "new" parts closely 

 related to it, he called the "neencephalon." 



In the reptilian pallium the bodies of the nerve-cells are fairly 

 definitely segregated into a superficial layer of "gray substance," the 

 cerebral cortex, the deeper "white substance" consisting mainly of 

 nerve-fibers. This cortex is much more prominent in mammals, the 

 "gray" layer being relatively thicker and more definitely delimited. 

 In ungulates, carnivores, and primates the cortical tissue is especially 

 voluminous and its manner of disposition in relation to the underlying 

 "white substance" is peculiar. The cortical layer is of approximately 

 uniform thickness throughout its extent but it, together with the 

 adjacent "white" material, is as if it had been elaborately folded in- 

 ward and outward, the adjacent projecting masses being separated by 

 narrow, deep depressions (Figs. 521, 523). The projecting masses are 

 called convolutions or gyri, and the intervening clefts are sulci. An 

 especially deep sulcus is called a fissure. Certain major fissures, 

 definitely located and of common occurrence, are regarded as dividing 

 the cerebral hemispheres into "lobes." Most conspicuous is the 



