THE NERVOUS SYSTEM 1465 



basal ganglia of the cerebral hemisphere^namely, the corpus striatum, 

 claustrum, and amygdaloid nucleus of either side. The superficial part of 

 the cortical substance represents the primitive insula, or island of Reil. The 

 Syl ian fossa gradually extends across the ventral aspect of the hemisphere- 

 vesicle towards the lower margin of its mesial surface, and the temporjil lobe 

 grows forwards and downwards towards it. In this manner the stent of the 

 Sylvian fissure is formed. The fossa also extends in a back\vard direction 

 upon the lateral surface of the vesicle, and in this manner the rudiment of 

 the posterior horizontal limb of the Sylvian fissure is formed. The anterior 

 ascending and anterior horizontal limbs are formed at a later period. The 

 elevated margins of the Sylvian fossa and of its back\vard extension, which 

 gradually approach each other, and meet over the insula, represent the 

 opercular region. 



The cerebral hemisphere on either side is disposed in the form of an in- 

 complete ring around the Sylvian fossa, the break in the ring being formed 

 by that fossa. The stalk of the hemisphere is located at the fossa, and cor- 

 responds to the corpus striatum, with which is associated the optic thalamus. 

 The remaining, and much larger, 

 part of the hemisphere constitutes 

 the neopallium. 



The insula, or island of Reil, as 

 stated, is developed from the super- 

 ficial part of the cortical substance 

 which forms the floor of the Sylvian 

 fossa. The grey matter becomes 

 almost completely surrounded by a 

 limiting fissure, and is then broken 

 up by sulci into the component 

 parts of the adult insula. _ _ 



The anterior part of the hemi- F^^. 612.— The Primitive Fissure of 

 sphere-vesicle corresponds to the Sylvius. 



frontal lobe ; the lower part, as far 



forwards as the stem of the Sylvian fissure, becomes the temporal lobe, and 

 the upper and posterior part represents the parietal lobe. The occipital lobe 

 is formed at a later period as the hemisphere grows backwards. The limbic 

 lobe is developed in connection with the mesial surface of the hemisphere. 

 The olfactory lobe is developed as a hollow protrusion from the ventral aspect 

 of the frontal portion of the hemisphere-vesicle. 



The surface of each hemisphere becomes very much broken up into gyri or 

 convolutions, this being effected by means of fissures. The fissures are of 

 two kinds — complete and incomplete. The incomplete fissures are merely 

 sulci produced by the growth of the gyri, and they do not involve the entire 

 thickness of the walls of the cerebral hemispheres. The complete fissures 

 are infoldings of the walls of the cerebral hemispheres, and involve their entire 

 thickness. They consequently give rise internally to certain prominences 

 upon the wall of each lateral ventricle — namely, the corpus striatum, lateral 

 choroid plexus, hippocampus major, calcar avis, and eminentia collateraUs. 

 The complete fissures are as follows : 



SyK-ian. Calcarine (anterior part). 



Choroidal. Parieto-occipital. 



Hippocampal. Collateral (central part). 



With the exception of the Sylvian fissure, already described in connection 

 with the Sylvian fossa, the other complete fissures appear on the mesial surface 

 of the vesicle of the cerebral hemisphere. 



The choroidal fissure is not really a fissure, but merely a groove or sulcus 

 produced by an infolding of the vesicular wall, which is here composed entirely 

 of ependyma. It commences above and behind the foramen of Monro of the 

 corresponding side, and it terminates close to the tip of the temporjil pole, 

 where the inferior or descending comu of the lateral ventricle ends. Between 

 these two points it passes backwards, downwards, and then forwards into 



