160 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
cerebrum against which it is folded. In the bottom of the Sylvian fissure is a part 
of the side wall of the cerebrum which has received the name of insula (island of 
Riel), while a hippocampal fissure causes the hippocampus to appear as a pro- 
nounced swelling on the floor of each ventricle. In the lower mammals these are 
the only fissures present, the rest of the cerebral surface being smooth. In the 
higher mammals other grooves (sulci) separating convolutions (gyri) appear. 
These convolutions increase the extent of cerebral surface and as a consequence 
they permit of more cortical gray matter upon which mentality depends. The 
number of gyri increases in the primates and reaches its extreme in man. The 
folding of the cerebrum also affects the cavities of the cerebrum as well as the course 
of the fibre tracts, especially of the fornix which becomes greatly bent on itself. 
In the ventricles distinct regions or ‘horns’ are recognized, an anterior cornu in 
Fic. 163. Fic. 164. 
Fic. 163.—Brain of Chrysothryx sciureus, after Weber. f, frontal lobe; z, interparietal 
fissure; 0, occipital lobe; ~, parietal lobe; s, Sylvian fissure; ¢, temporal lobe; ¢s, sulcus 
temporalis. 
Fic. 164.—Brain of Manis javanica, after Weber. ch, cerebellar hemispheres; h, 
hippocampal lobe; 0, olfactory lobe; ps, presylvian fissure; s, Sylvian fissure; ss, sulcus 
sagittalis; v, vermis; 1, optic nerve. 
the frontal lobe, a posterior in the occipital lobe and an inferior cornu in the 
temporal lobe. Associated with the cortical gray matter are nerve fibres (compara- 
tively few in the lower, extremely numerous in the higher mammals) which form 
a corona radiata and connect the cortex with the more posterior regions of the 
brain. In the non-placental mammals the anterior commissure is very large, 
forming the chief association tract between the two hemispheres, but in the higher 
groups the corpus callosum becomes greatly developed and largely replaces it. 
The diencephalon is greatly reduced, the hypophysis and infundibulum being 
small, the latter showing traces of the saccus vasculosus and inferior lobes so 
prominent in the lower vertebrates. The parietal organ is lacking, but the pinealis 
is relatively large. It is separated from the roof of the skull by the occipital 
lobes of the cerebrum. It is connected with the roof of the brain by two bands 
or peduncles and its cavity contains a quantity of so-called ‘brain sand.’ A 
transverse groove divides the optic lobes so that they consist of four lobes (corpora 
quadrigemina). 
