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 
Fic. 162.—Ventral surface of brain of Ornithorhynchus, after G. Elliot Smith.’ bo, 
bulbus olfactorius; c1, first cervical nerve; cl, cerebellum; cm, corpus mamillare; f, floc- 
culus; /p, lobus pyriformis; op, olfactory peduncle; rf, rhinal fissure; fc, tuber cinereum; fo, 
olfactory tubercle; ¢V, tuberculum quinti; Vm, Vmd, Vmx, motor root and maxillaris and 
mandibularis roots of trigeminal nerve; J-XJI, 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 
