THE CEREBRAL HEMISPHERES 



927 



the threshold or limen insulae (belonging to the rhinencephalon). An oblique 

 transinsular or central insular fissure divides this district into a larger preinsular 

 and a smaller postinsula. The postinsula is usually a single long gyre (gyrus 

 louyus in.<;ulae), while the preinsula is subdivided by shallow fissures into three, 

 four, or five shorter preinsular gyres built upon a radiate plan, converging in the 

 region of the insular pole. As already hinted, the island of Ileil represents an 

 area of the brain mantle whose growth did not keep pace with that of the surround- 

 ing parts; hence its submergence by them. The close apposition of the insular 

 region to the subjacent basal ganglia, and the failure of development of great 

 masses of projection fibres so prominent elsewhere, were doubtlessly factors 

 therein. The insular cortex is uninterruptedly continuous with the rest of the 

 cortex, but it has become specialized into the purest association centre in the 

 cerebrum, and we shall learn of its intimate relations to the faculty of speech at 

 a later stage. 



'i. IS79. The left inland of Reil schematically represented in a supposedly transparent cerebral hemisphere, 

 showing how it is concealed from view by the opercula. 



The Rhinencephalon, or Olfactory Lobe (lobus olfactorius) (Figs. 680, 681). 

 The grouping of the parts constituting the central olfactory structures under the 

 term "rhinencephalon" as distinguished from the rest of the fore-brain (pallium) 

 was first clearly made by Turner and proved by His to be embryologically well 

 founded and by Edinger to agree with phylogenetic development. More light 

 has been thrown upon the subject recently by Retzius and Elliott Smith. The 

 sense of smell, while highly useful in the quest for food in earlier and lower forms 

 of vertebrates, is relatively little used in the mental life of man. The enormous 

 preponderance of the cerebral mantle and the concomitant atrophy of the rhinen- 

 cephalon in the human brain afford one of the most striking contrasts in brain 

 morphology. This relatively feeble development in bulk of the olfactory apparatus 

 in the human brain by no means renders its description simple. In fact, not 

 until its development in lower macrosmatic animals was studied could anatomists 

 form even an approximately clear conception of the seemingly disjointed remnants 

 in the human brain of an olfactory apparatus so relatively huge in lower animals. 

 The great expansion of the cerebral hemispheres and of the great commissure 



