THE NERVOUS SYSTEM 417 



excessive development is mainly that of the roof and outer side 

 of each of the telencephalic lobes, which form enormous hemi- 

 spheres that extend forward over the rhinencephalon and 

 backward over di- and mesencephalon, usually coming almost 

 in contact with the cerebellum (metencephalon), from which 

 they are separated merely by a membranous or bony par- 

 tition, the tentorium. In addition to increase in bulk there 

 is also an important histological change; namely, the appear- 

 ance of large masses of ganglion cells over the outer surface, 

 arranged in definite layers and constituting the most important 

 nervous element, the seat of the highest faculties. This 

 ganglionic tissue forms a definite layer of gray matter of con- 

 siderable thickness, the cortex cerebri. In lower mammals, 

 such as the marsupials and rodents, the outer surface of the 

 hemispheres remains smooth, but in the higher Orders, such as 

 the ungulates, the carnivores, and especially the primates, it 

 becomes folded up into irregular rounded elevations, the gyri 

 or convolutions, separated from one another by grooves, the 

 deeper of which are termed fissures, (e. g., lateral cerebral fis- 

 sure [fissure of Sylvius] ) ; and the others, sulci (e. g., sulcus 

 centralis [fissure of Rolando] ). 



This folding of the surface has the evident effect of still 

 further increasing the physiological efficiency of the cerebral 

 cortex by extending its surface area within the same mass 

 limits. 



While the main mass of the hemispheres is derived from the 

 roof and the outer side of the telencephalic lobes, the inner side, 

 remaining thin at first, makes a contribution in the form of a 

 longitudinal invagination which thickens, and forms a ridge 

 that encroaches upon the lateral ventricle. This is the hippo- 

 campus [hippocampus major or Amman's horn}. It attains a 

 considerable development in Man, where it forms a conspicu- 

 ous elevation upon the inner side of the floor of the ventricle 

 and becomes prolonged posteriorly into a free rounded end, 

 terminating in digitations [pes hippocampi}. This intrudes 

 itself upon the thick outer portion and lies imbedded in it, 

 covered by the temporal lobe. 



