BRAIN. 



159 



brum covers the di- and mesencephalon, and in the primates even the whole of the 

 cerebellum from above. This increase of the cerebrum is largely an increase of 

 the nervous matter of the pallium, a portion the neopallium developing on the 

 lateral side of each hemisphere between the hippocampus and the basal structures 

 (pyriform lobes). This increase in cerebrum is limited in forward and backward 

 growth by the limitations of skull development. Hence it overlaps the olfactory 



FIG. 162. Ventral surface of brain of Ornithorhynchus, after G. Elliot Smith, bo, 

 bulbus olfactorius; c 1 , first cervical nerve; cl, cerebellum; cm, corpus mamillare;/, floc- 

 culus; Ip, lob us pyriformis; op, olfactory peduncle; rf, rhinal fissure; tc, tuber cinereum; to, 

 olfactory tubercle; tV, tuberculum quinti; Vm, Vmd, Vnix, motor root and maxillaris and 

 mandibularis roots of trigeminal nerve; I-XII, cranial nerves. See also fig. 152. 



lobes in front, so that they appear to rise from its ventral surface, while behind it 

 extends backward, then turns downward and lastly extends forward along the sides 

 of the mid- and 'twixt-brains, even overlapping a part of the cerebrum itself. In 

 this way the cerebrum becomes marked off into a series of regions called the frontal 

 lobes in front, the parietal above, the occipital behind, while the reflexed ventral 

 portion of either side makes a temporal lobe. 



This folding and overgrowth causes grooves or fissures in the surface of the 

 cerebrum, the most constant being a rhinal fissure between olfactory and frontal 

 lobes, a Sylvian fissure between the temporal lobe and the lower surface of the 



