1 70 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



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left of the letters d in the side view of the alligator brain, fig. 178) which is the 

 first appearance of the flocculus. 



AVES. — The brain of the bird is short, broad and more highly specialized, 

 and it retains all three flexures to some extent throughout life (p. 151). The 

 cerebrum is triangular and high, and is usually marked on the dorsal surface 

 by a groove, the vallecula (fig. 179). Its size is largely due to the development 

 of the corpora striata, in which there is an evident difi'erentiation of parts. The 

 hemispheres reach back nearly to the cerebellum, thus forcing the optic lobes 

 to a lateral position so that in some they are invisible from above. This ex- 

 tension of cerebrum and cerebellum results in the cover- 

 ing of the 'twixt-brain so that it cannot be seen in a dor- 

 sal view, though the rudimentary pinealis which it bears 

 is seen in the angle between the cerebral and cerebellar 

 hemispheres (fig. 179). The cerebellum is large and its 

 medial portion is transversely furrowed, this part con- 

 stituting the vermis, while the lower lateral parts form 

 the flocculi. The medulla oblongata is very short and 

 the fossa rhomboidalis is entirely covered by the hinder 

 end of the cerebellum. 



MAMMALS. — The mammalian brain is characterized 

 by the increase in the size of the cerebrum and cere- 

 bellum and the relative diminution of di- and mesen- 

 cephalon, the cerebral increase being almost wholly in 

 the pallium (neopallium) in correlation with the ad- 

 vance in mentality. In the lower mammals the olfac- 

 tory lobes are at the tip of the cerebrum, but in the 

 higher, the pallial increase causes a forward growth of 

 the frontal lobes (p. 159) so that the olfactory lobes 



Fig. 179. Brain of ^-^e brought to the lower surface, they and their pos- 



hen. c, cerebrum; cb, terior extensions being separated from the rest of the 

 mTopiiri4L°''(mS cerebrum by a rhinal fissure (fig. 164, r) on either side, 

 brain) ; wo, medulla ob- Behind, the cerebral hemispheres always cover the 

 longata; t;, vallecula; 'twixt-brain, but in the non-placentals and in some 

 the epiphysis shows be- , , i r ^1 • j u • • j 



tween cerebrum and lower orders more or less of the mid-brain is uncovered 



cerebellum. (fig. 180). Still higher in the scale, cerebrum and 



cerebellum all but meet, and in the primates the cere- 

 brum has grown so far back that it partially or completely covers the cerebellum. 

 In the lower groups (some rodents, edentates, etc.) there is no develop- 

 ment of a temporal lobe, and but little if any folding of the cerebrum. Higher 

 in the scale, and even in the monotremes and marsupials, the pallial increase 

 causes a folding so that the morphologically postero-ventral end of the cerebrum, 

 lateral to the pyriform lobe, is brought below and to the outside of the lateral 

 parts of the hemispheres, the line of folding being marked by a deep fissure 

 of Sylvius, at the bottom of which the covered parts of the side of the hemi- 

 sphere form the so-called island of Riel. This folding brings the pyriform or 

 hippocampal lobe to the medial side of the temporal lobe, where it may, in part. 

 be visible in a side view (fig. 181) or, as in man, it may be so covered as not to 



