i6o 



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 |jame 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 



^^^/ 



FIG. 163. FIG. 164. 



FIG. 163. Brain of Chrysothryx sciureus, after Weber. /, frontal lobe; i, interparietal 

 fissure; o, occipital lobe; p, parietal lobe; s, Sylvian fissure; t, temporal lobe; ts, sulcus 

 temporalis. 



FIG. 164. Brain of Manis javanica, after Weber, ch, cerebellar hemispheres; h, 

 hippocampal lobe; o, olfactory lobe; ps y presylvian fissure; s, Sylvian fissure; ss, sulcus 

 sagittalis; v, vermis; II, 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) . 



