THE BRAIN IN THE SEPARATE CLASSES 



171 



be seen without a partial dissection. The four lobes of the cerebrum — 

 frontal, parietal, occipital and temporal — are regions rather than sharply limited 

 parts. 



The olfactory lobes are enlarged at the end into an olfactory bxilb and in 

 mammals with well-developed powers of smell an extension of the lateral 

 ventricle extends into each bulb; but in the aquatic mammals (whales, sirenians 

 and seals) and in the primates the whole olfactory lobe is reduced. 



Of the two tracts connecting the olfactory lobe with the regions farther back 

 (p. 160) the ventral or olfactory tract persists, extending back to the hippocampaJ 

 or pyriform lobe. The more dorsal or hippocampal tract is well developed in 

 the monotremes (fig. 181, fd), but higher, with the development of the corpus 

 callosum, it becomes correspondingly reduced and is, in large part, rudimentary 

 in man (fig. 165). The cerebral commissures need no farther mention (see 

 p. 159), but the great development of the neopallium has introduced a new set 



Fig. 180. — At Dorsal view of brain of kangaroo {Macropus rufus), after Ziehen. 

 B and C, lateral and dorsal views of brain of a bat {Vespertilio murinus) after Haller. 

 c, cerebrum; /, flocculus; A, cerebellar hemisphere; m, mid-brain {Corpora quadrigemini); 

 mo, medulla oblongata; v, vermis. 



of fibres, the corona radlata, which connect the cortex with the more posterior 

 parts of the brain. 



The rich development of fibres connected with the cerebral cortex causes 

 an increase in the thickness of the thalamic region of the diencephalon, as the 

 connexion is made through this region with the parts farther back. One 

 result is the narrowing of the third ventricle, which is farther reduced by the 

 development of an intermediate mass, which, however, is largest in the lower 

 mammals (fig. 181). Prominent among the epiphysial structures is the pinealis 

 (conarium) which is completely covered by the cerebrum and can have no 

 visual value. Its stalk is hollow for a distance, but the distal portion contains 

 no nervous matter. In some mammals there is a dorsal outgrowth of epithelium 

 (of the roof plate) just in front of the pinealis (fig. 167). The paraphysis 

 appears in development, but is lost in the adult. 



The mid-brain is peculiar dorsally in the presence of two pairs of optic 



