STRUCTURAL ARRANGEMENTS OF CEREBRUM 417 



cortex extends over the whole of the roof of the cerebral hemispheres, 

 though even here a very large proportion of it is devoted to the association 

 of olfactory impulses. The importance of these olfactory association fibres 

 is well shown in the figure (Fig. 208) of a diagrammatic section through a 

 lizard's brain. Above the reptiles there is a divergence in the course of 

 development. The wider reactive powers of birds are based chiefly on an 

 enormous development of the corpus striatum, whereas in mammals the 

 corpus striatum remains relatively small and the chief development occurs 

 in the roof of the cerebral hemispheres, the so-called pallium or mantle. 

 With the increased entry of fibres from the optic thalamus into the cerebral 

 hemispheres, carrying impulses from the eyes, ears, and all the other sense- 

 organs of the body, the olfactory part of the brain diminishes in importance, 



FIG. 208. Schematic section through brain of lizard showing the chief 

 nerve-tracts. (After EDINGER.) 



and in the higher mammals and man is altogether overshadowed by the newly 

 formed part of the pallium. On this account those parts of the cerebral 

 hemispheres in special connection with the olfactory sense-organs are often 

 spoken of as the archipalUum, in distinction to all the rest of the more 

 newly formed brain substance, known as the neopallium. 



In man the cerebral hemispheres form a great ovoid mass exceeding 

 in size all the rest of the brain put together. The two hemispheres are 

 separated by a deep fissure, the great longitudinal fissure. Before and 

 behind, this fissure extends to the base of the cerebrum, but in the middle the 

 two hemispheres are connected by a mass of transverse fibres known as the 

 corpus callosum. On the outer side each cerebral hemisphere presents a 

 deep cleft, the Sylvian fissure. The whole surface of the brain is thrown 

 by fissures (or sulci) into convolutions, by which means a very large increase 

 of the surface grey matter is obtained. -By these fissures the brain surface 

 is divided into lobes. The general arrangement is shown in Figs. 209 and 210. 

 The chief lobes are the frontal, the parietal, the occipital, the temporal, the 

 insular, the limbic, and the olfactory. On the inner side, from before 

 backwards, we have the marginal, the paracentral, the pre-cuneus, the 

 cuneus ; and in close proximity to the corpus callosum, the cingulum or 



