STRUCTURAL ARRANGEMENTS OF CEREBRUM 473 



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 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 A and B. 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 back- 

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

 cuneus ; and in close proximity to the corpus callosum, the cingu- 

 lum or supra-callosal convolution above, and the hippocampal con- 

 volution and the uncus below. The chief fissures separating these are 

 the Sylvian fissure, the central sulcus or fissure of Rolando, the parieto- 

 occipital fissure, the calcarine fissure, the collateral fissure, and the 

 calloso-marginal fissure. Each of the main lobes mentioned above 

 is further subdivided by smaller fissures. The extent of these secondary 

 fissures varies from brain to brain, the higher types of brain being richer 

 in convolutions than those of the more primitive races. 



The gradual evolution of the cerebral cortex, and the concomitant 

 shifting of the chief afferent impulses, arising in the projicient sense- 

 organs from the lower ganglia to the higher educatable cortex, is well 

 shown in the accompanying diagrams from Monakow (Fig. 210). In 

 the lower fishes practically all the reactions to visual impressions are 

 carried out by the optic lobes. In the higher types the reflexes through 

 these lobes become subordinated, first to the more complex organ of 

 the optic thalamus (where representatives from all the afferent tracts 

 of the body assemble), and later to the still more complex occipital 

 cortex, when the reactions are determined not only by inherited 

 nerve-paths but also by the various blocks and facilitations imprinted 

 on the nerve- paths by the experience of the individual himself. 



The original cavities of the hemispheres form the lateral ventricles, 

 each of which, in the adult brain, is prolonged into the main divisions 

 of the hemispheres as the anterior horn, the posterior horn, and the 

 inferior horn. Each lateral ventricle is roofed over by the corpus 

 callosum and the adjoining white matter of the hemispheres. On 

 opening the ventricle we see on its floor the body of the fornix, a 

 flattened tract of white matter with longitudinal fibres, which in front 

 bifurcates into two cylindrical bundles which pass vertically down- 

 wards in front of the foramen of Monro into the mesial part of the 

 subthalamic tegmentum. Internal to the fornix is a layer of pia 



